DWDM Principle Training Manual

DWDM Principle Training Manual

ZTE CORPORATION CORPORATION

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ZTE UNIVERSITY ZTE University, Dameisha, Yantian District, Shenzhen, P.R.China Postcode: 518083 Tel: (+86755) 26778000 Fax: (+86755) 26778999 ZTE CORPORATION ZTE Plaza, Keji Road South, Hi-Tech Industrial Park, Nanshan District, Shenzhen, P.R.China Postcode: 518057 Technical Support Websit ：http://support.zte.com.cn Client Support Hot line：（+86755 ）26770800

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Version: 1.0 S.N.:PXJCSW200512097

800-830-1118

COPYRIGHT

Copyright © ZTE Corporation All rights reserved. All information contained herein are confidential information of ZTE and must be handled with highest care. care.

Nobody can, can, for any any purpose, copy, save, save, link to searching searching

tools, or distribute by any means (including but not limited to electronic, mechanical, photocopying, recording means) of the above mentioned information without prior written consent of ZTE.

Author: Randy Editor: Guo Yali *

*

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ZTE UNIVERSITY ZTE University, Dameisha, Yantian District, Shenzhen, P.R.China Postcode: 518083 Tel: (+86755) 26778000 Fax: (+86755) 26778999 ZTE CORPORATION ZTE Plaza, Keji Road South, Hi-Tech Industrial Park, Nanshan District, Shenzhen, P.R.China Postcode: 518057 Technical Support Websit ：http://support.zte.com.cn Client Support Hot line：（+86755 ）26770800

Fax: (+86755) 26770801 *

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Version: 1.0 S.N.:PXJCSW200512097

800-830-1118

Preface Thanks for using DWDM Principle Training Manual. In order to use the Manual properly, please read the Preface first.

1. Application This Manual should not be used for the purpose of on-site installation or trouble shooting.

2. About This Manual This manual is composed of one volume，and the table of contents of the volume is shown below:

Volume

I

Course Code

WM_001_E1

Course Name

DWDM Principle

This manual is based on DWDM fundamentals. We will update this manual while the product is upgraded. We apologize if there is any discrepancy between the manual and the products used in your company.

3. Conventions  Key points

Indicates the key points in one section.

O Note

A Note statement statement is used to alert the important.

reader of installation, installation, operation, or maintenance information that is

5 Caution

Indicates a potentially hazardous situation which, if not avoided, could result in damages to the equipment or personal injury. injury. It may also be used to alert against unsafe practices.

& Tips

Indicates a suggestion or hint to make things easier or more productive for the reader.

4. Manual Update history

Version

Date

Comments

1.0

Dec. 2005

New

5. From the Author Thank you for using this manual and your continuous support. We would appreciate your comments and suggestions on this Manual. We can be reached at Telephone：（+86755）26778806 Fax：（+86755）26778999

ZTE UNIVERSITY

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Man-machine

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WM_000_E1

DWDM Basic Principles

Course Objectives: z

z

Grasping the significance and application environment of the DWDM networks Grasping the DWDM principles and the key technology for implementing the DWDM

References: z

z

z

Optical Wavelength Division Multiplexing System Modern Communication Base and Technology Principle and Test of DWDM Transmission System

Contents

1 DWDM Overview.......................................................................................................................................1

1.1 Background of DWDM Technology ................................................................................................. 1 1.1.1 Development of Multiplexing Technology in Optical Network .............................................1 1.1.2 PDH, SDH and DWDM ......................................................................................................... 2 1.2 DWDM Technology Overview.......................................................................................................... 5 1.2.1 Different between DWDM Technology and Other Multiplexing Technologies..................... 5 1.2.2 Relationship between DWDM and SDH................................................................................7 1.2.3 Operation Wavelength Range...............................................................................................10 1.3 DWDM Features and Advantages ................................................................................................... 12 1.4 DWDM Development Trend ........................................................................................................... 13 2 Overview of Optical Fiber Communication ........................................................................................... 15

2.1 Basic Knowledge of Optical Fibers.................................................................................................15 2.1.1 Brief Introduction to Optical Fibers ..................................................................................... 15 2.1.2 Usage Overview of Applicable Frequencies of Optical Fiber .............................................. 18 2.1.3 Types and Features of Common SMFs.................................................................................19 2.2 Working Wavelength of DWDM System ........................................................................................ 20 2.2.1 Introduction to Working Wavelength Area ........................................................................... 20 2.2.2 Wavelength Allocation.......................................................................................................... 21 2.3 Fiber Transmission Features............................................................................................................25 2.3.1 Fiber Loss ............................................................................................................................. 25 2.3.2 Dispersion.............................................................................................................................26 2.3.3 Non-Linear Effect of Fiber ................................................................................................... 29 2.4 New Optical Fiber Types.................................................................................................................33 i

3 Key Technologies of DWDM System...................................................................................................... 35

3.1 Basic Structure of DWDM System ................................................................................................. 35 3.2 Light Source Technology ................................................................................................................ 36 3.3 Optical Wavelength Division Multiplexing and De-Multiplexing Technologies ............................ 39 3.3.1 Overview.............................................................................................................................. 39 3.3.2 Introduction to OM .............................................................................................................. 39 3.3.3 Key Performance Indices ..................................................................................................... 42 3.4 OTU Technology............................................................................................................................. 44 3.4.1 Overview.............................................................................................................................. 44 3.4.2 Working Principle and Performance Indices ........................................................................ 45 3.4.3 Classification and Applications of OTU .............................................................................. 48 3.5 Optical Amplifying Technology...................................................................................................... 49 3.5.1 EDFA Technology ................................................................................................................ 49 3.5.2 Raman Amplifying Technology ........................................................................................... 55 3.6 Supervision Technology..................................................................................................... ............. 56 3.6.1 Functions of Optical Supervision Channel (OSC) ............................................................... 57 3.6.2 Requirements for OSC ......................................................................................................... 57 3.6.3 Implementation of OSC ....................................................................................................... 58 4 Protection Principle of DWDM System ................................................................................................. 61

4.1 Brief Introduction to DWDM System Hierarchy............................................................................ 61 4.2 1+1 Protection................................................................................................................................. 62 4.2.1 Link 1+1 Protection ............................................................................................................. 62 4.2.2 Ring 1+1 Protection ............................................................................................................. 64 4.2.3 Features of 1+1 Protection ................................................................................................... 65 4.3 1: N Protection ................................................................................................................................ 65 4.3.1 Working Principle ................................................................................................................ 65

ii

4.3.2 Protection Implementation ................................................................................................... 66 4.3.3 Features of 1:N Protection....................................................................................................66 4.4 Bidirectional Optical Channel Protection........................................................................................67 4.5 Bidirectional OMS Protection ......................................................................................................... 69

iii

1 DWDM Overview  Highlights: z

DWDM definitions and generation background.

z

DWDM development trend.

1.1 Background of DWDM Technology Prior to the introduction to DWDM technology, we should know about the development of the optical network. This section will introduce the background of the DWDM technology in terms of multiplexing technology and transmission technology.

1.1.1 Development of Multiplexing Technology in Optical Network The communication network covers diversified transmission media, such as twisted pair, coaxial cable, optical fiber and wireless transmission. Among them, the optical fiber transmission features large transmission capacity, good quality, small attenuation, strict security and large trunk distance. Since the broadband high-speed service ceaselessly develops in the information age, the optical transmission system is not only expected to have larger capacity and longer distance, but also expected to be interactive, fast and convenient. Therefore, the multiplexing technology is introduced in the optical transmission system. The multiplexing technology means to use the broadband and large-capacity features of the optical fiber to simultaneously transmit multiple channels of signals on one optical fiber or cable. In the multi-channel signal transmission system, the multiplexing mode of signals greatly affects system performance and cost. The multiplexing technology of optical transmission network goes through three development phases: Space Division Multiplexing (SDM), Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). With simple design and practical feature, the SDM technology requires that the quantity of fiber transmission cores must be configured in accordance with quantity signal multiplexing channels, which means poor investment profit. The TDM 1

WM_000_ E1


technology is widely applied. It is the base of PDH, SDH, ATM and IP. But its disadvantage is low line utilization ratio. The WDM technology supports multiple wavelengths (channels) to be carried on one fiber. So it is the major measure for expanding the current fiber communication network and is mostly used in trunk network.

1.1.2 PDH, SDH and DWDM The traditional fiber transmission technologies, such as Plesiochronous Digital Hierarchy

(PDH)

and

Synchronous

Digital

Hierarchy

(SDH),

use

"one-wavelength-in-one-fiber" mode. They are so restricted by the characteristics of their own equipment that neither the transmission capacity nor expansion mode can meet the requirements of the communication network developing at a high speed, while leaving the massive bandwidth resources of fibers far from being fully exploited. The new Dense Wavelength Division Multiplexing (DWDM) becomes the most effective and practical means for the fiber expansion. With its unique technical advantages, the DWDM technology becomes a simple and economical means to expand the fiber transmission capacity in a rapid and effective manner. It can fully meet the current need of the network broadband service development and lays a solid foundation for the development of the future fully-optical transmission network. The development processes of PDH, SDH and DWDM are briefed below, as well as interface specifications of each technology. 1.

PDH The early optical transmission system uses PDH, which introduces in Pulse Coding Modulation (PCM) digital transmission technology based on the former analog telephone network. It multiplexes signals of low rate level into high-speed signals by means of bit filling and digit interleaving. The primary signals of the PDH system are in synchronous TDM mode, and the multiplexing of other high order groups are in plesiochronous (or called asynchronous) TDM mode. The PDH system covers three regional rate level standards in Europe, North America and Japan, as shown in Table 1.1-1.

2

Chapter 1 DWDM Overview

Table 1.1-1

Country/Region

Europe

and

China

Primary Group

Secondary

(Primary)

Group

2.048Mbit/s 30 channels

North America

1.544Mbit/s 24 channels

1.544Mbit/s

Japan

PDH Code Rate

24 channels

Tertiary Group

Quartus Group

8.448Mbit/s

34.368Mbit/s

139.264Mbit/s

120 channels (30

480

1920

× 4)

(120 × 4)

(480 × 4)

6.312Mbit/s

44.736Mbit/s

274.176Mbit/s

96 channels (24

672 channels (96

4032

× 4)

× 7)

(672 × 6)

6.312Mbit/s

32.064Mbit/s

97.728Mbit/s

96 channels (24

480 channels (96

1440

× 4)

× 5)

(480 × 3)

channels

channels

channels

channels

From early 1970's to 1980's, the PDH system and devices are popularly used in the digital network. However, along with the developing fiber communication technology and user's increasing demands for communication services, the PDH disadvantages are more and more remarkable. 1)

The compatibility between three rate standards is not available, which obstructs development of international interconnection.

2)

There is no worldwide standard optical interface specification. Private optical interfaces developed by different manufacturers are not compatible with each other, which limits networking flexibility and increases network complexity and operation costs.

3)

PDH is a kind of multiplexing structure based on point-to-point transmission. It only supports point-to-point transmission, but cannot accommodate complicated networking.

4)

The operation, management and maintenance must depend upon manual digital signal cross-connection and service-suspension test, which cannot meet monitoring and NM requirements of modern communication network.

5)

Along with rate increase, it is more and more difficult to implement multiplexing of high-order groups through PDH technology, and requirements of fiber

digital

communication

transmission cannot be met.

3

for

large-capacity

and

super-high

speed

WM_000_ E1


2.

SDH In mid-1980's, the Bell Communication Research Institute in America put forward the concept of Synchronous Optical Network (SONET). In 1988, the CCITT (former ITU-T) accepted the SONET concept, formed the worldwide unified technology standard for transmission network, and rename it as SDH. The SDH signals use synchronous multiplexing mode and flexible multiplexing and mapping structure. Code streams of different levels are arranged regularly in the payload of the frame structure. The payload is s ynchronous with the network, so software can be used to directly de-multiplex a high-speed signal into low-speed tributary signals at a time, called “one-step” de-multiplexing”. The rate specifications of the SDH system are shown in Table 1.1-2.

Table 1.1-2 SDH Level (ITU-T)

SDH Signal Levels

OC Level (SONET)

Line Rate (Mbit/s)

STM-1

OC-3

155.520

STM-4

OC-12

622.080

STM-16

OC-48

2488.320

STM-64

OC-192

9953.280

The SDH standardizes the features of the digital signals, such as frame structure, multiplexing mode, transmission rate level and interface code pattern. It provides a frame that is supported globally, on which a world-class telecom transmission network has been developed, featuring flexibility, reliability and convenient management. This kind of transmission network is easy to expand and is applicable to the development of new telecom services. In addition, it makes possible the interworking between the devices of different manufacturers. But, after the transmission rate is higher than 10 Gbit/s, the system dispersion and other negative influences will increase difficulty of long-distance transmission. Furthermore, the SDH system is the TDM system based on the single wavelength. The single-wavelength transmission cannot fully utilize the broad bandwidth of fiber. Therefore, the WDM technology is introduced in the backbone network, greatly enlarging the transmission capacity of fiber.

4


3.

DWDM DWDM is one of WDM technologies. Due to small interval (1 nm ~ 10 nm order) between adjacent wavelengths, it is called DWDM. At present, the practical DWDM system works in 1550 nm window for the purpose of using the gain spectrum feature of the EDFA to directly amplify the composite optical wavelength signals. To meet the horizontal compatibility between systems, the central wavelength of the optical channel must accord with G.692 standard. In the DWDM system, each optical channel can bear different customer signals, such as SDH signal, PDH optical signal and ATM signal. Due to unique advantages of fiber communication and its networking technologies for accommodating multi-service and broadband requirements, high-speed SDH system, N × 2.5Gbit/s DWDM system and N × 10Gbit/s DWDM system become majority and backbone of the core network.

1.2 DWDM Technology Overview The DWDM technology is this kind of fiber communication technology: Transmitting multiple optical carriers with information (analog or digital) on one fiber and expanding system only through wavelength (channel) increase. It can combine (multiplex) optical signals with different wavelengths and then transmit them. After the transmission, it can separate (de-multiplex) the combined optical signals and then send them to different communication terminals. In other words, it can provide multiple virtual fiber channels on one physical fiber.

1.2.1 Difference Technologies

between

DWDM

Technology

and

Other

Multiplexing

This section compares the multiplexing technologies often used in the fiber communication system. 1.

TDM TDM means that different channels of signals use different time intervals (timeslot) for signal transmission on the same fiber. The TDM has fixed timeslot allocation, which facilitate adjustment and control and is applicable to the digital information transmission.

5

WM_000_ E1


Its disadvantage is low line utilization, because when a signal source has no data for transmission, the corresponding channel will be idle but the other busy channels cannot occupy this idle channel. In addition, restricted by modulation capacity of high-speed electronic components and laser, it is difficult to implement the system with a capacity over 40 Gbit/s. The TDM technology is widely applied, such as PDH, SDH, ATM and IP. 2.

SDM SDM means the technology that divide space into different channels to implement wavelength multiplexing. For example, you can add the core quantity in the cable or use more fibers to form different channels. The SDH performs optical intensity modulation to each channel of baseband signals respectively. Each channel of signals are transmitted by one fiber, different channels will not affect each other, leading to best transmission performance. The SDM technology has simple design and practical feature, but it requires that the fiber transmission core quantity must be configured in accordance with the signal multiplexing channel quantity, leading to poor i nvestment profit.

3.

SCM Microwave Sub-Carrier Multiplexing (SCM) technology means to modulate multiple baseband signals into the microwave carriers with different frequency for the sake of electrical Frequency Division Multiplexing (FDM), and then use this bit stream to modulate a single optical carrier into the fiber. At the receiving end, pick the electrical FDM line signals through the photoelectrical detector and then use microwave technology to de-multiplex the different microwave carriers, to restore the former baseband signals. This technology is mostly used in the CATV multi-band transmission system of access network.

4.

WDM WDM means to bear multiple wavelength (channel) systems on one fiber and convert one fiber into multiple "virtual" fibers, each of which independently works on different wavelengths.

Boasting economical

efficiency and

practicability, the WDM system technology is the major wavelength

6


multiplexing technology commonly used in the current fiber communication network. The WDM is divided into three multiplexing modes: 1310 nm/1550 nm-wavelength multiplexing, Coarse Wavelength Division Multiplexing (CWDM) and DWDM. 1)

1310 nm/1550 nm-wavelength multiplexing In early 1970's, this multiplexing technology only uses two wavelengths: one in 1310 nm window and the other in 1550 nm window. It implements single-fiber dual-window transmission through the WDM technology. It is the initial wavelength division multiplexing case.

2)

DWDM In simple words, the DWDM technology refers to the WDM technology with small interval between adjacent wavelengths, with working wavelength in the 1550 nm window. It can bear 8 ~ 160 wavelengths on one fiber, mostly used in long-distance transmission system. For the details, please refer to other chapters in this manual.

3)

CWDM

The CWDM technology refers to the WDM technology with large interval (usually greater than 20 nm) between adjacent wavelengths. Usually, its wavelength quantity is 4 or 8 (16 at most). It uses 1200 nm ~ 1700 nm windows. It adopts non-cooling laser and passive amplifier component, leading to a lower cost than that of DWDM. Its disadvantages are small capacity and short transmission distance. Therefore, the CWDM technology applies to the communication cases with short distance, broad bandwidth and dense access points, for example, the network communication inside the building or between buildings.

1.2.2 Relationship between DWDM and SDH 1.

Relationship between DWDM and SDH on the transmission layer of optical network

7

WM_000_ E1


Both DWDM system and SDH system belong to the transmission network layer. They are the transmission means established on the fiber transport media. Their relationship in the transport network is shown in Fig. 1.2-1.

Circuit layer (for example, ATM and IP)

SDH channel layer

SDH system

ADM

DWDM system

OTM

DXC

DWDM optical channel layer

Fig. 1.2-1

OADM

Relationship between DWDM and SDH in Transport Network

The SDH system implements multiplexing, cross-connection and networking on the electrical channel layer. The WDM system implements multiplexing, cross-connection and networking on the optical domain. 2.

Multiplexing modes of DWDM and SDH for carrier signals The SDH is the TDM system based on a single wavelength (one fiber transmitting one wavelength channel). When the transmission rate exceeds 10 Gbit/s, the system dispersion and other negative influences will increase the difficulty of long-distance transmission. The DWDM technology simultaneously transmits multiple optical carrier signals of different wavelengths in the same fiber, fully utilizing the bandwidth resources of fiber and increasing system transmission capacity.

3.

DWDM capable of transmitting different types of signals At present, most customer-layer signals in the DWDM system are SDH signals. But the wavelengths used in the DWDM system are mutually separated and 8


irrelated with the service signal formats, so each wavelength can transmit the optical signals with totally different features, for the sake of hybrid transmission of multiple kinds of signals. The relationship between DWDM system and some common services is shown in Fig. 1.2-2.

IP

ATM

SDH

SDH

ATM

Ethernet

Other

Open optical interface

DWDM

Fiber physical layer

Fig. 1.2-2

4.

Relationship between DWDM and Other Services

Optical interface standards of DWDM and SDH signals The optical interfaces of the SDH device should accord with the ITU-T G.957 recommendation, which does not stipulate the working central wavelength. The optical interfaces in the DWDM system must accord with the ITU-T G.692 recommendation, which specifies the reference frequency of each optical channel, channel interval, nominal central frequency (central wavelength), central frequency offset and other parameters. Therefore, the DWDM system can be either open DWDM system or integrated DWDM system.

z

Open system: The transmitting side of the WDM system provides the optical wavelength converter (OTU) to converts the customer signals with non-standard wavelength into the standard wavelength compliant with G.692 standard. "Open" means that the DWDM system has no special requirement for the working wavelength of the input signals, for example, the signals accessed through "Open interface" shown in Fig. 1.2-2.

9

WM_000_ E1


z

Integrated system: All the customer signals accessing the DWDM system must accord with G.692 standard, for example, the signals accessed through "Non-open interface" shown in Fig. 1.2-2.

5.

Integrated application of DWDM and SDH The transmission capacity of the fiber network can be effectively improved through integrated application of DWDM and SDH.

1.2.3 Operation Wavelength Range The quartz fiber has three low-loss windows: 860 nm, 1310 nm and 1550 nm, as shown in Fig. 1.2-3. 3.0 ~140THz 2.5

~50THz OH- absorption peak

2.0

OH- absorption peak

Loss (dB/km) 1.5 OH- absorption peak

1.0 0.5

O

E

S

C

L

0 800

1000

1200

1400

1600 Wavelength (nm)

O: Original Band

E: Extend Band

Fig. 1.2-3

1.

S: Short Band

C: Conventional Band

L: Long Band

Low-Loss Windows in Fiber Communication

860 nm window Its wavelength range is 600 nm ~ 900 nm. It is used in multi-mode fiber, with large transmission loss (2 dB/km averagely). It is applicable to the short-distance access network, such as Fiber Channel (FC) service.

2.

1310 nm window The lower limit of the available wavelength here depends on the fiber cut-off wavelength and fiber attenuation coefficient, and the upper limit depends on OH

10


root absorption peak at 1385 nm. The working wavelength range is 1260 nm ~ 1360 nm. The average loss is 0.3 dB/km ~ 0.4 dB/km. This window is applied to intra-office, short-distance and long-distance communication of STM-N signal (N = 1, 4 or 16). The light source can be multi-longitudinal mode laser (MLM) and LED. Since the broadband optical amplifier working in 1310 nm window is not available at present, this window is not suitable for the DWDM system. 3.

1550 nm window The lower limit of the available wavelength here depends on OH root absorption peak at 1385 nm, and the upper limit depends on infrared absorption loss and bending loss. . The working wavelength range is 1460 nm ~ 1625 nm. The average loss is 0.19 dB/km ~ 0.25 dB/km. The loss in the 1550 nm window is the lowest, so it can be applied to short-distance and long-distance communication of SDH signals. In addition, the EDFA often used currently has sound gain flatness in this window, so this window is applicable to the DWDM system as well. The working wavelength in the 1550 nm window is divided into three parts (S band, C band and L band), band), with the wavelength range range shown in Fig. 1.2-4.

Fig. 1.2-4

1)

Division of Working Wavelength Wavelength in 1550 Window

S band (1460 (1460 nm ~ 1530 nm): Since the working working wavelength wavelength range of EDFA EDFA is in C band or L band, S band is not used in the DWDM system at present.

2)

C band (1530 nm ~ 1565 nm): nm): It is often used as the working wavelength area of the DWDM system under 40 waves (with band interval as 100 GHz), DWDM system under 80 waves (with band interval as 50 GHz) and SDH system.

3)

L band (1565 nm ~ 1625 nm): nm): Working Working wavelength wavelength area of the DWDM system above 80 waves. In this case, the band interval is 50 GHz. 11

WM_000_ E1


1.3 DWDM Features and Advantages 1.

Fully utilizing fiber bandwidth resources and featuring featuring large large transmission capacity The DWDM technology makes full use of the huge bandwidth (about 25 THz) resource of fibers to expand the transmission capacity of the system.

2.

Super-long Super-long transmission distance Through EDFA and other super-long distance transmission technologies, the channel signals in the DWDM system are amplified at the same time, for the sake of long-distance transmission of the system.

3.

Abundant service access types The wavelengths in the DWDM system are separated to each other, capable of transmitting different services in transparent way, such as SDH, GbE and ATM signals, for the sake of hybrid transmission of multiple kinds of signals.

4.

Saving fiber resource The DWDM system multiplexes multiple single-channel wavelengths for transmission in one fiber, greatly saving fiber resource and reducing line construction cost.

5.

Smooth upgrading and expansion Since the DWDM system transmits the data in each wavelength channel in transparent way and does no process the channel data, only more multiplexing wavelength channels should be added for expansion, which is convenient and practical.

6.

Fully utilizing well-developed TDM technology At present, the optical transmission technologies in TDM mode, such as SDH, have been well developed. Through the WDM technology, the transmission capacity can be enlarged by several times or even dozens of times, with expansion cost lower than that in the TDM mode.

7.

Forming full optical network Full optical network is the development trend of the optical transport network. In such network, the WDM system is connected with Optical Add/Drop Multiplexer (OADM) and Optical Cross-connection (OXC) device, to directly 12


performing optical channel channel adding/dropping and cross-connection cross-connection to the services services on the optical wavelength signals, and thus forming full optical network with high flexibility, reliability, survivability and economical efficiency to meet the requirements of future information society for the broadband transport network.

1.4 DWDM Development Trend 1.

Higher channel rate The channel rate of the DWDM system has developed to 10 Gbit/s from 2.5 Gbit/s, and the system at 40 Gbit/s rate is in experimentation and its technology becomes more and and more mature.

2.

Greater wavelength multiplexing quantity The DWDM system at early phase usually works at 8/16/32 wavelengths, with channel interval as 100 GHz and working wavelength in C band. Along with ever development of the technology, the working wavelength of DWDM system can cover C and L bands, with interval as 50 GHz. For example, ZTE's ZXWM M900 device can provide the multiplexing of up to 160 waves.

3.

Super-long fully-optical fully-optical transmission distance The initial cost and operation cost for network construction can be reduced through extending full optical transmission and reducing electrical regeneration points. The traditional DWDM system uses EDFA to extend the current-free delay transmission distance. At present, this distance can be extended from 600 km to above 2000 km, through distributed Raman amplifier and super-powerful Forward Error Correction (FEC) technology, technology, dispersion management technology and optical balancing technology and effective effective modulation formats.

4.

Evolving from point-to-point WDM to full optical network The ordinary point-to-point DWDM system consists of Optical Terminal Multiplexer (OTM). Although it has a huge transmission capacity, it only provides primitive transmission bandwidth and features inflexible networking capability. Along with ceaseless development of electrical cross-connection system, the node capacity ever increases, and the point-to-point networking obviously cannot keep up with the growth of network transmission link capacity. capacity. 13

WM_000_ E1


The further expansion opportunity depends on the optical nodes, that is, OADM and OXC. Through OADM, we can construct chain and ring optical networks. The OADM controls the optical signals of different wavelength channels to be sent to the proper locations, for the sake of protection and restoration of optical services. OXC is the route switch of next generation optical communication. In the full optical network, it provides these functions: Providing connection function based on wavelengths, providing wavelength add/drop function of optical channels, leading the wavelength channels for the sake of best utilization of fiber infrastructure, and implementing protection and restoration on wavelength, wavelength and fiber levels. The OXC is set at the important tandem point of the network, converging input of different wavelengths from different directions and then output signals with proper wavelengths. Through OADM and OXC, we can construct more complicated ring network. In the next generation IP Over DWDM telecom/network architecture, the OXC may replace the existing electrical switching/route by optical signals. 5.

Development of IP over DWDM technology The bandwidth of the Internet backbone network increases quickly, so if the DWDM technology is not used, Internet data streams alone will occupy the capacity of the whole single-wavelength fiber system (at present, the maximum transmission rate of the single-wavelength fiber system for commercial use is 10 Gbit/s). Therefore, the IP over DWDM will be the major technology of the network communication in the future.

14

2 Overview of Optical Fiber Communication  Highlights: z

Basic knowledge on optical fiber communication.

z

Types and applications of common optical fibers.

z

Transmission features of optical fibers.

2.1 Basic Knowledge of Optical Fibers 2.1.1 Brief Introduction to Optical Fibers 1.

Structure of optical fibers Optical fiber is a kind of cylinder glass fiber with sound light conducting performance and small diameter, consisting of fiber core, cladding and coating layer, as shown in Fig. 2.1-1. Coating layer Cladding

n2 n1: Refractive index of fiber core

n1

n2: Refractive index of cladding

Fig. 2.1-1

1)

Fiber core

Structure of the Optical Fiber

Fiber core It is made of SiO 2 (quartz). It also comprises few doped chemical, such as GeO 2, to improve refractive index (n1) of the fiber core. The diameter of the fiber core usually ranges 5 µm ~ 50 µm.

15

WM_000_ E1


2)

Cladding Usually, it is made of SiO 2, with outer diameter as 125 µm. The refractive index (n2) of cladding is less than that (n1) of fiber core.

3)

Coating layer It is made of high molecular materials, such as epoxide resin and silicone rubber, with outer diameter as about 250 µm. Through adding coating, we can improve flexibility, mechanical strength and aging-resistance features of the optical fiber.

2.

Fiber categories

1)

By distribution shape of refractive index When the light is transmitted in the fiber, each light shoots into the juncture between fiber core and cladding in proper angle. Since the refractive index (n1) of the fiber core is greater than that (n2) of the cladding, when the shooting-in angle of the light meets the full reflection condition, the light can be repeatedly reflected on the juncture and move forwards in "zigzag" way, and thus restricting the light inside the fiber core and forming transmission wave. Depending on the refractive index radio distribution on the fiber cross section, the fiber is divided into step-type fiber and graded fiber. The relationship between refractive index and fiber structure as well as the transmission of light in the fiber are shown in Fig. 2.1-2.

n2

Cladding Fiber core Light

n1

n2

Cladding Fiber core Light

n1

a: Step-type fiber

Fig. 2.1-2

b: Graded fiber

Comparison between Step-Type Fiber and Graded Fiber

16

Chapter 2 Overview of Optical Fiber Communication

2)

By fiber material Besides quartz fiber, the fibers can be divided into quartz fiber, glass fiber with many compositions, quartz-plastic fiber with quartz core and plastic cladding, and full-plastic fiber with plastic core and plastic cladding, by material. Such fibers have greater loss than the quartz fiber, so they are usually used by the short-distance systems inside buildings or rooms.

3)

By transmission mode Light is a kind of electromagnetic wave. Therefore, the light transmitted in the fiber should not only meet full-reflection condition between fiber core and cladding, but also meet the related conditions for electromagnetic wave in the transmission process. For the specified fiber structure, only a series specified electromagnetic wave can be effectively transmitted in the fiber. Such specified electromagnetic wave is called optical fiber mode. In the fiber, the conductible mode quantity depends on structure and refractive index radial distribution of the fiber. If the fiber supports only one conduction mode (base mode), this fiber is called Single-Mode Fiber (SMF) and its core transmits only one light. If the fiber supports multiple conduction modes, this fiber is called Multi-Mode Fiber (MMF), and each light in its core is in a transmission mode. Fig. 2.1-2 shows two typical multi-mode fibers. The differences between SMF and MMF are listed in Table 2.1-1.

17

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Table 2.1-1

Differences Between SMF and MMF

SMF

MMF

Transmission

Only supporting transmission in base

mode

mode

Fiber core

Smaller (about 5 µm ~ 10 µm) Caused

Large mode dispersion due to different

Dispersion

different frequency components in the

transmission rates of different modes,

influence

optical signal; increasing along with

directly

increased optical signal spectrum width

bandwidth and transmission distance

Type

Fiber

transmission

Greater (about 50 µm) of

Ordinary

by

Supporting multiple conduction modes

SMF,

rates

Dispersion

(DSF)

and

affecting

transmission

Shifted

Dispersion

Ordinary MMF

Compensation Fiber (DCF) Working

1310 nm and 1550 nm

window

Long-distance

Applications

fiber

850 nm and 1310 nm communication

system with large capacity

Short-distance

fiber

communication

system at low rate

2.1.2 Usage Overview of Applicable Frequencies of Optical Fiber Due to improving fiber manufacture technique, the fiber transmission loss is lower and lower. At present, there are five low-loss windows, as shown in Fig. 2.1-3. 3.0 ~140THz 2.5

~50THz OH- absorption peak

OH- absorption peak

2.0 Loss (dB/km) 1.5

I

II

V

III

IV

C

L

OH- absorption peak

1.0 0.5

O

E

S

0 800

O: Original Band

1000

E: Extend Band

1200

S: Short Band

Fig. 2.1-3

18

1400

C: Conventional Band

1600 Wavelength (nm) L: Long Band

Division of Low-Loss Windows


The optical signal mark, wavelength range, applied fiber types and application of each window are described in Table 2.1-2.

Table 2.1-2 Window Mark (nm) Wavelength range (nm) Fiber type

Applications

Feature Comparison between Low-Loss Windows

I 850

600~900

MMF

II

III

IV

1550 (C band)

1600 (L band)

1260~1360

1530~1565

1565~1625

1360~1530

MMF/G.652/

G.652/G.653/G

G.652/G.653/G

Full-wave

G.653

.655

.655

fiber

1310

(O

band)

Short

Short

distance and

distance and

low rate

low rate

V 1360 ~ 1530 (E + S bands)

Long distance and high rate

2.1.3 Types and Features of Common SMFs This section briefly introduces features and functions of three SMFs, G.652, G.653 and G.655. The fiber types applied in the DWDM system are also involved. 1.

G.652 (ordinary SMF) It is also called dispersion non-shifted SMF, used in 1310 nm and 1550 nm windows. In the 1310 nm window, it has dispersion close to zero. But in the 1550 nm window, its loss is the smallest, with dispersion of 17 ps/km nm. When it is used in the 1310 nm window, it is only applicable to the SDH system; when it is used in the 1550 nm window, it is applicable to both SDH system and DWDM system, requiring dispersion compensation when the single channel rate is over 2.5 Gbit/s.

2.

G.653 (dispersion shifted SMF) It has the smallest loss and the smallest dispersion in the 1550 nm. Therefore, it usually works in the 1550 nm window. It is applicable to the high-rate and long-distance single-wavelength communication system. When the DWDM technology is used, serious non-linear Four Wave Mixing (FWM) problem will occur in zero-dispersion wavelength area, resulting in optical signal attenuation in multiplexing channels and channel crosstalk. 19

WM_000_ E1


3.

G.655 (non-zero dispersion shifted SMF) In the 1550 nm window, the absolute value of its dispersion is not zero and within a certain range (ensuring smallest loss and small dispersion in this window). It is applicable to the high-rate and long-distance optical communication system. In addition, non-zero dispersion suppresses the influence of non-linear FWM over DWDM system. Therefore, this kind of fiber is usually used in the DWDM system.

2.2 Working Wavelength of DWDM System 2.2.1 Introduction to Working Wavelength Area Based on multiplexing channel quantity and frequency interval of the DWDM system, the working wavelengths of the systems below 40 wavelengths, 80-wavelength system and 160-wavelength system are introduce below. 1.

8/16/32/40-wavelength system Working wavelength range: C band (1530 nm ~ 1565 nm) Frequency range: 192.1 THz ~ 196.0 THz Channel interval: 100 GHz Central frequency offset: ±20 GHz (at rate lower than 2.5 Gbit/s); ±12.5 GHz (at rate 10 Gbit/s)

2.

80-wavelength system Working wavelength range: C band (1530 nm ~ 1565 nm) Frequency range: C band (192.1 THz ~ 196.0 THz) Channel interval: 50 GHz Central frequency offset: ±5 GHz

3.

160-wavelength system Working wavelength range: C band (1530 nm ~ 1565 nm) + L band (1565 nm ~ 1625 nm)

20


Frequency range: C band (192.1 THz ~ 196.0 THz) + L band (190.90 THz ~ 186.95 THz) Channel interval: 50 GHz Central frequency offset: ±5 GHz

2.2.2 Wavelength Allocation The working wavelength of the DWDM system complies with the specific central wavelength and central frequency values in the multi-channel system, as stipulated by the ITU-T Recommendation G.692. 1.

The wavelength allocation for C band 40-wavelength system with wavelength interval of 100 GHz is shown in Table 2.2-1.

Table 2.2-1 No.

Wavelength Allocation of 40CH/100 GHz Interval on C Band Central Frequency (THz)

Wavelength (nm)

1

192.1

1560.61

2

192.2

1559.79

3

192.3

1558.98

4

192.4

1558.17

5

192.5

1557.36

6

192.6

1556.55

7

192.7

1555.75

8

192.8

1554.94

9

192.9

1554.13

10

193.0

1553.33

11

193.1

1552.52

12

193.2

1551.72

13

193.3

1550.92

14

193.4

1550.12

15

193.5

1549.32

16

193.6

1548.51

17

193.7

1547.72

18

193.8

1546.92

19

193.9

1546.12

20

194.0

1545.32

21

194.1

1544.53

22

194.2

1543.73

23

194.3

1542.94 21

WM_000_ E1


No.

Central Frequency (THz)

Wavelength (nm)

24

194.4

1542.14

25

194.5

1541.35

26

194.6

1540.56

27

194.7

1539.77

28

194.8

1538.98

29

194.9

1538.19

30

195.0

1537.40

31

195.1

1536.61

32

195.2

1535.82

33

195.3

1535.04

34

195.4

1534.25

35

195.5

1533.47

36

195.6

1532.68

37

195.7

1531.90

38

195.8

1531.12

39

195.9

1530.33

40

196.0

1529.55

2.

The wavelength allocation for C/C+ band 80-wavelength system with wavelength interval of 50 GHz is shown in Table 2.2-2.

Table 2.2-2

Wavelength Allocation of 80CH/50 GHz Interval on C/C+ Band

Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

1

196.05

1529.16

41

194.05

1544.92

2

196.00

1529.55

42

194.00

1545.32

3

195.95

1529.94

43

193.95

1545.72

4

195.90

1530.33

44

193.90

1546.12

5

195.85

1530.72

45

193.85

1546.52

6

195.80

1531.12

46

193.80

1546.92

7

195.75

1531.51

47

193.75

1547.32

8

195.70

1531.90

48

193.70

1547.72

9

195.65

1532.29

49

193.65

1548.11

10

195.60

1532.68

50

193.60

1548.51

11

195.55

1533.07

51

193.55

1548.91

12

195.50

1533.47

52

193.50

1549.32

13

195.45

1533.86

53

193.45

1549.72

22


Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

14

195.40

1534.25

54

193.40

1550.12

15

195.35

1534.64

55

193.35

1550.52

16

195.30

1535.04

56

193.30

1550.92

17

195.25

1535.43

57

193.25

1551.32

18

195.20

1535.82

58

193.20

1551.72

19

195.15

1536.22

59

193.15

1552.12

20

195.10

1536.61

60

193.10

1552.52

21

195.05

1537.00

61

193.05

1552.93

22

195.00

1537.40

62

193.00

1553.33

23

194.95

1537.79

63

192.95

1553.73

24

194.90

1538.19

64

192.90

1554.13

25

194.85

1538.58

65

192.85

1554.54

26

194.80

1538.98

66

192.80

1554.94

27

194.75

1539.37

67

192.75

1555.34

28

194.70

1539.77

68

192.70

1555.75

29

194.65

1540.16

69

192.65

1556.15

30

194.60

1540.56

70

192.60

1556.55

31

194.55

1540.95

71

192.55

1556.96

32

194.50

1541.35

72

192.50

1557.36

33

194.45

1541.75

73

192.45

1557.77

34

194.40

1542.14

74

192.40

1558.17

35

194.35

1542.54

75

192.35

1558.58

36

194.30

1542.94

76

192.30

1558.98

37

194.25

1543.33

77

192.25

1559.39

38

194.20

1543.73

78

192.20

1559.79

39

194.15

1544.13

79

192.15

1560.20

40

194.10

1544.53

80

192.10

1560.61

23

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3.

The wavelength allocation for L/L+ band 80-wavelength system with wavelength interval of 50 GHz is shown in Table 2.2-3.

Table 2.2-3

Wavelength Distribution of 80CH/50 GHz Interval on L/L+ Band

Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

1

190.90

1570.42

41

188.90

1587.04

2

190.85

1570.83

42

188.85

1587.46

3

190.80

1571.24

43

188.80

1587.88

4

190.75

1571.65

44

188.75

1588.30

5

190.70

1572.06

45

188.70

1588.73

6

190.65

1572.48

46

188.65

1589.15

7

190.60

1572.89

47

188.60

1589.57

8

190.55

1573.30

48

188.55

1589.99

9

190.50

1573.71

49

188.50

1590.41

10

190.45

1574.13

50

188.45

1590.83

11

190.40

1574.54

51

188.40

1591.26

12

190.35

1574.95

52

188.35

1591.68

13

190.30

1575.37

53

188.30

1592.10

14

190.25

1575.78

54

188.25

1592.52

15

190.20

1576.20

55

188.20

1592.95

16

190.15

1576.61

56

188.15

1593.37

17

190.10

1577.03

57

188.10

1593.79

18

190.05

1577.44

58

188.05

1594.22

19

190.00

1577.86

59

188.00

1594.64

20

189.95

1578.27

60

187.95

1595.06

21

189.90

1578.69

61

187.90

1595.49

22

189.85

1579.10

62

187.85

1595.91

23

189.80

1579.52

63

187.80

1596.34

24

189.75

1579.93

64

187.75

1596.76

25

189.70

1580.35

65

187.70

1597.19

26

189.65

1580.77

66

187.65

1597.62

27

189.60

1581.18

67

187.60

1598.04

28

189.55

1581.60

68

187.55

1598.47

29

189.50

1582.02

69

187.50

1598.89

30

189.45

1582.44

70

187.45

1599.32

31

189.40

1582.85

71

187.40

1599.75

24


Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

Nominal

Nominal

Wavelength

Central

Central

No.

Frequency

Wavelength

THz

nm

32

189.35

1583.27

72

187.35

1600.17

33

189.30

1583.69

73

187.30

1600.60

34

189.25

1584.11

74

187.25

1601.03

35

189.20

1584.53

75

187.20

1601.46

36

189.15

1584.95

76

187.15

1601.88

37

189.10

1585.36

77

187.10

1602.31

38

189.05

1585.78

78

187.05

1602.74

39

189.00

1586.20

79

187.00

1602.17

40

188.95

1586.62

80

186.95

1603.57

2.3 Fiber Transmission Features 2.3.1 Fiber Loss Power transmission loss is a basic and important parameter of the fiber. Due to existence of fiber loss, the optical power transmitted in the fiber will attenuate by index with the increase of transmission distance. 1.

Generation of fiber loss and low-loss window The fiber loss covers two aspects:

1)

Loss coming from fiber, including inherent absorption loss of fiber materials, absorption loss of material impurity (especially the loss caused by the remained OH component in the fiber), Raileigh dispersion loss, and dispersion loss due to incomplete fiber structure.

2)

The fiber additional loss caused by optical cable layout, fiber connection and system coupling/connection in all kinds of environment, because the fibers are bundled into cable. This aspect involves bending loss and minor bending loss of optical fiber/cable, connection loss in the fiber line, and coupling loss between optical components. The fiber attenuation spectrum is shown in Fig. 2.1-3. The average loss in Window I is 2 dB/km, the one in Window II is 0.3 dB/km ~ 0.4 dB/km, and the one in Window III is 0.19 dB/km ~ 0.25 dB/km. The 1380 nm point in Window V exist OH absorption peak. 25

WM_000_ E1


2.

The line loss values of the common fibers are shown in Table 2.3-1.

Table 2.3-1 Fiber Type

SMF Loss

G.652

G.653

Typical loss value (1310 nm)

0.3 dB/km ~ 0.4 dB/km

-

Typical loss value (1550 nm)

0.15

0.19

Working window

2.

dB/km

~

0.25

G.655 -

dB/km

~

0.19

dB/km

dB/km

0.25dB/km

0.25 dB/km

1310 nm and 1550 nm

1550 nm

1550 nm

~

Relationship between fiber loss and OSNR OSNR means the ratio between optical signal power and noise power. It is very important for estimating and measuring bit error performance, engineering design and maintenance. Take the OSNR at the receiving end of the DWDM system as an example, the calculation formula is: log

log

OSNR = Pout – 10 M - L + 58 - NF - 10 N Here, Pout: In-fiber optical power (dBm). M: Number of multiplexing channels of the WDM system L: Loss between any two optical amplifiers, that is, sectional loss (dB) NF: Noise index of the EDFA. N: Number of

the

EDFAs

between optical

multiplexer and

optical

de-multiplexer of the WDM system. The formula shows this: When the other parameters keep unchanged, greater line loss leads to lower OSNR, which means decreased transmission quality of the optical line. In the initial design of the DWDM, except loss limit and dispersion limit, the OSNR at the receiving end, Q value and BER should also be considered. The design is qualified only when these three factors are qualified.

2.3.2 Dispersion After the optical pulse signals entering the fiber through input end are transmitted for a long distance, the pulse wave shape spreads by time at the fiber output end, this

26


phenomenon is called dispersion. We take the dispersion in the SMF as an example, as shown in Fig. 2.3-1. SMF

Optical power

Time

Optical power

Time Outgoing optical pulse waveform

Incoming optical pulse waveform Fig. 2.3-1

Dispersion in Fiber

Dispersion will cause inter-symbol interference, affects correct judgment of optical pulse signal at the receiving end, deteriorates BER performance and even affects information transmission. The dispersion in the SMF is caused by different transmission rates of different frequency components in the optical signal, and is called chroma dispersion. In the area with negligible chroma dispersion, the polarization mode dispersion is the major part of SMF dispersion. The phenomena and causes of chroma dispersion and polarization mode dispersion will be introduced below, as well as their influences to the DWDM system.

2.3.2.1 Chroma Dispersion 1.

Brief introduction to chroma dispersion Chroma dispersion is divided into material dispersion and waveguide dispersion.

1)

Material dispersion: The quartz glass, fiber material, has different refractive index for different optical wavelengths. While the light source has certain spectrum width, and different wavelengths results in different group rates, so the optical pulse spreading will occur.

2)

Waveguide dispersion: For a transmission mode of the fiber, the pulse spreading occurs due to different group rates in different optical wavelengths. This dispersion is related to the waveguide effect of fiber structure, so it is also called structure dispersion. Material dispersion is greater than waveguide dispersion. According to the dispersion calculation formula, the material dispersion at a specific wavelength

27

WM_000_ E1


may be zero, and this wavelength is called the zero dispersion wavelength of the material. Luckily, this wavelength is in the low-loss window near 1310 nm. For example, G.652 fiber is the zero dispersion fiber. Although the optical components are heavily affected by the dispersion, there is the tolerable maximum dispersion value (that is, dispersion tolerance). Normal transmission can be ensured when the generated dispersion is within the tolerance. 2.

Influence of chroma dispersion Chroma dispersion will result in pulse spreading and chirp effect.

1)

Pulse spreading Pulse spreading is the major influence of chroma dispersion to system performance. When the transmission distance is longer than the fiber dispersion length, the pulse spreading is too large. At this time, the system will have serious inter-symbol interference and bit errors.

2)

Chirp effect Dispersion not only results in pulse spreading but also makes pulse generate phase modulation. Such phase modulation makes different parts of the pulse make different offset from the central frequency with different frequencies, which is called chirp effect of pulse. Due to chirp effect, the fiber is divided into normal dispersion fiber and abnormal dispersion fiber. In the normal dispersion fiber, the high-frequency component of the pulse is located at the rear edge of the pulse and the low-frequency component is located at the front edge of the pulse. In the abnormal dispersion fiber, the low-frequency component of the pulse is located at the rear edge of the pulse and the high-frequency component is located at the front edge of the pulse. In the transmission line, we can properly use these two fibers to offset the chirp effect and remove the pulse dispersion spreading.

3.

Removing influence of chroma dispersion to DWDM system Since the DWDM system is mostly used in the 1550 nm window, if G.652 fiber is used, it is required to use the DCF with negative wavelength dispersion to compensate the dispersion and reduce the total dispersion value of the whole transmission value. 28


2.3.2.2 Polarization Mode Dispersion (PMD) PMD is a kind of physical phenomenon existing in optical fiber and optical component fields. The baseband in the SMF has two polarization modes that are orthogonal. In the ideal case, two polarization modes should have the same feature curve and transmission characteristics. Due to geometrical and pressure asymmetry, two polarization modes have different transmission rates, resulting in delay and PMD, as shown in Fig. 2.3-2. 1/2

Usually, the unit of PMD is ps/km . Optical fiber Incoming light

Outgoing light

Delay

Fig. 2.3-2

PMD in SMF

In the digital transmission system, the PMD will result in pulse separation and pulse spreading, degrade transmission signal and limit transmission rate of carriers. Compared with other dispersions, the PMD can almost be omitted. But it cannot be totally extinguished, but can be minimized through optical components. The narrower the pulse in the super-speed system is, the greater the PMD influence is.

2.3.3 Non-Linear Effect of Fiber In the common fiber communications system, the transmitting optical power is low and the fiber has linear transmission feature. But, for the DWDM system, the fiber has non-linear effect after the EDFA is used. The non-linear effect of the fiber will result in serious crosstalk between multi-wavelength channels of the DWDM system, which will lead to additional attenuation of the fiber communication system as well as restriction of optical power, EDFA amplifying performance and current-free regenerative relay distance.

29

WM_000_ E1


The non-linear effect involves Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), Four Wave Mixing (FWM), Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBC). 1.

SPM Due to dependency relationship between refractive index and light intensity, refractive index changes during optical pulse continuance, with the pulse peak phase delayed for both front and rear edges. With more transmission distance, the phase shift is accumulated continuously and represents large phase modulation upon certain distance. As a result, the spectrum spreading results in pulse spreading, which is called SPM, as shown in Fig. 2.3-3.

Intensity

Optical spectrum before transmission

Pulse width before transmission

Intensity

Optical spectrum after transmission

Pulse width after transmission

Fig. 2.3-3

SPM

When the system works in the fiber working area (for example, the short wavelength area of G.653 fiber or working area with negative dispersion of G.655 fiber) with negative dispersion index, the SPM will result in smaller dispersion limit distance. When the system works in the fiber working area (for example, the long wavelength area of G.652/G.653 fiber or working area with positive dispersion of G.655 fiber) with positive dispersion index, the SPM will result in greater dispersion limit distance.

30


The SPM effect occurs in certain distance from the transmitter end. In addition the low-dispersion fiber can also reduce the influence of SPM to the system performance. 2.

XPM When two or more optical waves with different frequencies are simultaneously transmitted in the non-linear media, the amplitude modulation of each frequency wave will result in the corresponding change of the fiber refractive index, resulting in non-linear phase modulation of the optical wave with other frequencies, which is called XPM. XPM often occurs along with SPM. XPM will result in a series of non-linear effects, such as signal interference between DWDM system channels and non-linear dual-refraction of the fiber, leading to unstable polarization of the fiber transmission. Meanwhile, the XPM will also affect wave shape and spectrum of pulse. Adding dispersion properly can reduce the XPM influence.

3.

FWM FWM refers to a physical process of energy exchange between multiple optical carriers caused by non-linear effect of the fiber, when multiple frequencies of optical carriers with high power are simultaneously transmitted in the fiber. FWM results in optical signal energy attenuation in multiplexing channels and channel crosstalk. As shown in Fig. 2.3-4, a new optical wave appears on another wavelength, due to FWM. Incoming light

Outgoing light

New optical wave Fig. 2.3-4

FWM

The generation of FWM is related to the fiber dispersion. For zero dispersion, the mixing efficiency is the highest. Along with dispersion increase, the mixing

31

WM_000_ E1


efficiency reduces fast. The DWDM system uses G.655 fiber to slider over the FWM effect in 1550 nm zero-dispersion wavelength area. 4.

SRS SRS belongs to the stimulated non-elastic scattering process caused by non-linear effect. It comes of mutual action and energy exchange between photon and optical phonon (molecular vibration status). SRS effect results in attenuation of signals with short wavelength and reinforcement of signals with long wavelength, as shown in Fig. 2.3-5. Incoming light

Outgoing light

Power

Power ...

λ1

...

λ2 λ3

λ1

λ

Fig. 2.3-5

λ2 λ3

λ

SRS

SRS effect is widely applied in the fiber communication, for example, making distributed Raman amplifier based on Raman gain, to provide distributed broadband amplifying for optical signals. The DRA board of ZTE DWDM device implements the optical amplifying function through the SRS effect. On the other hand, SRS introduces in negative influence to the communication system. In the DWDM system, light in the short-wavelength channel will serve as pump light to transfer energy to the long-wavelength channel, resulting in Raman crosstalk between channels. 5.

SBS SBS belongs to the stimulated non-elastic scattering process caused by non-linear effect. It comes of mutual action and energy exchange between photon and acoustic phonon (crystal vibration status). SRS effect can be used to make fiber Brillouin laser and amplifier. On the other hand, SBS will result in unstable signal light source and crosstalk between reverse transmission channels. However, along with increase of system transmission rate, the SBS peak gain obviously reduces. So, SBS will not greatly affect the high-speed fiber transmission system.

32


2.4 New Optical Fiber Types This section will briefly introduce features and applications of some new-type fibers. 1.

G.654 optical fiber The G.654 fiber works in the 1550 nm window, with average loss as 0.15 dB/km ~ 0.19 dB/km, which is less than those of the other types of fibers. The zero dispersion point is still in the 1310 nm window. It is applicable to the long/medium-distance optical transmission system.

2.

Full-wave fiber -

The full-wave fiber, water peak free fiber, eliminates OH ions near the 1385 nm wavelength and thus also eliminates the appended water peak attenuation caused -

by the OH ions. In this way, the fiber attenuation is only determined by the internal scattering loss of the silicon glass. Full-wave fiber is numbered as G.652 C&D in ITU-T Recommendations. It is one kind of G.652 fiber. Its full name is wavelength-expanded dispersion non-shifted single-mode fiber.

The attenuation of the full-wave fiber becomes even at the band of 1310 nm~ -

1600 nm. As internal OH ions are already eliminated, no water peak attenuation will occur even when the fiber is exposed to hydrogen gas and the long-term attenuation is reliable. Full-wave optical fiber can provide a complete transmission band from 1280 nm to 1625 nm. The available wavelength range is about 1.5 times of the wavelength range of ordinary fibers. 3.

Real-wave fiber Real-wave fiber is a kind of non-zero dispersion shifted single-mode fiber (G.655 fiber) widely used at present. Its fiber characteristics are similar to those of G.655 fiber. Its zero dispersion point is in short-wavelength area below 1530 nm. In 1549 nm ~ 1561 nm band, the dispersion index is 2.0 ps/nm·km ~ 3.0 ps/nm·km. It has small dispersion slope and dispersion coefficient, capable of tolerating higher non-linear effect. It is applicable to large-capacity optical transmission system to reduce network construction cost. 33

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4.

Fiber with large effective fiber core area It also belongs to non-zero dispersion shifted single-mode fiber (G.655 fiber). Essentially, it improves non-linear resistance capability of the system. The super-speed system performance is mostly limited by dispersion and non-linear effect. Usually, dispersion can be distinguished through dispersion compensation. But the non-linear effect cannot be distinguished only through linear compensation. The effective area of the fiber determines the fiber non-linear effect. Larger effective area means high affordable optical power, that is, better resistance to non-linear effect.

34

3 Key Technologies of DWDM System  Highlights: z

Basic structure of the DWDM system.

z

Light source technology.

z

Optical wavelength division multiplexing and de-multiplexing technologies.

z

OTU technology.

z

Optical amplifying technology.

z

Supervision technology.

3.1 Basic Structure of DWDM System The DWDM system multiplexes several or dozens of optical channel signals with different nominal wavelengths to one fiber for transmission, with each optical channel bearing one service signal. The basic structure of a unidirectional DWDM system is shown in Fig. 3.1-1. Optical transmitter TX1 TX2 TX3

TXn

Optical receiver Optical relay amplifier

G.692 Optical transponder Optical transponder Optical transponder

... Optical transponder

λ1

λ1

λ2

λ2

λ3

OM

OBA

OLA

OPA

OD

λ3

λn

λn

Receiver/transmitter of optical supervision channel Transmitter of optical supervision channel

Fig. 3.1-1

1.

Receiver of optical supervision channel

Composition of DWDM System

Optical transmitter end

35

Optical transponder Optical transponder Optical transponder


RX1 RX2 RX3

RXn

WM_000_ E1


TX1…TXn, the optical transmitters of all the multiplexing channels, respectively transmit the optical signals (λ 1, λ 2 …λ n, with the corresponding frequencies as f1, f2…fn) with different nominal wavelengths. Each optical channel bears different service signals, such as standard SDH signal, ATM signal and Ethernet signal. After that, the optical multiplexer combines these signals into one beam of optical wave, which will be output by the OBA to the fiber for transmission. 2.

Optical receiver end After the line fiber goes through amplifying of OPA, the optical channel signals are de-multiplexed by the optical de-multiplexer and then respectively input to the corresponding multiplexing channel optical receivers, RX1…RXn.

3.

Optical regenerating amplifier end Located in the middle of the optical transmission section, it uses OLA to amplify the optical signals.

4.

Optical monitoring channel In the DWDM system shown in Fig. 3.1-1, an independent wavelength (1510 nm) is used as the optical monitoring channel for transmitting optical monitoring signals. The optical monitoring signals are used to bear NE management and monitoring information of the DWDM system, for the sake of effectively management of network management system over the DWDM system.

5.

Network management system This module is omitted in Fig. 3.1-1. The DWDM NMS should be capable of managing optical amplifying units (such as OBA, OLA and OPA), wavelength division multiplexer, Optical Transponder Unit (OTU) and channel performance supervision on one platform. It can manage the device in terms of performance, fault, configuration and security. The information in the NMS is borne by the monitoring signals in the optical monitoring channel.

3.2 Light Source Technology 1.

Type of optical sources At present, the semi-conductor optical sources widely used now are Laser Device (LD) and Light-Emitting Diode (LED). 36

Chapter 3 Key Technologies of DWDM System

LD is coherence light source, with large in-fiber power, small spectral line width and high modulation rate. It is applicable to the long-distance high-speed system. The LED is non-coherence light source, with small in-fiber power, large spectral line width and low modulation rate. It is applicable to short-distance low-speed system. The light source of the DWDM system adopts the semi-conductor laser. 2.

Features of DWDM system light source

1)

Providing standard and stable wavelength The DWDM system has very strict requirements for the working wavelength of each multiplexing channel. Wavelength drift will cause unstable and unreliable operation of the system. The common wavelength stabilization measures are temperature feedback control method and wavelength feedback control method.

2)

Providing rather large dispersion tolerance Fiber transmission may be limited by system loss and dispersion. Along with increased transmission rate, the dispersion influence is larger. Here, dispersion limit can be solved through using optical fibers/cables with small dispersion coefficient or semi-conductor laser with small spectral width. After the optical cables are laid, minimizing spectral width of light source devices is an effective measure for solving dispersion limit.

3.

Modulation modes of DWDM system laser At present, there are two methods of light source intensity modulation: Direct modulation and indirect modulation (that is, external modulation).

1)

Direct modulation Direct modulation means directly controlling the working current of semi-conductor laser through electrical pulse code stream, and thus making it generate the optical pulse stream corresponding to the electrical signal pulse. For example, when the electrical pulse signal is "1", the working current of the laser is greater than its current threshold, therefore it generates an optical pulse. When the electrical pulse signal is "0", the working current of the laser is smaller than its current threshold, therefore it does not generate optical pulse.

37

WM_000_ E1


The direct modulation mode is simple, with low loss and low cost. But, the super-speed change of working current of the laser will make modulation chirp easily. And chirp will limit transmission rate and distance of the system. The direct modulation mode is often used in the transmission system composed of G.652 fiber, with transmission shorter than 100 km and rate lower than 2.5 Gbit/s. 2)

Indirect modulation (external modulation) The external modulation mode refers to indirectly control (modulate) the continuous light generated by the laser which is in the continuous light emitting status, and thus obtaining optical pulse stream. Therefore, in external modulation case, the laser nit generates stable high-power laser, and the external modulator will modulate it in low chirp, to obtain the maximum dispersion value much greater than that in the case of direct modulation.It is applicable to the long-distance transmission system at rate over 2.5 Gbit/s. At present, the external modulators often used are electrical absorption modulator (EA) and waveguide Mach-Zehnder (MZ) modulator.

z

EA modulator It uses absorber controlled by electrical pulse signal to absorb or not absorb the optical wave transmitted by the continuous wave semi-conductor laser (CW), and thus making optical pulse stream under indirect control of electrical pulse signal stream. The EA light source features small size, high integration, low driving power and low power consumption. The maximum dispersion can reach 12 000 ps/ nm.

z

Waveguide M-Z modulator At the input end, the CW is in continuous wave working status. The optical wave emitted by it is divided by the optical de-multiplexer into two equal signal channels, which will respectively enter two optical tributaries of the modulator. Under control of electrical pulse stream, it performs phase modulation to the optical signals. At the output end, two optical tributaries are combined by the optical multiplexer. When the signal phases in two optical tributaries are reverse to each other, the optical multiplexer has no optical signal output; when the 38


signal phases in two optical tributaries are the same, the optical multiplexer has optical signal output. In this way, the optical pulse stream is controlled by the electrical pulse stream. The M-Z light source features high modulation rate, large maximum dispersion value, and large extinction ratio. Its chirp coefficient can be zero in theory. Its disadvantage is that polarization maintaining fiber must be used to connect the laser and the modulator, because modulation status is related to light polarization status.

3.3 Optical Wavelength Division Multiplexing and De-Multiplexing Technologies 3.3.1 Overview The optical wavelength division multiplexer and de-multiplexer, also called optical multiplexer and de-multiplexer, belong to optical wavelength division multiplexer, which is actually a kind of optical filter. At the transmitting end, the Optical Multiplexer (OM) is used to combine the optical signals in each multiplexing channel with nominal wavelength into a beam of optical wave, and then transmit it into the fiber for transmission, that is, multiplexing optical wave. At the receiving end, the Optical De-multiplexer (OD) is used to divide optical wave in the fiber into optical signals of each multiplexing channel with formal nominal wavelength, and then input them into the corresponding optical channel receivers, that is, de-multiplexing optical wave. Since the performance of OM and OD determine the system transmission quality, the attenuation, offset and channel crosstalk of OM and OD must be small values.

3.3.2 Introduction to OM Four types of common OMs are briefly introduced below, as well as OM types often used in the DWDM systems with different wavelength numbers. 1.

Brief introduction to common OMs

1)

Grating type of OM

39

WM_000_ E1


The grating type of OM is an angular dispersion type of device. Since the optical signals with different wavelengths have different refractive angles on the grating, it divides and combines the optical signals with different wavelengths. Its working principle is shown in Fig. 3.3-1. λ 1 ,2,3,...n

λ1 λ2 λ3 λ4 λn

Fig. 3.3-1

Principle of Grating Type of OM

It has sound wavelength selection performance, capable of narrowing wavelength interval to about 0.5 nm. However, the grating features precise manufacture requirements and is not suitable for large-batch manufacture. It is often used in research in the laboratory. 2)

Dielectric thin film type of OM It is composed of Thin Film Filter (TFF). TFF consists of dozens layers of dielectric films with different materials, different refractive indexes and different thickness values. One layer features high refractive index and the other layer features low refractive index, therefore TFF emerges a passband within certain wavelength range while a stopband within other wavelength ranges, and thus forming the desired filtering performance. The working principle is shown in Fig. 3.3-2.

40


λ 1,2,3,...n λ1

λ3

λ2

λ5 λ4

λ7

λ6

Fig. 3.3-2

Principle of Dielectric Thin Film Type of OM

It is a kind of compact passive optical device with stable structure, featuring flat signal passband, low insertion loss and sound channel interval. 3)

Array Waveguide OM (AWG) AWG OM is the flat waveguide device based on optical integration technology. Its working principle is shown in Fig. 3.3-3.

Fig. 3.3-3

Principle of AWG OM

Due to compact structure and low insertion loss, it is the best scheme for optical wave multiplexing/de-multiplexing in the optical transport network. 4)

Coupling type of OM It is a kind of surface interactive device with two or more fibers which are closed to each other and are properly melted. It is used for OM, with the working principle shown in Fig. 3.3-4.

41

WM_000_ E1


λ1 λ2 λ3 λ4 λ5

λ1，2，3……

λ6 λ7 λ8

Fig. 3.3-4

Principle of Coupling Type of OM

It can only implement multiplexing function, with low cost and large insertion loss. 2.

Multiplexer/de-multiplexer of DWDM system The relationship between the system with different wavelengths and the corresponding optical wavelength division multiplexers is shown in Table 3.3-1.

Table 3.3-1

Relationship between DWDM Systems and Corresponding Optical Wavelength Division Multiplexers

Type of Optical

OM

OD

Wavelength Division

Below 32

40

Above 80

Below 32

40

Above 80

Multiplexer

wavelengths

wavelengths

wavelengths

wavelengths

wavelengths

wavelengths

Coupling type

√

-

-

-

-

-

Array waveguide type

√

√

-

√

√

-

Dielectric thin film type

√

√

-

√

√

-

Grating type

-

-

√

-

-

√

3.3.3 Key Performance Indices 1.

Multiplexing channel quantity It

represents

the

quantity

of

optical

channels

of

multiplexing

and

de-multiplexing made by the optical wavelength multiplexer, closely related to resolution and isolation of the device. 2.

Insertion loss The attenuation effect of wavelength division multiplexer to optical signals directly affects system transmission distance. 42


Different types of wavelength division multiplexers have different insertion loss values. Smaller insertion loss value is preferable. 3.

Isolation It represents the isolation distance between multiplexing optical channels in the optical devices. The higher the channel isolation is, the better is the frequency selection performance of the wavelength division multiplexer. Consequently, the crosstalk suppression ratio becomes higher and the mutual interference between multiplexing optical channels becomes lower. It is meaningful only for the wavelength sensitive devices (TFF type and AWG type devices). It is not meaningful for the coupling devices.

4.

Reflection coefficient At the input end of the wavelength division multiplexer, the ratio between the reflection optical power and incidence optical power is the reflection coefficient. Smaller coefficient is preferable.

5.

Polarization Dependent Loss (PDL) PDL means the maximum change value of the insertion loss caused by the change of optical wave polarization status. Light is the electromagnetic wave with extremely high frequency, therefore, there is the problem of wave vibration direction (polarization). For the optical signals of the multiplexing channels input to the wavelength division multiplexer, their polarization statuses will not be totally consistent. And the same wavelength division multiplexer has different attenuation effects to the optical waves in different polarization statuses. Smaller PDL value is preferable.

6.

Temperature coefficient It means the offset of central working frequency of the multiplexing channel caused by ambient temperature change. Smaller temperature coefficient of the wavelength division multiplexer is preferable. Smaller coefficient means more stable central working frequency of the multiplexing channels.

43

WM_000_ E1


7.

Bandwidth It is a parameter of the wavelength sensitive devices (TFF type and AWG type devices). It is not meaningful for the coupling type of wavelength division multiplexer. Bandwidth is divided into channel bandwidth @-0.5 dB and channel bandwidth @-20 dB.

z

Channel width @ -0.5 dB It refers to the corresponding change value of the working wavelength when the OD insertion loss decreases by 0.5 dB. It describes the bandpass feature of the OD. Sound bandpass feature curve should be flat and wide. Greater bandpass value is preferable.

z

Channel width @ -20 dB It refers to the corresponding change value of the working wavelength when the OD insertion loss decreases by 20 dB. It describes the stopband feature of the OD. The stopband feature curve should be sharp. Smaller bandwidth value is preferable.

3.4 OTU Technology 3.4.1 Overview The Optical Wavelength Transponder Unit (OTU) technology is used to implement wavelength conversion. It converts the non-nominal wavelength of the optical channel signal into the nominal optical wavelength in accordance with G.692 stipulation in ITU-T recommendations, and then makes it access the DWDM system. OTU also provides other functions: 1.

Standard and stable light source The DWDM system needs to multiplex multiple wavelengths in a low-loss window, with small wavelength interval, so the central frequency of the DWDM light source must stably work in the nominal central frequency sequence specified by ITU-T standards.

44


2.

Light source with rather large dispersion tolerance Increase of current-free relay distance in the DWDM system requires greater dispersion tolerance distance of the light source, for the sake of solving non-linear effect of the fiber.

3.

Functioning as regenerator When the transponder serves as the regenerator, it has the data regeneration function, which is an optional function of the OTU.

3.4.2 Working Principle and Performance Indices 1.

Working principle The working principle of the OTU is shown in Fig. 3.4-1.

G.957

Shaping, timing, (regeneration)

O/E

tical input

Fig. 3.4-1

E/O

G.692 Optical out

Working Principle of OTU

OTU performs O/E conversion to the multiplexing optical channel signals which accord with the G.957 recommendation, and implements shaping, timing extraction and data regeneration (this can be omitted) to the converted electrical signals, and then performs E/O conversion, to output DWDM optical channel signals whose wavelength, dispersion and optical transmitting power accord with G.692 recommendation. After O/E conversion, if only shaping and timing processing (that is, 2R functions) are implemented, this OTU only implements wavelength conversion function with a small transmission distance. After O/E conversion, if shaping, timing processing and regeneration (that is, 3R functions) are implemented, actually this OTU also has the function of the regeneration repeater (REG). 2.

Key performance indices

1)

System working wavelength area It is in the 1550 nm low-loss window, divided into C band and L band.

45

WM_000_ E1


z

C band (conventional band) Wavelength range: 1530 nm ~ 1565 nm Working frequency: 196.05 THz ~ 192.10 THz (1 THz = 1000 GHz)

z

L band (long-wavelength band) Wavelength range: 1565 nm ~ 1625 nm Working frequency: 190.90 THz ~ 186.95 THz



Hint:

Usually, the working range of the DWDM system is represented through frequency. 2)

Channel interval Channel interval means the nominal frequency difference between two adjacent multiplexing channels, covering even channel interval and uneven channel interval. At present, even channel interval is used mostly. The minimum channel interval of the DWDM system is integer times of 50 GHz.

z

When the multiplexing channels are 8 wavelengths, the channel interval is 200 GHz.

z

When the multiplexing channels are 16/32/40 wavelengths, the channel interval is 100 GHz.

z

When the multiplexing channels are above 80 wavelengths, the channel interval is 50 GHz. Smaller channel interval requires higher resolution of the OD and means more multiplexing channels.

3)

Nominal central frequency It refers to the central wavelength (frequency) corresponding to each multiplexing channel in the DWDM system. For example, when the multiplexing channels are 16/32/40 wavelengths, the central frequency of wavelength 1 is 192.1 THz, the channel interval is 100 GHz and frequency increases in ascending order.

46


4)

Central frequency offset It is also called frequency offset. It refers to the offset between the actual working central frequency of the multiplexing optical channel and nominal central frequency. According to the national standards, the system with frequency interval as 100 GHz, the maximum central frequency offset is ±20 GHz (about ±0.16 nm) when the rate is below 2.5 Gbit/s or ±12.5 GHz when the rate is 10 Gbit/s.

For the

system with frequency interval as 50 GHz, the maximum central frequency offset is ±5 GHz. The maximum central frequency offset refers to the value which can be met when the designed life cycle of the system expires, with temperature, humidity and other factors taken into consideration. 5)

Dispersion tolerance Dispersion reflects the spreading of the optical pulse in the transmission in the fiber. The pulse spreading will result in decreased distinguish ratio of signal pulse at the receiving end, that is, levels of bit "1" and bit "0" are similar, leading to mistaken judgment of the receiver.

To avoid bit error, it is required to take

proper measures to compensate the optical pulse spreading in fiber transmission process. The pulse spreading will be more and more serious along with increased transmission distance. The requirements of DWDM system for the fiber chroma dispersion coefficient are basically those of a single multiplexing channel rate signal for fiber chroma dispersion coefficient. In addition, since the current-free relay distance of the DWDM system is much greater than that of a single SDH system, the dispersion tolerance distance of the system light source must be prolonged. 6)

Receiver sensitivity The receiver sensitivity refers to the minimum value of the average receiving optical power on the OTU input port, when input signals are located in the 1550 -12

nm window and the BER reaches 10 .

47

WM_000_ E1


7)

Overloaded optical power The overloaded optical power refers to the maximum value of the average receiving optical power on the OTU input port, when input signals are located in the 1550 nm window and the BER reaches 10

-12

.

3.4.3 Classification and Applications of OTU Depending on the locations in the DWDM system, the OTU can be classified into OTUT, OTUR and OTUG. Their applications in the transmission system are shown in Fig. 3.4-2. λ1

λ2

OTUT OTUT

OTUG

OM

OA

OA

OTUG

OD

Line fiber OM: Optical multiplexer

1.

OM

OA

OA

OD

OTUR

λ1

λ2

Internal fiber

OD: Optical de-multiplexe

Fig. 3.4-2

OTUR

OA: Optical amplifier

OTU Applications

OTUT It is located between customer devices and OM. The signals output from it to the OM should accord with G.692 standards. This kind of OTU not only has E/O and O/E conversion functions, but also has re-shaping and re-timing functions (that is, 2R functions) as well as B1 byte detection function.

2.

OTUR It is located between OD and customer devices. The optical signals output from the OD to the OTUR should accord with G.692 standards. This kind of OTU has the similar functions as OTUT functions. It implements wavelength conversion, 2R functions and B1 byte detection function.

3.

OTUG It is located between OM and OD. Its input and output signals should accord with G.692 standards.

48


This kind of OTU not only has E/O and O/E conversion functions, but also has re-shaping, re-timing and data regeneration functions (that is, 3R functions). Therefore, it is equal to a conventional REG. It also has B1 byte detection function.

3.5 Optical Amplifying Technology For the long-distance optical transmission, optical power gradually decreases along with transmission distance increase. The light source output of the laser usually is not over 3 dBm, otherwise, the laser life cycle may be unqualified. In addition, to ensure correct signal receiving, the receiving power at the receiving end must always be certain value, for example, -28 dBm. Therefore, the optical power limit turns into the major factor determining transmission distance. Optical amplifier is the technology for solving optical power limit. Without O/E/O conversion, it directly amplifies the optical signals. Its classifications are shown in Fig. 3.5-1.

{

Resonance type

Semi-conductor OA

{

Progressive wave type 1550 n m fiber amplifier, for example, EDFA

{

Lanthanon doped fiber amplifier

{

Fiber amplifier

Non-linear optical amplifier

Fig. 3.5-1

1310 nm fiber amplifier, for example, PDFA

{

Raman fiber amplifier (SRA) Brillouin fiber amplifier (SBA)

Fiber Amplifier Classifications

EDFA and Raman fiber amplifier are introduced below.

3.5.1 EDFA Technology 3.5.1.1 EDFA Technology Principle 1.

Working principle Erbium (Er) is a lanthanon. In the fiber manufacture process, certain quantity of 3+

3+

Er ions are doped to form Erbium Doped Fiber (EDF}. The Er ions in such fiber will absorb photon energy to make own energy level change, which is called stimulation. The light source for stimulation is called pump light source, 49

WM_000_ E1


and the corresponding transmitting stimulation optical wave is called pump light. Its working principle is shown in Fig. 3.5-2.

N3~0 1550 nm signal light

N2

1550 nm stimulated emission

1480nm

980 nm pump light

N1

Fig. 3.5-2

Working Principle of EDFA

3+

The Er ion free from stimulation is at the lowest energy level. When the pump 3+

light is shot in, the Er ion absorbs energy of the pump light and transits to the 3+

higher energy level. At the higher energy level, the Er

ions are in instable

status, therefore they continuously converge to metastable energy level in non-radiant transition format, and thus implementing population inversion distribution. When the optical signals with 1550 nm wavelength pass this segment of EDF, the metastable particles transit to the ground status in stimulated emission format, and photons which are the same as those in the incoming signal light are generated, and thus implementing amplifying of optical signals. 2.

Composition The EDFA consists of EDF, bump light source, coupler and isolator, as shown in Fig. 3.5-3.

50


λ1 λ2

λn

WDM coupler

Isolator

...

Erbium doped fiber

Isolator

λ1 λ2

λn

...

Pout

Pin

Pump laser Fig. 3.5-3

EDFA Composition

The coupler is used to combine signal light with pump light. The isolator is used to suppress light reflection, to ensure stable working of the optical amplifier. The pump laser generates pump light source. 3.

Key performance indices

1)

Gain (G) It is the ratio between output optical signal power and input optical signal power. Greater gain means more powerful amplifying capability.

2)

Noise Fig. (NF) It is the ratio between SNR at EDFA input end and SNR at output end. EDFA noise comes from many factors, such as signal shot noise, internal reflection noise and Amplified Spontaneous Emission (ASE) noise. But the major source of EDFA noise is ASE noise.

&

Note:

ASE means the emission noise caused by such own factors as unbalance between EDFA optical transmitting area and absorption area, different population inversion degrees (quantity of ions in stable energy level E2 and the quantity of ions in ground energy level E1 are different), EDFA gain and EDFA working status. Since the EDFA can amplify both optical signals and noise, NF is used. NF value is closely related to the ASE noise of the EDFA. It greatly affects the system performance, especially the OSNR of the whole system. Smaller NF is preferable, for example, below 5.0 dB.

51

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3)

Bandwidth The working wavelength range of the DWDM system covers C band and L band. The optical amplifier needs to amplify all the multiplexing channel signals of the system, so its bandwidth should be wide enough.

4)

Gain flatness Gp-p It means the allowed fluctuation of EDFA gain within the specified working band range. For the sake of sound flatness, aluminum doped technology is usually used in the EDF. In the DWDM system, to minimize the difference between output optical power signals of different multiplexing channels and facilitate optical power estimation, smaller EDFA gain flatness is preferable.

5)

Total input/output power range It is the optical power range at input/output end of the EDFA. In the WDM system application, one EDFA is responsible for amplifying all the multiplexing optical channel signals in the system. Therefore, its input/output optical power range should be large enough, especially for the WDM system with enormous multiplexing channels. On the other hand, to ensure EDFA gain flatness and low noise performance, the DEFA should work in small signal working range. In other words, the input/output power range of the EDFA cannot be too large. More importantly, to avoid fiber non-linear effect, the EDFA output power cannot too large. To avoid fiber non-linear effect, the optical power of a signal channel cannot be too large. It is required to determine the proper power according to signal rate and transmission fiber type.

6)

Polarization Dependent Gain (PDG) Since the EDFA generates different gains for the optical waves in different polarization statuses. So, we call the maximum change value of the EDFA gain caused by polarization status change of the optical wave as PDG. Smaller value is preferable.

52


7)

Pump light leakage Although optical isolators are set at input and output ends of the EDFA, a few pump light leakage occurs. Smaller leakage is preferable. Pump light leakage means the ratio between pump light leakage power and input/output pump light power.

8)

Input/output optical reflectance It is the ratio between optical power at the EDFA input/output end and reflection optical power. Greater value is preferable.

4.

Importance of EDFA for DWDM system To ensure the transmission quality of DWDM system, the EDFA used in the DWDM system must have sufficient bandwidth, flat gain, low NF and high output power. Proper gain flatness is especially important, which is special requirement of DWDM system for EDFA.

3.5.1.2 EDFA Classifications Depending on locations of EDFA in the DWDM system and pump source types, two EDFA classification modes are introduced below. 1.

By location By locations in the optical transport network, the EDFA is divided into Booster Amplifier (BA), Line Amplifier (LA) and Pre-Amplifier (PA).

1)

BA: It is located behind the OTM or transmitting light source of relay device, and in the front of the relay segment. It is used to boot the transmitting power for the sake of extending transmission distance.

2)

LA: It is located in the middle of the relay segment. It is used to directly insert the EDFA into the fiber transmission link for amplifying signals. A relay segment can be equipped with multiple LAs as required.

3)

PA: It is located at the relay segment end and ahead of the optical receiving device. It is used to pre-amplify the small signals going through line attenuation, to boost the power of optical signals entering the receiver and meet the sensitivity requirements of the receiver.

53

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The locations of all kinds of amplifiers in the optical line are shown in Fig. 3.5-4. Relay segment OTM

BA

Fig. 3.5-4

2.

LA

LA

PA

OTM

Locations of all Kinds of Amplifiers in Relay Segment

By pump source At present, the pump sources often used cover 980 nm and 1480 nm, because these two types of pump sources have high pump efficiency. The 980 nm pump light source has lower NF; the 1480 nm one has higher NF, so a larger output power is obtainable (about 3 dB higher than that of the 980 nm pump light source). In the actual LA applications, most 8-channel WDM system uses the 980 nm pump source, because the WDM system of G.652 fiber mostly features dispersion limit other than loss limit. If such WDM system uses the 1480 nm pump source, the system power attenuation will increase, which is unnecessary for boosting EDFA output power. However, the above-16-channel WDM system uses the 1480 nm pump source, because enormous tributaries dwindle the available power range and the pump source with higher power is necessary. The two-level pump can also be used, for the sake of both NF improvement and output power increase.

3.5.1.3 Problems of EDFA to Be Solved When solving the problems of fiber transmission system, the EDFA also introduces in some new problems. 1.

Non-linear effect EDFA amplifies the optical power through increasing the optical power shot into the fiber, but optical power increase should be proper. When the optical power is increased to certain degree, fiber non-linear effect will occur. Therefore, in the usage of fiber amplifier, it is required to control the value of the in-fiber optical power in a single channel.

54


2.

Bandwidth Bandwidth refers to the range of the optical wavelength which can be amplified flatly. The working wavelength range of the EDFA in C band is 1530 nm ~ 1561 nm, and the one of the EDFA in L band is 1565 nm ~ 1625 nm. The gain flatness filter is used inside the EDFA, so that the EDFA has almost the same gain for each multiplexing optical channel signal within the corresponding wavelength range. The gain fluctuation should be limited within the allowed rage, for example, ±1 dB. Therefore, the bandwidth is closely related to the gain flatness.

3.

Optical surge When the optical line is normal, the erbium ions stimulated by the pump light are carried off by the signal light, and thus implementing amplifying of signal light. If the input light is interrupted, the metastable erbium ions still converge continuously, so energy transient will occur, leading to optical surge. The way of solving optical surge is to implement Automatic Power Reduction (APR) or Automatic Power ShutDown (APSD) function in the EDFA. In other words, the EDFA will automatically reduce power or shut down power upon no input light, and thus suppressing surge.

4.

Dispersion Along with transmission distance increase, the total dispersion increases correspondingly. Therefore, the current-free relay segment in the WDM system cannot be prolonged limitlessly. We can prolong the current-free relay distance of the multiplexing section through dispersion compensation measures.

3.5.2 Raman Amplifying Technology 1.

Working principle The Raman fiber amplifier uses the gain mechanism generated by non-linear SRS in the fiber to amplify the optical signals. The SRS converts the energy of short-wavelength pump light into the energy of long-wavelength signal light, to amplify the signal light.

55

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2.

Features

1)

Based on dozens of kilometers of line fibers, it implements distributed amplifying, with low NF and effectively improve system SNR.

2)

With the same SNR, it can reduce the optical power at the transmitting end and minimize the non-linear effect.

3)

It can generate gain for all the wavelengths, serving as full-band amplifier (required to be divided into C band amplifier and L band amplifier).

4)

It has flat gain. The gain wavelength range depends on the pump wavelength.

5)

Since the noise of Raman fiber amplifier reduces along with fiber distance increase, the fiber should be long enough. There is no requirement for the fiber type.

6)

The pump conversion efficiency is low, so the high-power pump laser source is required.

7)

The amplifying gain is low, so it needs to work with the EDFA to form combined amplifier, for the sake of compensation of line attenuation and node insertion loss.

3.

Application If the DWDM system above 40 G only uses EDFA for amplifying, spontaneous emission will be accumulated, restricting overall system performance. Compared with EDFA, the SRA has such advantages as low noise, introducing in no additional loss upon removal of pump light, and no transient effect. Therefore, the combination of EDFA and SRA can form the important optical amplifying technology for the transmission system above 40 G or of super great distance.

3.6 Supervision Technology Detection, control and management are basic requirements of all the network operations. To ensure secure operation of the DWDM system, physically, the monitoring system is designed into the independent system separated from working channels and devices.

56


For example, ZTE DWDM system uses an independent wavelength (1510 nm) and relays on no service channel, to ensure that no active amplifying is required for the long distance transmission and reliability is improved, and thus eventually implementing monitoring over the NE devices of the system.

3.6.1 Functions of Optical Supervision Channel (OSC) Different from the conventional SDH system, the DWDM system with optical amplifier can monitor and management the EDFA. Since the EDFA only amplifies optical signals without electrical signal input. Especially when it uses as the optical amplifier regenerator, it has no electrical interface connection because there is no add/drop of service signals, leading to higher difficulty of monitoring on it. In addition, there is no special byte for monitoring the EDFA in the SDH overhead, so an electrical signal must be added to monitor the EDFA status. The OSC is used to transmit the NE management and supervision information related to the DWDM system on a long wavelength, involving fault alarm, fault location, quality parameter supervision in the operation, control over backup line upon line interruption and EDFA supervision. In this way, the network operator can effectively manage the DWDM system.

3.6.2 Requirements for OSC The DWDM system has the following requirements for the OSC: 1.

The OSC cannot restrict the optical wavelengths (980 nm and 1480 nm) of the pump light source in the optical amplifier.

2.

The OSC cannot restrict the transmission distance between two LAs.

3.

The OSC cannot restrict the services on the 1310 wavelength in the future.

4.

The OSC should still be available upon failure of the LA. The supervision information transmitted on the OSC is the information related to all kinds of optical amplifiers, such as input/output optical power of the optical amplifier and working wavelength of pump light source. Therefore, when the optical amplifier is failed, the O SC cannot work normally. At this time, the subversion is meaningless.

57

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5.

The OSC transmission is bidirectional. Bidirectional transmission ensures the supervision information can be received by the line terminal when one fiber is broken.

6.

OSC transmission segment can be dropped on each optical amplifier relay station and DWDM system office station and added with new supervision signals.

3.6.3 Implementation of OSC The implementation principle of OSC is shown in Fig. 3.6-1. λ1

λ2

OTUT

OTUR

OTUT

OM

OBA

OM2

OD2

λosc

Fig. 3.6-1

1.

OTUR

λ2

OSC information

Line fiber OD: Optical de-multiplexer

OD

λosc

OSC information

OM: Optical multiplexer

OPA

λ1

Internal fiber OBA and OPA: Optical amplifiers

OTUT and OTUR: Optical transponders

Implementation Principle of OSC

Dropping and adding of OSC information As shown in the above diagram, to ensure that the supervision information transmitted on the OSC can be dropped or added on each optical amplifier relay station and DWDM system office station without influence from optical amplifier, at the transmitting end, it is required to use a 2-wavelength OM OM2 behind the OBA to add the OSC information into the main channel; at the receiving end, it is required to use a 2-wavelength OD OD2 ahead of the OPA to drop the OSC information.

2.

Working wavelength of OSC For the DWDM system working with LA, an additional OSC is required, which should be able to perform adding/dropping with low enough BER in each optical relay/amplifier.

58


According to ITU-T recommendation, a specific wavelength can be used as the OSC. Such wavelength can be 1,310 nm, 1,480 nm or 1,510 nm when out of service information transmission band. The 1,510 nm is preferable. Since this channel is out of the gain bandwidth of the EDFA (also called external OSC), the supervision signals must be drop (dropping optical channel) ahead of EDFA and added (adding optical channel) behind the EDFA. As shown in Fig. 3.6-1, OSC is added behind OBA and dropped ahead of OPA. 3.

Transmission rate of OSC In the actual DWDM system, most of the information really requiring supervision involves EDFA working status, so the supervision information is not huge. In addition, to ensure normal operation of the supervision channel upon optical amplifier fault, the receiving sensitivity should be high. As a result, the supervision channel signals without amplifying of optical amplifier can cover the maximum transmission distance of the service major signals. Therefore, the working rate of the OSC is set to 2 Mbit/s. At present, since the technology ceaselessly develops, OSC rate improves as well. For example, ZTE DWDM can provide supervision rate of 10 Mbit/s or 100 Mbit/s.

4.

Frame structure of OSC information For the supervision system at working rate of 2 Mbit/s, thirty-two 64 kbit/s bytes are used to bear supervision information, which is transmitted and exchanged in PCM32 frame format. For the system at supervision rate of 10 Mbit/s or 100 Mbit/s, for example, ZTE DWDM, the supervision channel uses 10/100 M Ethernet technology to encapsulate data in IP packet format and transmits and exchanges them in Ethernet data frame.

5.

Line coding The 2 Mbit/s OSC uses Code Mark Inversion (CMI) as the line code type. The 10/100 Mbit/s OSC uses 4B/5B code.

59

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6.

OSC protection Upon the OSC bidirectional transmission interruption caused by totally break-off of the fiber, the NE management system cannot obtain the supervision information normally. At this time, the backup route (for example, Data Communication Network (DCN)) should be used for transmitting supervision information, for the sake of OSC protection.

60

4 Protection Principle of DWDM System  Highlights: z

Principle of 1+1 protection.

z

Principle of 1:N protection.

z

Principle of optical channel protection.

z

Principle of optical multiplexing section protection.

4.1 Brief Introduction to DWDM System Hierarchy &

Note:

In this chapter, we take ZTE DWDM as an example for introduction. The DWDM system protection involves protection of optical channel layer and optical MS layer. First of all, we will introduce the location of each layer in the system. The DWDM system is divided into Optical Multiplexing Section (OMS) layer, Optical Transport (OTS) layer, Optical Channel (OCH) layer and Optical Access (OAC) layer. The locations of the layers in the system are shown in Fig. 4.1-1, and their functions are listed in Table 4.1-1 OTM

OTM OLA

G.692 Optical transponder

λ1

λ1

Optical transponder

TX2

Optical transponder

λ2

λ2

Optical transponder

TX3

Optical transponder

λ3

Optical transponder

TX1

TXn


λ3

OM

OBA

OLA

OPA

OD

λn

λn

OTS


RX1 RX2 RX3

RXn

OTS OMS

OAC

OCH

Fig. 4.1-1

Hierarchy of DWDM System

61

OAC

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Table 4.1-1 Layer

Hierarchy Meaning of DWDM System

Location

OMS

Function Multiplexing

Between OTMs

between OLAs At

OCH

the

line

side

of

optical

transponder platform At

OAC

the

client

channel

signals

and

de-multiplexing multiplexed optical channel signals

Between OTM and OLA, and

OTS

optical

side

Transmitting optical signals on all kinds of fibers Supporting OAC to convert customer signals into optical

signals

in

accordance

with

G.692

recommendation for transmission of

optical

transponder platform

Accessing customer signals

4.2 1+1 Protection In the 1+1 protection changeover, the optical signals are simultaneously transmitted on working line and protection line. In other words, the signals are permanently connected (bridged) with working line and protection at the transmitting end. At the receiving end, the protection function monitors the statuses of the signals received from these two lines and selectively connects with the line with better signal quality. So this protection mode is called "concurrent transmitting and priority receiving". For ZTE DWDM, 1+1 protection is implemented by the OP board.

4.2.1 Link 1+1 Protection Depending on locations, the OP board can implement 1+1 protection of OCH or OMS. 1.

1+1 protection of OCH One OP board is used to protect a pair of bidirectional services. In channel 1+1 protection, the number of OP boards configured must be consistent with that of the channels to be protected. The protection channel and protected channel are transmitted in the same fiber. Channel 1+1 protection in the chain networking can only protect device other than route, as shown in Fig. 4.2-1.

62

Chapter 4 Protection Principle of DWDM System

Optical transponder

Protected channel

Protection channel O P

Optical transponder Optical transponder


λ1

λ1 λ2 λ3

O

O

M

D

U

U

λ2 λ3

... λn

λn

Optical transponder Optical transponder

Optical transponder

O P

Protected channel Optical transponder

Optical transponder

Optical transponder

Protection channel

O

O

D

M

U

U

Fig. 4.2-1

2.

Optical transponder

Optical transponder

1+1 Protection of OCH

1 +1 protection of OMS The 1+1 protection of OMS is in segment-by-segment 1+1 protection mode, as shown in Fig. 4.2-2.

OTU OTU OTU OTU OTU OTU OTU

OTU

λ1

λ1

λ2 λ3

O M D

Line 1 in direction A

OBA

Line 2 in direction A

λn

O P

λ1

O D D

OPA

O D U

λ3

λn

O P Line 1 in direction B

λ2 λ3

OPA

λ2

λ1 λ2

OBA

Line 2 in direction B

λn

O M U

λ3

λn

OTU OTU OTU OTU OTU OTU OTU

OTU

Line 1 is working channel and line 2 is protection channel.

Fig. 4.2-2

1 +1 Protection of OMS

When the OP board supervises the major optical channel, changeover is implemented through optical switch inside the board when changeover condition is met.

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4.2.2 Ring 1+1 Protection When the ring network uses 1+1 protection, the protection can be divided into 1+1 protection of OCH and 1+1 protection of OMS. In the ring network, the protection channel and protected channel reaches the receiving end through different routes. 1.

1+1 protection of OCH The 1+1 protection of OCH can protect not only route but also devices. We assume the ring network is as shown in Fig. 4.2-3.

C Protection channel B

D

Working channel

A

Fig. 4.2-3

Ring Networking

The optical connection between Node A and Node B is shown in Fig. 4.2-4.

O M U OTU O P OTU

O D U Working channel

O D U

O M U

O M U

O D U

OTU O P OTU

Protection channe

O D U

Site A

Fig. 4.2-4

2.

O M U

1+1 Protection of OCH (Ring Networking)

1+1 protection of OMS

64

Site B


In the ring network, the 1+1 protection of OMS protects the multiplexed signals. When the fiber is broken off, the two nodes close to the broken points implement "loop-back" function, and thus protecting all the services. It is similar to the protection mode shown in Fig. 4.2-2.

4.2.3 Features of 1+1 Protection 1.

Protection line is special and cannot be shared with other working lines.

2.

Signaling support is not required. It can be implemented easily.

3.

It can be used in any network structure (point-to-point, ring or grid network).

4.

It is restorable protection without signaling support.

5.

Its bandwidth utilization ratio is low and cost is high.

4.3 1: N Protection 4.3.1 Working Principle In the 1:N protection changeover, multiplex working lines share one protection line. N working lines are bridged to the protection line at both ends. The protection function monitors and judges the received signal status, and changes over the services on this working line to the protection line, upon deterioration or failure of service signals on the working line. This mode is called "transmitting-receiving changeover". Its working principle is shown in Fig. 4.3-1. Service can be transmitted Protection l ine Transmitting of working line 1 Transmitting of working line 2

Protection line Receiving of working line 1 Receiving of working line 2

This service is discarded. Protection line Transmitting of working line 1 Transmitting of working line 2

Protection line Receiving of working line 1 Receiving of working line 2

Changeover protocol

Fig. 4.3-1

Working Principle of 1:N Protection

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4.3.2 Protection Implementation At present, ZTE DWDM can provide 1:N protection for OCH. We take the protection implemented by the Electrical Switching Board (SWE) as an example. The SWE implements changeover in electrical switching mode. At the transmitting end, N channels of service signals are input to input ports 1 ~ N of the SWE, and then output to OTU through output ports 1 ~ N of the SWE. At the receiving end, input ports 1 ~ N of the SWE respectively receive the signals from OTU, and output ports 1 ~ N of the SWE output such signals to the user terminal. The protection function is shown in Fig. 4.3-2. 1 1 Incoming SDH optical signal

SWE

OTU

OTU

Outgoing OTU optical signal

Incoming SDH optical signal OMU

16

17

1

SWE

Outgoing OTU optical signal

ODU

OTU

Fig. 4.3-2

1

OTU

16 17

Functional Block Diagram of 1+1 Protection of OCH

If any channel in the N channels of services becomes faulty, once the receiving end detects the faulty service, it notifies the SWE boards at the transmitting end and receiving end through protocols, and then the receiving/transmitting end changes this channel of service to the port N + 1 to protect the service. When multiple channels of services are faulty at the same time, the service in high priority will take priority of protection. The protection priority is set in the NM.

4.3.3 Features of 1:N Protection 1.

The protection line is shared by multiple working lines.

2.

Signaling support is required. The implementation process is relatively complicated.

3.

It can be used in ring and grid networks.

4.

Protection is restorable.

5.

Its bandwidth utilization ratio is high but protection reliability is low. 66


4.4 Bidirectional Optical Channel Protection 1.

Working principle In the 2-fiber bidirectional channel shared protection ring, λ 1 of the external ring forms the working channel, and λ 1 of the internal ring forms the protection channel. The working channel allows wavelength multiplexing of multiple unidirectional services, and the protection channel shares protection of all services on the working channel. As shown in Fig. 4.4-1, when a cross-section fiber is faulty (× means faulty), the services passing this span are damaged, and thus the access switch starts operation at the service transmitting end, and services are transmitted along the protection route. Meanwhile the two changeover switches at the receiving end start operations, and services are received from the protection route. In this way, service protection is implemented.

T1（λ1）

A

B

C

Reserved wavelength channel ( λ1) inIntern al ring

F

E

T2（λ1）

T3（λ1） Changeover node

Fig. 4.4-1

2.

D

Adding channel, dropping channel T4（λ1） Changeover node

Principle of 2-Fiber Bidirectional Channel Shared Protection

Implementation mode of OPCS board The ZXMP M800, ZTE DWDM, implements bidirectional channel shared protection through the Optical Channel Shared Protection (OPCS) board. Besides channel protection of the ring network, the OPCS board also controls the adding status of adding protection wavelength through connecting with the

67

WM_000_ E1


optical switch, to avoid conflict of multiple services that use the same working wavelength on the protection ring. Fig. 4.4-2 shows a networking example.

H

A

λ21(B→A) λ21(B→A)

B

B

λ22(A→B) λ22(A→B)

G

F

λ22(E→F) λ22(E→F)

E

D

λ21(F→E)

λ21(F→E)

Fig. 4.4-2

Wavelength Configuration of Channel Shared Protection

We assume that a pair of bidirectional services between Site A and Site B need protection. First of all, we install the OPCS board at Sites A and B and connect fibers. In the configuration, the required service wavelength is differential-wavelength transmission. Service from A to B is borne by λ 21 (external ring), and service from B to A is borne by λ 22 (internal ring). In this way, the working wavelength formed by λ 21 and λ 22 can be repeatedly used between other nodes in the ring network, and λ 21 of internal ring serves as the protection wavelength of external ring λ 21. Similarly, λ 22 wavelength serves as the protection wavelength of internal ring λ 22, and thus implementing shared protection of multiple services in the ring network. The wavelength allocation can be flexibly adjusted. But service bidirectional feature and differential-wavelength of working wavelengths must be guaranteed. For the convenience of project debugging and maintenance, the default principle is allocating wavelengths by adjacent odd and even wavelengths. 3.

Application features

1)

It is used for loop protection.

68


2)

Service protection is based on channels. Changeover depends on quality of the signals in a channel leaving the loop.

3)

In the loop, the transport directions of node receiving information and node transmitting information are two reverse directions. The resource utilization ratio is high.

4)

In the changeover, changeover is implemented in adding channel node and dropping channel node of the service.

5)

Wavelength allocation is flexible.

4.5 Bidirectional OMS Protection 1.

Working principle In the 2-fiber bidirectional MS protection, the system uses the same wavelength in internal ring and external ring for mutual protection. For example, for the 32-wavelength system, the first 16 wavelengths of the internal ring serve as working wavelengths, and the last 16 wavelengths serve as protection wavelengths. The first 16 wavelengths of the external ring serve as protection wavelengths, and the last 16 wavelengths serve as working wavelengths. The wavelengths are complementarily distributed. Only 8 wavelengths of the 32-wavelength system can be protected as well, and the scheme of mutual protection can be employed for the 8 wavelengths of the internal and external rings, that is, the actual working wavelengths of the system are 24 wavelengths. The working wavelengths usually transmit services while the protection wavelengths usually not. Fig. 4.5-1 shows the schematic diagram of MS protection for mutual protection of the wavelengths in the internal and external rings, with 16 working wavelengths. The solid lines indicate working routes, and the dotted lines indicate protection routes of the external ring in case of fault between D and E.

69

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B

A

H

λ17 C

Fig. 4.5-1

2.

λ17

D

Adding channel, dropping channel

G

E

λ1

F

λ1

Adding channel, dropping channel

Schematic Principle Diagram of 2-Fiber Bidirectional MS Shared Protection

Implementation mode of OPMS board The ZXMP M800, ZTE DWDM, implements bidirectional OMS shared protection through the Optical MS Shared Protection (OPMS) board. Fig. 4.5-2 shows a networking example.

H

A

λ21(B→A) λ21(B→A)

B

B

λ43(A→B)

λ43(A→B) λ43(E→F)

G

F

λ43(E→F)

E

D

λ21(F→E)

λ21(F→E)

Fig. 4.5-2

Wavelength Configuration of MS Shared Protection

We assume that a pair of bidirectional services between Site A and Site B need protection. First of all, we install the OPMS board at Sites A and B and connect fibers. In the configuration, the service wavelengths are in differential-wavelength transmission mode, and both working bands and protection bands of internal/external

ring

are 70

distributed

symmetrically.

For

example,

16


wavelengths (192.1 THz ~ 193.8 THz) of the external ring serve as working wavelengths of external ring, and 16 wavelengths (194.3 THz ~ 196.0 THz) of the internal ring serve as working wavelengths of internal ring. We assume that service from A to B is borne by λ 21 (external ring), and service from B to A is borne by λ 43 (internal ring). In this way, the working wavelength formed by λ 21 and λ 43 can be repeatedly used between other nodes in the ring network, and λ 21 of internal ring serves as the protection wavelength of external ring λ 21. Similarly, λ 43 wavelength of external ring serves as the protection wavelength of internal ring λ 43, and thus implementing shared protection of multiple services in the ring network. 3.

Application features

1)

It is used for loop protection.

2)

Service protection is based on MS. Changeover depends on quality of the MS signals between adjacent nodes.

3)

In the loop, the transport directions of node receiving information and node transmitting information are two reverse directions. The resource utilization ratio is high.

4)

Upon fault, changeover is executed between adjacent nodes of faulty span.

5)

In the configuration of MS shared protection, at least one Optical MS Shared Protection board (with preventing resonance switch) is configured in the loop, to avoid self-stimulation of the loop.

71

Appendix A Abbreviations Abbreviation

Full Name

AFR

Absolute Frequency Reference

AFEC

Advanced FEC

AGENT

-

AIS

Alarm Indication Signal

APR

Automatic Power Reduction

APS

Automatic Protection Switching

APSD

Automatic Power Shutdown

APSF

Automatic Protection Switching for Fast Ethernet

ASE

Amplified Spontaneous Emission

AWG

Array Waveguide Grating

BER

Bit Error Ratio

BLSR

Bidirectional Line Switching Ring

BSHR

Bidirectional Self-Healing Ring

CDR

Clock and Data Recovery

CMI

Code Mark Inversion

CODEC

Code and Decode

CPU

Center Process Unit

CRC

Cyclic Redundancy Check

DBMS

Database Management System

DCC

Data Communications Channel

DCF

Dispersion Compensation Fiber

DCG

Dispersion Compensation Grating

DCN

Data Communications Network

DCM

Dispersion Compensation Module

DCF

Dispersion Compensating Fiber

DDI

Double Defect Indication

DFB-LD

Distributed Feedback Laser Diode

DSF

Dispersion Shifted Fiber

DGD

Differential Group Delay

DTMF

Dual Tone Multi Frequency

DWDM

Dense Wavelength Division Multiplexing

DXC

Digital Cross-connect

EAM

Electrical Absorption Modulation

ECC

Embedded Control Channel

EDFA

Erbium Doped Fiber Amplifier

EFEC

Enhanced FEC

73

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Abbreviation

Full Name

EX

Extinction Ratio

FDI

Forward Defection Indication

FEC

Forward Error Correction

FPDC

Fiber Passive Dispersion Compensator

FWM

Four Wave Mixing

GbE

Gigabits Ethernet

GUI

Graphical User Interfaces

IP

Internet Protocol

LD

Laser Diode

LOF

Loss of Frame

LOS

Loss of Signal

MANAGER

-

MDI

Multiple Document Interface

MCU

Management and Control Unit

MOADM

Metro Optical Add Drop Multiplexer Equipment

MBOTU

Sub-rack backplane for OTU

MQW

Multiple Quantum Well

MSP

Multiplex Section Protection

MST

Multiplex Section Termination

NCP

Net Control Processor

NDSF

None Dispersion Shift Fiber

NE

Network Element

NNI

Network Node Interface

NMCC

Network Manage Control Center

NRZ

Non Return to Zero

NT

Network Termination

NZDSF

Non-Zero Dispersion Shifted Fiber

OA

Optical Amplifier

OADM

Optical Add/Drop Multiplexer

OBA

Optical Booster Amplifier

Och

Optical Channel

ODF

Optical fiber Distribution Frame

ODU

Optical Demultiplexer Unit

OGMD

Optical Group Mux/DeMux Board

OHP

Order wire

OHPF

Overhead Processing Board for Fast Ethernet

OLA

Optical Line Amplifier

OLT

Optical Line Termination

OMU

Optical Multiplexer Unit

ONU

Optical Network Unit

OP

Optical Protection Unit

OPA

Optical Preamplifier Amplifier 74

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