DWDM Planning

Project P709

Planning of Full Optical Network Deliverable 1 Considerations on Optical Network Architectures: Functionalities, Configurations and Client Signals

Suggested readers:

• • •

Managers, PNO Optical Network planners Experts on Standard Bodies (ITU T SG-13/15 and ETSI TM1 WG2/3) Optical systems and equipment manufacturers

For full publication

January 1999

EURESCOM PARTICIPANTS in Project P709 are:

• • • • • • • • •

Finnet Group Swisscom AG Deutsche Telekom AG France Télécom

MATÁV Hungarian Telecommunications Company TELECOM ITALIA S.p.a. Portugal Telecom S.A. Telefonica S.A. Sonera Ltd.

This document contains material which is the copyright of certain EURESCOM PARTICIPANTS, and may not be reproduced or copied without permission All PARTICIPANTS have agreed to full publication of this document The commercial use of any information contained in this document may require a license from the proprietor of that information. Neither the PARTICIPANTS nor EURESCOM warrant that the information contained in the report is capable of use, or that use of the information is free from risk, and accept no liability for loss or damage suffered by any person using this information. This document has been approved by EURESCOM Board of Governors for distribution to all EURESCOM Shareholders.

© 1999 EURESCOM Participants in Project P709

EURESCOM PARTICIPANTS in Project P709 are:

• • • • • • • • •

Finnet Group Swisscom AG Deutsche Telekom AG France Télécom

MATÁV Hungarian Telecommunications Company TELECOM ITALIA S.p.a. Portugal Telecom S.A. Telefonica S.A. Sonera Ltd.

This document contains material which is the copyright of certain EURESCOM PARTICIPANTS, and may not be reproduced or copied without permission All PARTICIPANTS have agreed to full publication of this document The commercial use of any information contained in this document may require a license from the proprietor of that information. Neither the PARTICIPANTS nor EURESCOM warrant that the information contained in the report is capable of use, or that use of the information is free from risk, and accept no liability for loss or damage suffered by any person using this information. This document has been approved by EURESCOM Board of Governors for distribution to all EURESCOM Shareholders.


Deliverable 1

Considerations on Optical Network Architectures

Preface (Prepared by the EURESCOM Permanent Staff) Network traffic is increasing at an unprecedented rate, driven by the dramatic growth of the Internet and corporate data communications. The evolution of photonics makes the development of optical switching and routing structures in the core and metropolitan part of the transport network possible. This brings an increase in capacity and reduces transport costs. The Wavelength Division Multiplexing (WDM) technique jointly with optical crossconnect (OXC), and Optical Add-Drop Multiplexing (OADM) equipment, will permit the realisation of a switched optical layer based on wavelength routing of semipermanent paths and fast protection/restoration mechanisms for the large amount of information flows carried on the optical links. As a consequence, the development of an optical network infrastructure will enable the flexible, reliable and transparent provision of transport services for any type of traditional and innovative services and applications. Taking into consideration the current trends, the objective of network planning is to find the best possible balance between network implementation cost, network flexibility, network availability and survivability, subject to service requirements and topological constraints. The aim of the P709 EURESCOM Project is to investigate a number of alternative strategies for the planning of the optical transport network - with massive deployment of WDM, OADM, and small size OXC - that will be used in a middle term future. This is the first Deliverable (D1) of P709. D1 provides an overview over network architectures, which potentially may be used in the future. It also summarises the requirements on optical networks as well as maturity and availability of optical functionalities. It should be noted that the Deliverable could not include all new functionalities of optical devices since it is an ongoing technology and due to the limited study period, this was not possible. P709 is a logical continuation of the P615 Project (Evolution towards an optical network layer) and some input from this Project was used in D1. D1 is a very useful study for Managers, Optical Network planners, and experts on Standard Bodies of ITU-T SG15 and ETSI TM1 (WG2 & WG3).


page i (ix)


page ii (ix)

Deliverable 1


Deliverable 1


Executive Summary Optical WDM network is gaining more and more attention and is being implemented in a number of field trials. Several commercial products are appearing on the market market with certain maturity. In USA, Europe and Japan, most of PNOs are planning to increase the capacity of their transport network with massive deployment of WDM point-to-point system as well as fixed OADM and small size OXC. The aim of EURESCOM Project P709 ‘Planning of full optical network’ is to investigate a

number of alternative strategies for the planning of optical transport network. This Deliverable D1, the first one of P709 Project, concludes the results of Task 2 ‘Considerations on Optical Network Architectures’ activities. This document is aimed at those people who work on Network Planning for PNOs, Experts on Standard Bodies related to the optical technologies, systems and networks, or manufacturers building equipment for WDM networks. The fundamental idea of the document is to show how the WDM technique could bring new network architectures through the use of novel optical functions, and how the latter optical network layer could transport multi-client signals. It could provide companies with an overview of the state-of-the-art of optical functions, and useful considerations on optical network architectures impacting the network planning process. The first part of this Deliverable discusses the general characteristics of optical functions as they are available now or will be in the near future. Commercial WDM point-to-point systems are also described and compared. Different classes of network architecture, from the simple topologies to more complex structures are presented in Section 2 in order to select reference network architectures. The possible combinations of basic optical network architectures are collected, in relation with work carried out in P615 Project. The resulting selection of reference two-level network architectures is the following:

• • • •

CS-Ring architecture OMS-SP Ring architecture mesh-ring architecture ring-mesh architecture

The document goes on to discuss important network parameters which characterise the WDM networks in terms of architecture, demand, physical limitation, topology and survivability. In the last Section, the possibility to plan an optical network using a non SDH client signal is proposed. After a brief investigation into ATM and IP client signals performance and functionalities, multi-layer network configurations are proposed using IP, ATM, SDH and WDM network functionalities. A first evaluation of ATM over WDM, IP over SDH, IP over ATM and IP directly over WDM configurations is discussed. Main achievements of the Deliverable are:

•

identification of new optical functions considered necessary in order to enable the migration to WDM future optical networks or desirable to enhance offered network functionality


page iii (ix)


Deliverable 1

• •

selection of optical network architectures

•

contribution to determine the ability of planning an optical layer carrying non SDH signals.

contribution to determine the physical limitation and typical values of network characteristic parameters

While addressing considerations on optical network architectures, the elementary optical functionalities, network configurations and the possibility of carrying non SDH client signals are discussed. The selected optical network architectures will be used in Task 3 and Task 4 in comparative studies of planning methodologies.

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Deliverable 1


List of Authors Jamil CHAWKI

France Télécom BD-CNET

Task 2 & PIR 2.4 Leader

António Jaime Ramos: Portugal Telecom/ CPRM-Marconi Hélder Gaspar:

Portugal Telecom/ CPRM-Marconi

Eduardo Sampaio:

Portugal Telecom/ CPRM-Marconi

Reinald Ries:

Deutsche Telekom AG

Ralf Herber:

Deutsche Telekom AG

Paulette Gavignet:


André Hamel:


François Tillerot:


Géza Paksy:

Hungarian Telecom MATAV

Teresa Almeida:

Portugal Talcum

PIR 2.3 Leader

PIR 2.2 leader

António Jaime Ramos:

Internal Reviewer

Dag Roar Hjelme (Sintef, Norway):

External Reviewer

Martjin Luyten (NL):

External Reviewer


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Table of Contents Preface .............................................................................................................................i Executive Summary...................................................................................................... iii List of Authors................................................................................................................v Table of Contents ..........................................................................................................vi Abbreviations ............................................................................................................. viii Introduction ....................................................................................................................1 1 Assessment of optical functionalities and WDM point-to-point systems ...................3 1.1 Description of available functions....................................................................3 1.1.1 Signal Transport (Single mode fibre)..................................................3 1.1.2 Transmitter ..........................................................................................3 1.1.3 Receiver...............................................................................................4 1.1.4 Transponder.........................................................................................4 1.1.5 Dispersion compensation ....................................................................4 1.1.6 Optical Amplifier OA, 1R (EDFA) .....................................................5 1.1.7 Filters...................................................................................................5 1.1.8 Optical Add Drop Multiplexer OADM...............................................6 1.1.9 Space switch (matrix)..........................................................................6 1.2 WDM point-to-point Systems...........................................................................7 1.2.1 Description of a WDM point-to-point link..........................................7 1.2.2 N x 2.5Gbit/s systems..........................................................................8 1.3 Identification of new/desirable optical functions ..........................................10 1.3.1 Wavelength conversion .....................................................................11 1.3.2 Optical signal monitoring functions (QoS, optical spectrum, and Failure detection) ....................................................................11 1.3.3 Optical 3R regeneration.....................................................................11 1.3.4 Network survivability (protection, restoration).................................11 1.3.5 Management functions ......................................................................12 1.3.6 Optical time domain multiplexing OTDM (Long term function) ........................................................................................12 1.3.7 Optical packet switching (Long term function) ................................12 1.4 Conclusion ......................................................................................................12 2 Assessment of optical network architectures ............................................................13 2.1 Complex topologies ........................................................................................13 2.1.1 Connected rings.................................................................................13 2.1.2 Meshed domains interconnected by a ring trunk...............................13 2.1.3 Ring domains interconnected by a meshed trunk..............................13 2.2 Characteristic parameters ...............................................................................13 2.2.1 Specific characteristics of optical network .......................................14 2.2.2 Parameters related to topology ..........................................................15 2.2.3 Parameters related to physical limitations.........................................16 2.2.4 Parameters related to demands ..........................................................17 2.2.5 Parameters related to architecture .....................................................18 2.2.6 Parameters related to the survivability approach ..............................19 2.3 Selection of reference network architectures .................................................19 2.3.1 Two-level CS-Ring architecture........................................................20

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2.3.2 Two-level OMS-SP Ring architecture .............................................. 20 2.3.3 Two-level mesh-ring architecture ..................................................... 21 2.3.4 Two-level ring-mesh architecture ..................................................... 22 2.3.5 Characteristics of the selected optical network architectures ........... 22 2.4 Identification of physical network parameters limitation .............................. 23 2.4.1 Identification of mechanisms originating limitations [4, 5].............. 23 2.4.2 Identification of systems/components which introduce limitations...................................................................................... 24 2.4.3 Processes to overcome limitations at present and solve them in the future ....................................................................................... 25 2.5 Identification of Ranges of values.................................................................. 25 2.5.1 Functional layer characteristics ........................................................ 26 2.5.2 Ranges of values ............................................................................... 26 2.6 Conclusions .................................................................................................... 28 3 Potential of WDM routing for different client signals.............................................. 29 3.1 ATM client signal........................................................................................... 29 3.1.1 ATM Network functionalities and physical layer............................. 29 3.1.2 ATM Services ................................................................................... 29 3.1.3 ATM Performance Parameters.......................................................... 29 3.2 IP client signal................................................................................................ 30 3.2.1 Internet network layers and services................................................. 30 3.2.2 IP protocols : IP v4/v6, RTP and RSVP ........................................... 31 3.3 Network configurations required by ATM/IP client signals.......................... 32 3.4 Impact of non SDH client signal on the planning of optical network ........... 33 3.4.1 ATM over SDH over WDM : SDH protection vs. WDM protection....................................................................................... 33 3.4.2 Configuration ATM over WDM....................................................... 35 3.4.3 Configuration IP over ATM [9]........................................................ 36 3.4.4 Configuration IP over SDH [10, 11, 12] ........................................... 37 3.4.5 Configuration IP over WDM ............................................................ 37 3.5 Conclusion...................................................................................................... 38 4 Conclusion ................................................................................................................ 39 References.................................................................................................................... 40 Appendix 1: Recent Progress in the Performance of Optical Transmission System Components ............................................................................................. 41


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Abbreviations AAL

ATM Adaptation Layer

ABR

Available Bit Rate

ACK

ACKnowledgement

APS

Automatic Protection Switching

ATM

Asynchronous Transfer Mode

AWG

Arrayed Waveguide Grating

BER

Bit Error Rate

CBR

Constant Bit rate

CBFG

Chirped Bragg Fibre Grating

CDV

Cell Delay Variation

CER

Cell Error Ratio

CLR

Cell Loss Ratio

CMR

Cell Miss-insertion Rate

CS Ring

Coloured Section Ring

DA

Dispersion Accommodation

DCF

Dispersion Compensating Fibre

DFF

Dispersion Flattened Fibre

DSF

Dispersion Shifted Fibre

EDFA

Erbium Doped Fibre Amplifier

FTP

File Transfer Protocol

FWM

Four Wave Mixing

HDLC

High level Data Link Control

IP

Internet Protocol

IPv4 / v6

Internet Protocol version 4 / version 6

LAN

Local Area network

LLC

Logical Link Control

MAPOS

Multiple Access Protocol Over SDH

MCTD

Mean Cell Transfer Delay

MS

Multiplex Section

MSP

Multiplex Section Protection

MS-SP Ring

Multiplex Section Shared Protection Ring

OA

Optical Amplifier

OADM

Optical Add Drop Multiplexer

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OC or OCH

Optical Channel

OC-DP Ring

Optical Channel Dedicated Protection Ring

O/E

Opto-Electronic

OMS

Optical Multiplex Section

OM-SDP Ring

Optical Multiplex Section Dedicated Protection Ring

OMS-SP Ring

Optical Multiplex Section Shared Protection Ring

OPS

Optical Protection Switching

OSC

Optical Supervision Channel

OTDM

Optical Time Division Multiplexing

OTS

Optical Transmission Section

OXC

Optical Cross Connect

PDU

Protocol Data Unit

POH

Path Over Head

PPP

Point-to-point Protocol

QoS

Quality of Signal / Service

RSVP

Resource Reservation Protocol

RTP

Real Time Protocol

SDH

Synchronous Digital Hierarchy

SDXC

Digital Cross Connect

SHR

Self-Healing Ring

SMF

Single Mode Fibre

SNAP

Sub Network Attachment Point

STM

Synchronous Transport Module

TCP

Transfer Control Protocol

UBR

Unspecified Bit Rate

UDP

User Datagram Protocol

VC

Virtual Circuit of ATM or Virtual Container of SDH

VP

Virtual Path of ATM

WDM

Wavelength Division Multiplexing

WWW

World Wide Web


page ix (ix)

Deliverable 1


Introduction In order to cope with the increasing network traffic driven by the dramatic growth of the Internet and corporate data communications, the evolution of the optical technologies makes the development of optical switching and routing structures in the core and metropolitan part of the transport networks possible. The Wavelength Division Multiplexing (WDM) technique jointly with optical nodes will permit the wavelength routing of semi-permanent paths and fast protection/restoration mechanisms in the optical layer. The purpose of this document is to show how the WDM technique could bring new network architectures through the use of available and forthcoming optical functions, and how the latter optical network layer could be compatible with the transport of signals with various formats. As a logical continuation of the P615 Project (Evolution towards an optical network layer), some input from this Project was used in this document. Beyond the scope of the P615 Project, this document provides results from the investigation of the commercially available and the desirable optical functionalities especially concerning optical amplifiers, advanced functions such as wavelength conversion and optical nodes. It also provides more complex network configurations, based on network interconnections, and identifies characteristic parameters. The first part of this Deliverable discusses the general characteristics of optical functions as they are available now or will be in the near future. Commercial WDM point-to-point systems are described and compared. The optical functions needed for the future WDM networks are progressing rapidly. A set of new optical functions considered necessary in order to enable the migration to WDM future optical networks or desirable to enhance offered network functionality, is also identified. From these possible network functionalities described in Section 1, different classes of network architecture, from the simple topologies to more complex structures, are presented in Section 2 in order to select reference network architectures. The resulting selection of reference two-level network architectures is the following :

• • • •

CS-Ring architecture OMS-SP Ring architecture mesh-ring architecture ring-mesh architecture

The selected optical network architectures will be used in Task 3 and Task 4 in comparative studies of planning methodologies. In Section 2, the most important network parameters which characterise the WDM networks from the point of view of architecture, demand, physical limitation, topology and survivability, are summarised. Physical limitation and typical values of network characteristic parameters are presented in the last part of this section. Finally, in Section 3, the possibility to plan an optical network using a non SDH client signal is proposed. After a brief investigation into ATM and IP client signals performance and functionalities, multi-layer network configurations are proposed using IP, ATM, SDH and WDM network functionalities. A first evaluation of ATM


page 1 (42)


Deliverable 1

over WDM, IP over SDH, IP over ATM and IP directly over WDM configurations is discussed.

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1


Assessment of optical functionalities and WDM pointto-point systems Optical network planning activities have to reflect a variety of physical as well as practical conditions and constraints in order to produce useful results. Among these is the set of the available optical functions which is to be used for the construction of the network under consideration. In this first section of the Deliverable, the state-of-the-art of optical functions, as they are available now or will be in the near future, will be presented. The performance of available WDM systems (point-to-point) will be compared. In this part such results are already available from EURESCOM P615 Project. The state-of-the-art of optical components and of realised functions which can be used in WDM optical networks, however, is progressing very rapidly. Thus it is necessary to update the information on the presently available optical functions. Finally, a list of desirable optical functions will be discussed in the last paragraph.

1.1

Description of available functions

1.1.1

Signal Transport (Single mode fibre) All-optical networks are based on a passive fibre infrastructure which serves as the physical transport medium between the network nodes. The most relevant properties of transmission fibres are attenuation, dispersion and non-linearity. Standard single mode fibres (SMF) as well as dispersion shifted or flattened fibres (DSF, DFF) are commercially available with standardised properties according to ITU-T recommendations G.652 ...655. TrueWave® fibre also is a kind of dispersion manipulated fibre. With attenuation values close to that of SMF its dispersion, however, is kept non zero at a value optimised in order to produce minimum distortions due to the combined effects of non-linearity and dispersion. In table 1 the characteristic data of various fibre types are summarised [1], [2] . Fibre type

SMF

DFF

DSF

True wave

Zero dispersion wavelength (µm)

1.312

1.535 – 1.565 ?

>1.56

1.518

Dispersion coefficient at 1.55µm (ps/(nm.km)

17

<2.7

0.1 – 3.5

-1 – 5.5

Table 1: Characteristic data for some types of single mode fibres

1.1.2

Transmitter Laser diode with direct and integrated modulation The standard optical transmitter element in WDM systems is a laser diode. Integrated laser modulation, ILM, offers a high dispersion tolerance through the use of electroabsorption modulator on the same laser chip. Key features of these devices are :

• • •

optical output power

0 to +10 dBm

modulation bandwidth

10 GHz

dispersion tolerance

1000 to 10000 ps @ STM-16


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•

stability & accuracy of optical frequency

Deliverable 1

0.05 nm

Laser diode with external modulation External modulation is used in order to reduce the chirp of the optical transmitter and thus increase its dispersion tolerance. Transmitter modules are available for 10 Gbit/s transmission and in laboratory systems modulation bandwidth of 100 GHz has been demonstrated. Apart from reduced width of the optical spectrum the other properties are the same as for directly modulated laser diodes.

1.1.3

Receiver Optical receivers are found in optical line terminations and in transponders where they convert the signal from the optical into the electrical domain. State-of-the-art

optical receivers reach sensitivities close to –30 dBm for 2.5 Gbit/s and around –20 dBm for 10 Gbit/s (BER 10 E-10). 1.1.4

Transponder

Transponders are opto-electronic frequency converters which basically consist of an optical receiver and transmitter. The receiver converts the optical input signal into the electrical domain where it is amplified and sometimes even reshaped and re-timed. This signal is used to modulate a laser diode optical transmitter which produces the required optical carrier frequency. Most WDM system manufacturers rely on transponders as input interface into the WDM system. They are available now for bit rates up to 10 Gbit/s. The input sensitivity varies considerably (-5 dBm ... –20 dBm) for devices from different manufacturers. Transponders accept 1.3 µm- as well as 1.55 µm input signals and their output powers are around 0dBm. Some manufacturers still offer devices with output frequencies not matching the ITU-T recommendations concerning the WDM channel frequencies. Transponders which do not regenerate the input signal will work with any type of intensity modulated digital (binary) client signal independent of signal format (e.g. SDH, ATM...) and bit rate (155 Mbit/s, 622 Mbit/s, 2.5 Gbit/s, 10 Gbit/s) within the limits of the specifications. Frequency- or phase modulated optical input signals cannot be used with transponders. 1.1.5

Dispersion compensation

Besides fibre attenuation it is the effect of fibre chromatic dispersion which mainly limits the achievable repeater spacing in optical links. The origin of the latter effect is the variation of the group delay as a function of the optical frequency. In fibre optical transmission lines the dispersion effect increases linearly with fibre length and width of the optical spectrum and causes pulse distortion and bit interference. As chromatic dispersion is a linear effect it can be compensated by inserting additional appropriate optical elements into the transmission link. Dispersion compensating fibre

Dispersion compensating fibre (DCF) is a special type of fibre which for light in the 1.55 µm wavelength region has negative dispersion coefficient in the order of –80 ps/(nm.km). Thus 1km of DCF is needed to compensate the dispersion of about 5 km of SMF as the corresponding value is 17 ps/(nm .km) in SMF.

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The value of the dispersion coefficient varies as a function of optical frequency in DCF as well as in SMF. Therefore it is not possible to achieve perfect dispersion compensation in a large frequency range. The attenuation of DCF with typical values around 0.6dB/km is still considerably larger than that of SMF. Chirped Bragg fibre grating Using chirped Bragg fibre gratings (CBFG) is another option for dispersion compensation. These devices provide a low loss solution. However, they work in reflection mode and therefore optical circulators or fibre couplers are necessary to separate input and output signal. Presently compensation bandwidth of only some hundreds of GHz achievable with one CBFG is more limited compared to DCF. A wider bandwidth can be achieved through the use of longer gratings or cascaded gratings. But the requirement of additional circulators or couplers may be regarded as a drawback. On the other hand, their non-linearity is practically zero which may be particularly important in very high bit rate systems with 10 Gbit/s and more.

1.1.6

Optical Amplifier OA, 1R (EDFA) Erbium doped fibre amplifiers (EDFAs) are one of the key building blocks of WDM systems. They allow the economical power amplification of all the signals in the different WDM channels. The system relevant optical properties of EDFAs are: power gain, saturated output power, noise figure, optical bandwidth and polarisation mode dispersion. The power gain is calculated as the ratio of output to input signal power of the amplifier. This value directly determines the maximum link segment attenuation between consecutive EDFAs. It depends on the number of channels and on total link length. In practical links this value varies from below 20 dB to 30 dB. The saturated output power is the upper limit of the total output power from the amplifier for high input power. Typical values range from 13 dBm to 17 dBm while EDFAs with output powers of up to 30 dBm are commercially available.

1.1.7

Filters Fibre Bragg Grating Filters These filters are based on photosensitivity in Ge-doped core optical fibres; reflection gratings are written by illuminating the fibre with a standing wave interference pattern. In recent years, fibre Bragg Gratings have proven successful as in line filters. This device has the advantages of being low loss, with a narrow pass-band characteristic (0.5 nm) and potentially low cost. This is a very promising technology for fixed filtering with a channel spacing in the nm range. Tuneable filtering is obtained by stretching the fibre where the Bragg filter is deposited. They are commonly used with optical circulators to obtain the OADM functionality. Diffraction Gratings and Phased Arrayed Gratings Grating devices are suited to address several wavelengths simultaneously because they pass a discrete set of predefined wavelengths. Two types are available; the first one is a micro-optic diffraction grating. The typical insertion loss per channel of a diffraction grating device is 3 dB with a -30 dB adjacent crosstalk level. The other type is an integrated optic device (SiO2 or InP) called Arrayed Waveguide Grating; for an AWG, the typical insertion loss is 5 dB and the crosstalk is 25 dB (figure 1). Components are commercially available for 8, 16 or 32 channels with 100


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Deliverable 1

or 200 Ghz channel spacing and the insertion losses for a transit wavelength are 6 to 10 dB.

Figure 1: Spectral response of a wide pass-band AWG

1.1.8

Optical Add Drop Multiplexer OADM Concerning optical add and drop facilities, two main suppliers are offering them : Ciena and Pirelli. Indeed 8 channels can be added or dropped at each amplifier site in the Ciena Multiwave 4000 and 12 in the Pirelli Wavemux. Figure 2 indicates where this function can be introduced in a WDM point-to-point system. It shows the example of insertion and extraction of channels at an amplifier site. The available add/drop functions are fixed but selectable add/drop facilities have already been announced as well as reconfigurable OADMs and OXCs. This is planned for the next years but to our knowledge no exact dates have been given. W DM

W DM

terminal

terminal

amplifiers transit

d ro p

add

Figure 2: Possible position of an add/drop function in a WDM point-to-point system

1.1.9

Space switch (matrix) Switching matrices are available which are suited for realising of flexible OADMs and OXCs. Various approaches have been followed to perform the switching function. Devices relying on mechanical operation contain actuators, e.g. motors, electro-statically or piezo-electrically deflected micro mirrors for the switching of the optical signal. Due to the required mechanical movements of part the switching times

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achieved so far range from 30ms to 500ms. Wave-guide devices which make use of thermal or electro-optic effects are considerably faster as can be seen in table 2 which contains data of various switching matrices. With respect to insertion losses and channel crosstalk wave-guide devices do not perform as well as mechanical switching matrices. Technology

mechanical

thermal

actuator

motor

size

64 x 64 8 x 8 16 x 16 100 x 100 64 x 64

8 x 8 8x8 27 x 27

8x8

insertion loss (dB)

2

2

4

8

8

8

switching time (ms)

500

30

40

2

2

<2

channel isolation (dB)

60

60

60

50

40

30

mirror

Electro-static

polymer

Electro- optic thermal

Table 2: Characteristic features of optical switching matrices

1.2

WDM point-to-point Systems First, we will describe what is generally called a WDM point-to-point system, then the performances of the WDM products will be compared and finally we will give some elements concerning the future functionalities (OADMs and OXCs).

1.2.1

Description of a WDM point-to-point link Figure 3 shows what is commonly called a WDM point-to-point system (in the broken-line rectangle). We have considered a system which processes a bi-directional transmission over two fibres (which is more often the case) with in-line amplification. The non-amplified systems will not be considered in this document. Each of the optical signals coming from SDH (or SONET) terminals (or ADMs) is sent into a transponder (optional or not) used to provide a wavelength compatible signal to the WDM terminal. Moreover, the signals which feed the WDM terminal must be able to be transmitted on dispersive fibre (standard fibre G.652) over several hundreds of kms (typically 500). The WDM terminal contains a multiplexer (or a coupler) and a booster for one direction and a preamplifier and a demultiplexer in the other direction. Then, the optical multiplex is launched into the transmission fibre and regularly amplified (each 80 to 120 km) by optical amplifiers (so without opto-electronic conversion). Another terminal, similar to the first one is placed at the end of the line and the signals are either directly received by the SDH terminal or via a transponder.

SDH interfaces SDH interfaces

) - s s r n e d a r t n ( o p

WDM terminal

WDM terminal

) - s s r n e d a r t n ( o p

amplifiers

SDH interfaces SDH interfaces

Multi-wavelength system (WDM) Figure 3: WDM point-to-point system description


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The management of the system is made using an Optical Supervisory Channel (OSC) at a wavelength (very often outside the multiplex) which is dedicated to transporting fault, configuration and performance information. This channel is processed in the WDM terminal sites and also in each amplifier site (demultiplexing, detection, processing, emission, multiplexing). The systems with transponders in the WDM

terminal are sometimes called ‘open systems’ (because they can theoretically accept various input signal formats). Systems without this feature are called ‘embedded systems’ (they are less flexible but the cost can be reduced compared to the open systems). Today, the maximum bit rate per channel at the input of the WDM terminal (or the transponder) is 10 Gbit/s. This is the maximum bit rate available with existing SDH TDM equipment (STM-64). Moreover, the transmission of 10 Gbit/s optical signal is difficult, due, essentially, to chromatic dispersion and polarisation mode dispersion (PMD) limitations. In fact, many WDM systems use the multiplexing of 2.5 Gbit/s signals. Some systems are already announced for a total capacity of 200 Gbit/s and more with a number of wavelengths reaching 80 (and even 96). Some of the systems accept both 2.5 and 10 Gbit/s input rates (they will be indicated hereafter) and also lower rates (155 and 622 Mbit/s). Concerning the Nx2.5 Gbit/s WDM systems with in-line amplification, the ITU-T recommendation G.692 addresses 4 or 8 channels multiplexed together (with a possible extension to 16 or 32). The target transmission distance over G.652, G.653 or G.655 fibre is 640 km with nominal span lengths of 80 (with 8 spans), and 120 km (5 spans). The channel grid is defined with 100 GHz interchannel spacing (193.1 THz is the reference frequency). 1.2.2

N x 2.5Gbit/s systems

Table 3 shows the performance of the WDM systems on which information is available. It indicates the supplier and the name of the product, the availability, the maximum bit rate capacity, the distance between amplifiers and the maximum distance that can be reached (it depends on the number of spans that can be cascaded). The systems are designed for G.652 standard fibre but some suppliers like Lucent also recommend G.655 fibre. The channel spacing is also indicated, giving an idea of the optical bandwidth that is used for the transmission. We can see that the increase in the number of wavelengths has led to the reduction of the channel spacing from 200 GHz to 50 GHz (0.4 nm).

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λ

P r o d u c t n a m e

A v a ila b i l it y

C a p a c i ty ( G b i t/ s )

M a x . d i s t a n c e (k m )

n u m b e r (N )

C h a n n el s p a c i n g (n m )

A lc a te l 1 68 6W M *

y e s

4 0

6 4 0

1 6

1 ,6

8 0

B o sc h F le x P le x T M O -4

y e s

1 0

1 6 0

4

5 ?

4 0

C ie n a M u ltiw a v e 1 6 0 0

y e s

4 0

6 0 0

1 6

0 ,8

1 2 0

C ie n a M u ltiw a v e 4 0 0 0

y e s

1 0 0 / ( 2 4 0 )

4 0 0

4 0

0 ,4

1 0 0

y e s ?

1 0 ( to 4 0 )

5 0 0

4 ( to 1 6 )

_

E r ic ss o n E R IO N , A X D 8 W

y e s

2 0

6 0 0

8

1 ,6

_

E r ic ss o n E R IO N , A X D 1 6 W

2 n d S em . 9 8 ?

4 0

_

1 6

0 ,8 ?

_

L u c e n t / W a v e S ta r O L S 8 0 G

y e s

4 0

6 4 0

1 6

0 ,8

8 0

L u c e n t / W a v e S ta r O L S 4 0 0 G *

4 è Q . 98

2 0 0

6 4 0

8 0

0 ,4

_

yes?

8 0

4 0 0

3 2

_

8 0

P i r e l li T 3 1

yes

2 0

5 0 0

8

P i r e l li W a v eM u x

2 n d S em . 9 8 ?

8 0 / (160)

6 0 0

3 2 /(6 4 )

D S C C o m . D S C F o c u s W D M

N E C

S p e c tra lw a v e

(9 6 )

S p a n

7 0

(k m )

to

8 0 ?

1 0 0 0 ,8 / ( 0 ,4 )

1 2 0

* 10 Gbit/s allowed inputs

Table 3: Description of the main point-to-point WDM systems

The management of these systems is done using software interfaces which allow access to the system state (alarms, configuration,...), the OSC channel being used to transport the required data. Each terminal or amplifier site can be accessed either directly (craft terminal) or remotely. Today the supervision data of the WDM system (in the optical layer) are processed independently of those of the SDH (or SONET) equipment (in the SDH layer). The new generation of WDM systems propose evolution concerning management in order to have a global management of the two layers. For that purpose, Ciena transports a few overhead bytes of the SDH layer in the OSC channel. The protection is implemented at the multiplex level (MSP 1+1) but by duplication of the whole WDM system (terminals and amplifiers), the switching being done by the SDH interfaces (in the SDH layer). This is the case for all the equipment suppliers except Ericsson, which proposes a switching from the working transmission line to the protection one in the WDM terminal (in the optical layer). Moreover Ericsson has introduced the "Flexing Bus" concept which applies to a ring network topology with an unused section dedicated to the protection. It allows a rapid protection in the optical layer.


page 9 (42)


manufacturer

no. of channels available (plan)

optical ADM fixed available (plan)

Deliverable 1

optical ADM flexible available (plan)

OXC available (plan)

Alcatel

16 λ

(40 λ 4Q98)

- (?/40 4Q98)

- (?)

- (?)

Bosch

8λ

(16 λ 1Q99)

- (4/16 4Q99)

- (?)

- (16 x 16 ?)

Ciena

40 λ

(80 λ 4Q98)

8/16

(16/40 4Q98)

- (? 4Q99)

- (? 4Q99)

DSC

8λ

(32 λ 2Q99)

?/8

(?/32 4Q98)

- (?/32 2Q99)

- (?)

ECI

-

(8 λ 3Q98)

? (?)

- (?)

- (?)

Ericsson

16 λ

(32 λ 1Q99)

- (4/16 3Q98)

16/16

Fujitsu

8λ

(? λ ?)

? (?)

- (?)

- (?)

Lucent

16 λ

(80 λ 2Q99)

- (?/16 4Q98)

- (40/80 2Q99)

- (? 2Q99)

NEC

32 λ

(64 λ 4Q98)

? (?)

- (?)

- (8 x 8 ?)

Pirelli-Quante

32 λ

(64 λ 1Q99)

12/32

- (?/32 4Q99)

- (16 x 16 4Q00)

Siemens

8λ

(32 λ 3Q99)

- (32/32 3Q99)

- (?/32 4Q99)

- (16 x 16 4Q99)

Tellium

32 λ

(64 λ 1Q99)

4/16

(?)

(?)

(32/32 1Q99)

(64/64 3Q99)

- (?)

- (128 x 128 3Q99)

Table 4: Specifications of WDM systems presently available and under development

Table 4 presents data of available systems as well as future plans of various manufacturers concerning number of WDM channels, fixed and flexible OADM and OXC. The first entry in each column is the value characteristic for the systems as they are presently available from the corresponding manufacturer. The second entry in brackets in the column indicates the manufacturers future plans. The planned performance as well as the estimated time of realisation are written whenever possible. A minus sign indicates that the function is not available at present and a (?) indicates that manufacturer information is not available.

1.3

Identification of new/desirable optical functions In this part of the Deliverable, WDM optical functions considered either strictly necessary or just desirable for enabling or enhancing optical networking, are identified. The following functions were identified:

• • • •

OXC nodes

• • •

Optical 3R regeneration

Flexible OADMs Wavelength conversion Optical signal monitoring functions (QoS, optical spectrum, and Failure detection)

Wide band (80 nm) Optical Amplifiers Network survivability (protection, restoration)

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Deliverable 1

1.3.1

• •

Management functions

• •

OTDM


Dense WDM systems (64 and 128 λ)

Optical packet switching

Wavelength conversion Definition: to convert the optical frequency of an optical channel on a WDM comb from its original position in the input signal comb to another position in the output signal comb; to convert a full comb from its original position in the fibre window to another position in the same window . Application: necessary function in order to improve flexibility in the optical network by the use of wavelength domain switching on OXCs and OADMs. Allows optical spectrum dynamic allocation that can be used for re-routing and survivability. Function that could be used to implement transponder function. Function transparency desirable, regarding signal origin, format and bit rate - all-optical wavelength conversion. Some realisations provide 2R regeneration properties. Availability: from 2000 onwards

1.3.2

Optical signal monitoring functions (QoS, optical spectrum, and Failure detection) Definition: monitor the optical signal, in order to determine BER, frequency accuracy and stability, and detect failures. Integration with existing network management systems desirable Application: necessary function to assure QoS and activate survivability mechanisms in the network. Availability: from 2000 onwards

1.3.3

Optical 3R regeneration Definition: perform optically, Amplification, Reshaping and Retiming of the optical signal Application:

necessary function to overcome “opacity” of optical systems, allowing higher cascadability numbers.

Availability: from 2005 onwards

1.3.4

Network survivability (protection, restoration)

ensures network fast recovery from a state of failure, using optical layer mechanisms

Definition:

Application: necessary function to assure provision of service with QoS, by re-routing

traffic on the network via alternative available routes, either pre-allocated (survivability dedicated resources -protection) or dynamically allocated (restoration) Availability: protection: from 1998 onwards; restoration: from 2000 onwards


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1.3.5

Deliverable 1

Management functions Definition: allow management of optical network elements by an integrated network management system, via standardised management interfaces whenever possible. Application: management of optical network elements by existing integrated network management systems. Availability: from 2000 onwards

1.3.6

Optical time domain multiplexing OTDM (Long term function) Definition: optically multiplex digital optical signals in the time domain. This optical digital signal will be characterised by the optical wavelength, by technology (e.g. soliton, soliton +WDM), and by its bit rate. May be associated to WDM: N x OTDM signals on a N optical channel WDM signal. Application: increase the transmission bit rate beyond electronic limit (currently 40GHz). Allow optical packet transmission and switching. Needs improving of optical signal processing techniques (clock recovery, all-optical regeneration) and necessary components (short pulse optical sources, optical timing systems, OTDMOADMs, OTDM-WDM converts, optical memories) Availability: > 5 years

1.3.7

Optical packet switching (Long term function) Definition: performs dynamic routing (switching) of optical packets Application: enable very high-speed digital optical packet networks. Needs improvement of optical signal processing techniques like optical addressing, optical header procession... and components such as fast optical switches, optical memories... Availability: 5 years

1.4

Conclusion After a presentation of available optical functions and WDM point-to-point systems, a set of new optical functions considered either strictly necessary in order to enable the introduction/ migration to WDM future optical networks or desirable to enhance offered network functionality, was identified. A possible definition and the application context were analysed. Most of these functions have already been developed and tested either in laboratory or in field trials. The use of these functions can however not be considered independently of the architectural network context. The next Section proposes reference network architectures for the optical layer, where the above described optical functions could find their place.

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Deliverable 1

2


Assessment of optical network architectures In this section we provide an overview of classes of network architectures, from the simple topologies to more complex structures which potentially may be used in future optical networks. It points out most important network parameters, which characterise the WDM networks in terms of architecture, topology and survivability. The aim of this section is to select and propose complex optical network structures for further study of Task3 and Task4. Typical values of parameters, related to topology, physical limitations, are listed at the end of this section.

2.1

Complex topologies A network characterised by a complex topology is composed of sub-networks which are directly interconnected by sharing nodes. In such a network topology the aim is to optimise the capacity of the network by mixing the different types of traffic on as few network elements as possible. The sub-networks will have basic topologies.

2.1.1

Connected rings The combined advantages of good protection performance, low cabling costs and efficient use of the network elements can be achieved by a network structure composed of connected rings.

2.1.2

Meshed domains interconnected by a ring trunk The ring trunk network gives excellent protection capabilities with a minimum of interconnections. However, the requirements of transport capacity between neighbouring nodes are high compared to the mesh, and therefore the trunk ring is mainly advantageous in areas where the cost of installing cables is high.

2.1.3

Ring domains interconnected by a meshed trunk The meshed trunk network has the advantage of providing excellent node-to-node physical connectivity and, thereby, provides many alternative routes for traffic. The traffic on the cables in a meshed network can, to a large extent, be considered pointto-point and, therefore, the requirements to the transmission capacity on the links are easy to predict.

2.2

Characteristic parameters The purpose of this section is to present and discuss various parameters to be taken into account to allow the future routing and dimensioning of the optical networks. Techniques and principles commonly applicable to existing networks do not necessarily apply to the dimensioning of networks based on wavelength routing, and this justifies the need for a specific analysis of network requirements.


page 13 (42)


2.2.1

Deliverable 1

Specific characteristics of optical network

•

High capacity: the core network has to deal with many applications and services envisaged for the future, which will probably have different bandwidths. The overall traffic volume is expected to be large and to increase as applications and services become cheaper and easier to use. Hence, the network has to have a large capacity and to be able to handle the granularity of optical channels.

•

Transparency: in order to take into account most of the assets of optical functions and to reduce the complexity of equipment, the signal should not be converted to the electrical domain wherever it is possible. Several levels of transparency could be specified such as signal format, bit-rate, transfer mode and service. Full transparency usually does not exist, as physical constraints always cause transparency limitations.

•

Flexibility : refers to the ability of the network to accommodate changes in traffic patterns. This could be easier in optical networks since the granularity of handled signals is higher.

•

Connectivity is the network ability to establish connections independently of the actual state of the network. Full connectivity means that any connection between any two points of the network can be established at any time. In optical networks the availability of wavelengths, OXC blocking or limited number of OADM wavelengths are the main barriers to full optical connectivity.

•

Scaleability: is the possibility of capacity or functionality upgrade of a network by adding new facilities in uniform steps. In optical networks the gradual increase of available wavelengths without changing the whole WDM terminal is a key scaleability feature.

The relationship between characteristic parameters of different network layers and other network related requirements is shown in figure 4. During a network planning process topological, architectural and general requirements are to be considered and fulfilled.

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Deliverable 1


Input data

Traffic demands Network planning

Geographical

Topological

topology

characterisics

Basic

Architectural

network structures

characteristics

Service

Functional

Network characteristics availability

layer

requirements

characteristics

Capacity Transparency Connectivity Fexibility

Transmission performances

Granularity Scalability

Functional layer characteristics

Available equipment

Network implementation

hw,sw Network engineering Operation and Maintenance

Figure 4: relationships of network characteristics and network requirements

2.2.2

Parameters related to topology The definition of network design rules for WDM optical networks, the evaluation of topology optimisation algorithms and network planning tools have to be done using numerical simulations. However, the demonstration of general results can be achieved in some cases, which provide simple design rules that could be used for guidelines in network planning. Some of the most important network parameters are:

•

The number of nodes (N), a node being either a source of traffic (optical channel) or a pure transit node

•

The node degree (D), defined as the mean number of nodes directly (i.e. without any transit) connected to a node via one or more fibres.

• • • •

The link length (LF) normalised to the node spacing. The number of fibre per link (F) The shape of the network (to be defined ) The network density


page 15 (42)


Deliverable 1

From those parameters, it has been demonstrated [3] that a good estimate of W, number of wavelengths required to meet the traffic demand (T being the number of channels per connection) can be expressed as :

W ≈

1

 2 1 − 2 

N

32

T

FDLF   N  π

However, those parameters may not be fully sufficient for describing a network, since topological particularities can be noticed.

D = d The network density d is defined by the formula: N − 1 . This parameter reflects the depth of the mesh in the network. Given a fixed number of nodes in the network, extreme values are obtained with full mesh (d=1) and ring (d=2/(n-1)). The figure below illustrates various situations for network topologies with a given link density.

Full mesh

Mesh

Ring

Mesh

D = 4; N = 5

D = 3; N = 6

D = 2; N = 6

D = 2.5; N = 8

d=1

d = 0.6

d = 0.4

d = 0.357

Figure 5: Examples of various graph densities

2.2.3

Parameters related to physical limitations The quality of the signal across the network dictates engineering rules for network planning. In an all-optical network, the transmitted data remain as optical signals all along the path in the optical layer. However, each path cannot be fully considered as a point-to-point WDM link because : signals on different paths may travel through a different number of optical devices, the number of wavelengths and signal characteristics on fibres can differ with links. The physical limitations lead to degradation of signal quality through cross-talk, signal distortion and noise accumulation. The design of optical cross-connects and the definition of the architecture of the nodes should ensure the nodes provide the necessary functionality. However, there is the issue of how many nodes can be cascaded along one optical path while keeping the signal quality to an acceptable level. As long as the regeneration of the signal has to be performed by the electrical layer, a limitation on the optical path length should be addressed as well. Another issue is related to the limited number of wavelengths per fibre due to cross-talk problems and limited amplifier bandwidth. The problem can be smoothed by wavelength re-use, which complicates the wavelength allocation and routing problem. In summary, the following parameters should be considered, which are closely related to the signal quality:

•

the average and maximum link length in the network

page 16 (42)


Deliverable 1

• • • • • • • 2.2.4


the maximum number of optical nodes that can be crossed without regeneration the maximum optical path length allowed without any regeneration the number of wavelength conversions along the path the number of wavelengths, optical channels, per fibre being used BER Degradation and system bandwidth optical channel individual integrity (Optical power level, Wavelength stability...) reconfiguration time (re-routing and switching time)

Parameters related to demands The demand matrix (or matrices) has an important role in the planning process, because it can affect the selection of the architecture, the grooming and routing policy, the transport characteristics of the optical layer, and so on. In principle the demand matrix for the optical layer should be expressed in wavelengths, but often it is expressed in the typical unit of the client layer that leaves to the optical layer planner a higher degree of freedom in the optical layer design . Demand distribution The distribution of demands among optical nodes is an important characteristic of the network. Different distributions can lead the planner towards different network architectures and allow him to use different methodologies and algorithms in the planning process. Both single wavelength and multi-wavelength demands should be addressed. Optical channel granularity The design of present transport networks is based on static traffic conditions. A large set of objectives can be taken into account in the optimisation. Entries, generally, are given in Mbit/s. The granularity, or channel capacity, defines the correspondence between the demand matrix in the optical layer and the wavelengths matrix. The choice of a granularity iss of courses dictated by technology. Most of multiwavelength systems recently available use STM-16 granularity. However, announcements were made towards STM-64 granularity, allowing mixed granularity on the same fibre. Capacity per link The physical limitations lead to a limit in the number of optical channels per fibre. The transmitted power per wavelength must be large enough to provide an acceptable Signal to Noise Ratio at the receiver. However, it is not possible to increase signal power since optical amplifier gain may saturate and non-linear effects like four-wave mixing will degrade signal transmission performance. Thus, there is a compromise between the capacity per link, the distance between amplifiers and the number of cascaded amplifiers (link length). Demand grooming and consolidation Depending on the network services offered, various kinds of traffic demands can be carried in the optical layer : voice, video signals, data, leased lines... The bit-rate allocated to each kind of signal may be different. The planning process also includes the grooming and consolidation of those signals towards the optical layer. Different


page 17 (42)


Deliverable 1

grooming alternatives can be taken into account, mainly based on the demand distribution, their requirements and the target value of utilisation of the optical layer. For that purpose, grooming optimisation should also take into account the impact in the optical layer. O p t ic a l l a y e r Grooming Demands (lower level)

Figure 6: Grooming process

2.2.5

Parameters related to architecture Flat or hierarchical networks A single layer network can be structured in two different ways: flat or hierarchical. A flat network does not impose any constraints on the demand routing, while a hierarchical one allows, in principle, a communication between two peer hierarchical nodes through one or more nodes of a highest hierarchy. Very often traffic in large telecommunication networks is allocated to hierarchical levels or tiers. Two, three or, in some special cases, four level networks are designed. The main advantages of traffic hierarchisation are the clear routing rules and easy manageability. However, sometimes this results in longer paths for individual demands. In the near future, alloptical networks with two hierarchical levels will probably be common for core network applications. Many European projects considered a two-level all-optical network, which included an upper level meshed configuration based on OXCs, and a lower level based on WDM rings. Number of sub-networks on each hierarchical level Both a flat and a hierarchical network can be split into several sub-networks. Generally a sub-network is defined by the node connectivities and by the independence of its own survivability mechanism (that is the reason why these subnetworks are often called Survivable Sub-Networks). Generally, in a hierarchical network, interconnections of sub-networks in the same network level are allowed only via the next higher hierarchical level. The number of sub-networks on a network level is not limited, and it depends only on the actual network size. Types of sub-networks A sub-network can be any network partition, but usually consists of some kind of basic network topology, like rings or small meshes. Candidates for sub-networks are in optical networks WDM rings and optical meshes. Number of transiting nodes (hubs) per sub-network Unless the demand distribution does not need to cross more than one sub-network, each sub-network must have at least one special node where the outgoing traffic is transited. These nodes (often called hub nodes) have special functionalities for interworking with other sub-networks.

page 18 (42)


Deliverable 1

2.2.6


Parameters related to the survivability approach A protection or a restoration mechanism (or both) is generally applied to a network in order to increase the demand survivability against failures. The recovery mechanism often adds some new constraints to the network planning activity such as restoration time, length of paths, routing ... These constraint will be addressed in the Deliverable D2.

2.3

Selection of reference network architectures Having studied network topologies we now have to select the reference network architectures that will be implemented in those topologies. In practice the PNO’s core

network is divided into hierarchical levels from the traffic routing point-of-view. This hierarchisation can be naturally translated to appropriately interconnected topological domains, resulting in complex network topologies. The different domains can then be implemented using several network architectures. In the following we investigate those kinds of hierarchical optical architectures which consist of the basic optical network domains like:

• • •

CS-Rings, OMS-SP Rings OXC-based optical mesh

The reasons for choosing this particular set of network architectures were the following: -

the CS-Ring was considered by the previous EURESCOM Project P615 as a good first step in introducing optical functionalities in SDH networks since this architecture combines SDH functionalities of existing equipment (routing and linear MS protection) with optical routing for logical node ordering.

-

the OMS-SP Ring is a very advanced full-optical architecture, where both routing and protection are implemented optically. Therefore, it is expected that the OMSSP Ring will bring specific problems that must be taken into account in the planning of optical networks.

-

the OXC-based mesh is seen as an advanced optical architecture, where the introduction of optical routing and restoration might contribute to simplify the complexity of the electronic equipment in high-capacity mesh networks.

The hierarchical network configurations selected are seen as interesting possibilities for the partitioning of real networks in more manageable domains. These configurations are expected to be useful in different network scenarios and will, possibly, be deployed at different stages in time, in the evolution from existing networks to networks based on the new optical network architectures. In Table 5 possible combinations of these domains are collected. The investigation is limited to two-level architectures only because these architectures can be considered to be a realistic solution for core networks. Complex optical network architectures consisting of these domains will have a performance dependent on the different protection schemes and re-routing strategies applied over individual sub-networks. Only the dual node interconnecting architectures are selected because in large capacity core networks the disjoint alternative routing is an essential requirement. In


page 19 (42)


Deliverable 1

the following paragraph, we will provide a qualitative comparison since more precise results on network planning for the various architectures proposed here should be provided in Deliverable D3. Upper level

CS-Ring

OMS-SP Ring

OMS-SP Ring

Optical mesh

Lower level

CS-Ring

OMS-SP Ring

Optical Mesh

OMS-SP Ring

Selected architecture

1

2

4

3

Study

P615

P709

P709

P709

Table 5: Selected reference architectures for comparison of two-level optical networks

2.3.1

Two-level CS-Ring architecture A two or more level (tier) CS-Ring architecture consists of hierarchically interconnected CS-rings. Interconnection and cross-connections are carried out at the SDH client layer. The main advantages of this architecture are: Wavelength allocation can be planned for each ring independently. Equipment is available now.

Figure 7: Two-level CS-Ring architecture

2.3.2

Two-level OMS-SP Ring architecture The main advantage of this architecture is the optical connectivity between the rings. In the hubs the optical OCH level flexibility depends on the optical cross-connect capability of the applied OADMs. First generation OADMs only have fixed add/drop capability. In this case wavelength conversion could be necessary for the interconnections.

page 20 (42)


Deliverable 1


Figure 8: Two-level OMS-SP Ring architecture

2.3.3

Two-level mesh-ring architecture This is probably the most promising architecture for the future. Traffic in lower capacity rings is collected and transported by the very high capacity upper level, like in a traditional SDH network. In case of large network sizes OXCs probably have to have wavelength conversion functionality in order to establish large numbers of optical paths. Further study is necessary in order to develop suitable optical protection and restoration mechanisms for this complex architecture. Analogously to similar SDH network architectures, the Optical Sub-network Connection Protection (OSNCP) (1+1 optical path protection) can be a candidate for this purpose.

Figure 9: Two-level Optical Mesh-Optical Ring architecture


page 21 (42)


2.3.4

Deliverable 1

Two-level ring-mesh architecture In some special cases this architecture could be an optimal solution. In this case a very high capacity optical ring is necessary for interconnection of meshed sub-networks on the lower level. The protection and restoration problems are similar to the mesh-ring architecture.

Figure 10: Two-level Optical Ring-Optical Mesh architecture

2.3.5

Characteristics of the selected optical network architectures The following table characterises the reference networks in terms of some of the parameters discussed above. Reference network architectures Characteristic Parameters

CS-Ring CS-Ring

OMS-SPRingOMS-SPRing

OMS Mesh - OMSSPRing-Mesh SPRing

Selected architecture

1

2

4

3

transparency

no

limited

yes

yes

connectivity

full SDH

limited optical

optical

optical (OC)

(VC-4/3/12) (OC)

(OC)

(VC-4-3-12)

restoration on optical layer

no

yes (limited)

yes (limited)

yes

flexibility

SDX DXC

limited

good

good

granularity

VC-4

wavelength

wavelength

wavelength

scaleability

low

low

good

Hub equipment

SDXC

OADM

OXC

OXC (SDXC)

No. Of hierarchical levels

2(3...)

2(3...)

2

2

Flexibility on optical layer

no

limited

limited

yes

Types of subnetworks

rings

rings

rings/OXCs/ links

rings/OXC/ links

by 2000

by 2000

after 2000

General

Architectural

Equipment availability now

Table 6: Some characteristic parameters of the reference network architectures

page 22 (42)


Deliverable 1

2.4


Identification of physical network parameters limitation In this part of section 2, an attempt is made to identify physical network parameters limitation. By this, we mean the limitations of real existing optical components and subsystems, on realising complete aimed functionality, and its impact on optical networking. These limitations can be grouped into two sets, regarding its origin: intrinsic limitations and technological immaturity limitations. The first set comprises the limitations imposed by the optical nature of used technology, which in spite of the degree of perfection of used components or systems, cannot be eliminated (theoretical limits). The second set groups the limitations mainly due technological immaturity, which result in a non-ideal behaviour or the components; these limitations are expected to be greatly reduced with the improvement of technological aspects.

2.4.1

Identification of mechanisms originating limitations [4, 5] Some effects, their respective causes and originated limitations are listed below:

2.4.1.1

Optical channel individual integrity Crosstalk Crosstalk in WDM systems arises due to filter/Demux imperfections and due to fibre non-linear effects. Both can be kept small enough by appropriate system design so that no significant penalties are expected. The following types of induced crosstalk were identified: Non-linear crosstalk : due to non-linear effects in the fibre; Four Wave Mixing FWM is the most relevant non-linear mechanism in the networks considered.

FWM generated cross-talk: depending on the optical power, the wavelength spacing of optical channels, on the fibre dispersion values, and on the transmission distance, it is a limitation on the number of channels for HD-WDM, as it becomes higher for channels more closely spaced and for lower dispersion values. SRS generated crosstalk: depending on the optical power, the wavelength spacing of optical channels, and on the transmission distance, it is a limitation for the number of optical channels and the range of the system. Linear crosstalk : due to non-perfection of components such as filters and switches; can limit cascadability. Two types of linear crosstalk: inter-band crosstalk and intraband crosstalk. Inter-band crosstalk is induced by other wavelength optical channels due to imperfect filtering. Intra-band crosstalk is due to the presence of residual levels of optical power in other wavelengths of the used comb (non perfect filtering), which will add to the signal in those wavelengths. Thus it is a consequence of switching. Crosstalk induced by this process cannot afterwards be removed as it has the same wavelength of the optical signal to which it was added. This constitutes therefore a limitation to cascadability of nodes with crossconnect/ switching functionality. BER degradation:

Physical mechanisms that produce pulse broadening with travelled distance, lead to BER degradation, and limit system bandwidth and range. The higher the transmitted bit rate the higher importance these effects have.


page 23 (42)


Deliverable 1

Dispersion: the first cause of pulse broadening and consequent BER penalty, is fibre chromatic dispersion coefficient. This is due to the dependence of propagating rd velocity on the wavelength. For SMF, in the 3 window, even for narrow linewidth optical sources this effect becomes important. PMD: Polarisation Mode Dispersion, results from the fact that due to non-perfect geometry of the fibre and induced mechanical stress, the fibre presents birefringence and therefore the two polarisation states have slightly different propagating velocities. This Birefringence varies randomly along the fibre. Special fibres as DCF and EDF (used in EDFAs) present higher values for PMD than SMF. So for long links using DCF and EDFAs the total PMD can result in significant BER degradation. SPM : this is a non-linear mechanism, due to the dependence of fibre refractive index on the optical propagating field intensity. Intensity modulation results into refractive index modulation and therefore phase (and frequency) modulation of the optical signal occurs. This causes linewidth broadening and thus BER degradation.

2.4.1.2

Network element cascadability Optical Amplifier OA Optical amplifier cascadability limitation due to optical amplifier generated noise; this noise accumulates with the number of transverse optical amplifiers therefore limiting cascadability of OAs. Optical amplifier cascadability limitation due to non-flatness of optical amplifier gains, which causes different amplification of individual optical channel power. In case of a change in the number of channels passing through the OA, power transients may cause degradation. OXCs, optical Switches Cascadability limited due to optical power losses, intra-band linear crosstalk and bandwidth reduction by filtering. Optical Mux/Demux Cascadability limited due to optical power losses and bandwidth reduction by filtering

2.4.2

Identification of systems/components which introduce limitations Various elements contribute to the physical limitations mentioned above. A relation between the physical nature of the introduced degradation, depending on the component, and the impact on the optical transmission is established in the following list. Optical Fibre Crosstalk: Non-linear effects, in particular FWM System bandwidth limitation: Dispersion BER degradation: Attenuation, Non-linear effects, in particular FWM OXC, Optical Switches BER degradation: Insertion losses Linear crosstalk: optical filtering, optical switching System bandwidth narrowing: optical filtering; filter misalignment; wavelength instability

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Optical Filters , Optical Mux/demux BER degradation: Insertion losses Linear crosstalk: non-ideal optical filtering System bandwidth narrowing: optical filtering; wavelength instability Optical Sources System bandwidth narrowing: Line width, Wavelength accuracy, Wavelength stability, Wavelength tuning accuracy (if tuneable sources) BER degradation: optical power efficiency coupling Optical inline amplifiers BER degradation: amplified spontaneous emission noise BER degradation of less amplified channels: Different channel power

amplification due to gain curve (non–flat) Limited cascadability: build-up of optical signal degradation as a result of above mentioned mechanisms, with number of transverse elements. 2.4.3

Processes to overcome limitations at present and solve them in the future

Most of these limiting mechanisms arise from the use of high optical power levels and WDM signals; i.e., already known and existing, their influence only became important in the present context. Some of them cannot be eliminated (however they can be compensated) such as dispersion effect, attenuation, non-linear effects. Others, with techological improvements on the manufacturing processes of the components, can be greatly reduced, almost eliminated, such as linear crosstalk due to filtering and switching, maintaining system bandwidth, by improving wavelength accuracy, stability, tuning, filtering and misalignment etc.

2.5

Identification of Ranges of values In this point, the number of parameters related to the layers of the optical channel, the optical multiplex section and the optical transport section in WDM networks are specified. These values are necessary in the planning process of such networks. Optical layer performance of today’s commercially available WDM systems for terrestrial optical networks, however, still keeps improving. In order to take into account the expected technical progress of optical WDM transmission systems as much as possible there are up to three values specified for various parameters. The first of these values represents presently available commercial WDM systems. The second parameter value has been derived from manufacturers’ publications or advertisements regarding product development in the near future. Whenever possible a third value is presented that reflects results from laboratory experiments or theoretical calculations and which may indicate what will be achieved in future systems.


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2.5.1

Deliverable 1

Functional layer characteristics According to Rec. G.otn an optical network will consist of the following three optical layers:

• • •

Optical Channel (OCH) layer Optical Multiplex Section (OMS) layer Optical Transmission Section (OTS) layer

These functional layers can be characterised in terms of the following parameters: Layer OCH

Characteristics Transparency Maximum number of cascaded optical nodes Optical cross-connect capacity Optical protection OMS Number of available wavelengths Wavelength conversion Optical protection OTS Power budget Dispersion budget Accumulated noise Cross-talk Number of cascaded optical amplifiers Table 7: Characteristics of optical layers

2.5.2

Ranges of values Values reflecting limitations of present implementation of optical functionality and its impact on optical networking, either obtained by calculation, simulation or experimentation, will be presented in this section. Power budget The achievable power budget depends mainly on the optical output power of the EDFAs and on the number of wavelengths used in the network as well as on the maximum number of EDFAs cascaded in any transparent link, which may happen to come into existence due to optical switching, configuration, or restoration processes within the network. Power budget of present systems is limited to a value of 30 dB of attenuation between EDFAs. This value is optimised with respect to a dispersion limited system (no dispersion accommodation) at a bit rate of 2.5 Gbit/s on standard single mode fibre resulting in a total link length of 500 km to 600 km which corresponds to 150 dB of optical power budget. If a larger transparent total link length is desired the power budget between neighbouring EDFAs must be reduced. Dispersion accommodation necessary in this case introduces additional optical attenuation. Dispersion budget If standard single mode fibres (G.652) are to be used without dispersion accommodation (DA) techniques the dispersion budget is limited to values of about 12000 ps/nm for 2.5Gbit/s transmission. 10 Gbit/s transmission without DA is too

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severely limited in length to be attractive. With DA methods the limitations in dispersion budget are relaxed considerably. A values of 75000 ps/nm is experimentally estimated. Accumulated noise The amount of accumulated noise depends on the system design. With a transmission link design, using a shorter span length, that takes into account the network requirements the accumulated noise effects can be kept small enough so that no significant penalties are to be expected Number of cascaded OAs Available WDM point-to-point systems are designed to use a maximum number of up to 6 EDFAs in cascade as outlined above. It is possible to increase this number considerably without running into ASE noise accumulation problems if the optical budget between succeeding EDFAs is reduced. Transmission of 8x5 Gbit/s over a total distance of 4500 km of standard single mode fibre using dispersion accommodation technique has been demonstrated. With dispersion shifted fibre 20x5 Gbit/s over 9100 km have been achieved. With soliton techniques transmission of 8x10 Gbit/s over 10000 km was shown using more than 300 cascaded EDFAs. Parameters values for OCH, OMS and OTS sections are summarised in table 8. parameter

available announced performance performance

Optical channel (OCH) layer characteristics transparency number of optical nodes in cascade number of wavelength conversions Optical Multiplex Section (OMS) layer

number of available wavelengths optical cross-connect capacity protection methods supervision channel Optical Transmission Section (OTS) layer power budget (for link between consecutive EDFAs) in EDFA cascade number of cascaded EDFAs (2.5Gbit/s client)

6-7 1-2

5-10

16-32 ? ?

80-100 16x16 ? ?

<200 128x128 ? ?

<30dB

<30dB

<30dB

4-6

Min. 30 at reduced power budget of 20 dB, 300 possible Min. 15 at reduced power budget of 20 dB, 300 possible

number of cascaded EDFAs (10Gbit/s client) 4-6 G.652 dispersion budget (2.5Gbit/s client) dispersion accommodation (DA) dispersion budget (10Gbit/s client) accumulated noise cross-talk Inter-band Intra-band

limits

no 12000 ps/nm DA req. ?

DA req.

DA req.

25 25

35 30

50 35

Table 8: Parameter values for OCH, OMS and OTS layers


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2.6

Deliverable 1

Conclusions In this Section, network configurations have been proposed, consisting of interconnected basic topologies, either in flat or hierarchical arrangements. Four reference network architectures have been selected, which will be considered further in Task 3 and Task 4 of the Project. Various parameters have been presented, related to topology, physical limitations, demands architecture and survivability. Their relationship with the network planning and network engineering processes has been mentioned. A particular attempt has been made to identify physical parameters limitation and ranges of value for power and dispersion budget, accumulated noise and number of cascaded optical amplifiers. However the application of the described architecture and the study of network planning problems should also consider the client layers. The next Section is dedicated to the potential of the optical layer for routing various formats, which could be promising, when considering the actual growth of different client signals such as ATM and IP.

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Deliverable 1

3


Potential of WDM routing for different client signals The objective of this part is to describe the various client signals, which could be carried in the optical networks. In order to pinpoint the possibilities to plan an optical network using a non SDH client signal, the requirements of client layers is analysed. Recent development of ATM and IP physical interfaces could have an impact on the future optical network design. After a brief investigation on ATM and IP client signals performance and functionalities, multi-layer network configurations are proposed using IP, ATM, SDH and WDM characteristics. A first evaluation of such configurations is discussed.

3.1

ATM client signal

3.1.1

ATM Network functionalities and physical layer ATM is a connection oriented technology with end-to-end QoS supporting multiple service. Main advantage of ATM are:

• • • • 3.1.2

fast switching high-speed interfaces efficient use of bandwidth through statistical multiplexing at VP and VC levels ATM network functions are : VP or VP-VC switch, VP or VC Cross-Connect and ATM Multiplexer

ATM Services An ATM Service Category is intended to represent a class of ATM connections that have homogeneous characteristics in terms of traffic pattern, QoS requirements and possible use of control mechanisms, making it suitable for a given type of resource allocation. A first classification of these services/capabilities may be seen from a network resource allocation viewpoint. We can identify (ATM Forum), Constant Bit Rate (CBR), Variable Bit Rate (VBR), Available Bit Rate (ABR) and Unspecified Bit Rate (UBR) categories.

3.1.3

ATM Performance Parameters The ATM performance parameters are defined in ITU-T recommendation I.356, and they are:

• • • • •

Cell Error Ratio (CER) Cell Loss Ratio (CLR) Cell Mis-insertion Rate (CMR) Mean Cell Transfer Delay (MCTD) Cell Delay Variation (CDV).

It is necessary to identify the network degradation sources that have a direct or indirect impact on the ATM performance parameters. These sources have a global


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Deliverable 1

effect on all services that are supported by the network. Table 9 shows the impact of the network degradation sources on the ATM performance parameters. Item

CER

CLR

CMR

CDV

ä

Propagation delay Statistical physical errors

MCTD

ä

ä

ä

Switching architecture

ä

ä

ä

Buffers capacity

ä

ä

ä

ä

ä

ä

ä

Traffic load

ä

ä

ä

ä

Resources allocation

ä

ä

ä

Number of nodes between ends

ä

ä

Faults

ä

Table 9: Impact of degradation sources on the ATM parameters Transmission degradation sources have been highlighted

The referred network degradation sources can be grouped into two main areas: ATM degradation sources and Transmission degradation sources. The ATM degradation sources are directly dependent on the ATM technology and equipment. The transmission degradation sources are directly dependent on the physical medium or technology used to transport an ATM signal. In this section only the transmission degradation sources will be analysed. They are: Propagation delay, Statistical physical errors and Faults.

3.2

IP client signal

3.2.1

Internet network layers and services The Internet offers global connectivity for data communications between different computers networks. As shown in Figure 11, the Internet is based on a four-layer protocol. The lowest layer is called the link layer which contains protocols for LANs and sub-network communications such as Serial Line IP and Point-to-point Protocol PPP [6], [7] and [8].

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Application layer SMTP, FTP, SNMP, HTTP, TELNET, MIME, NNTP Transport Layer TCP, UDP Internet layer IPv4, IPv6 IP ng

• • • • • • •

SMTP :Simple Mail Transport Protocol FTP : File Transport Protocol SNMP: Simple Network Management Protocol HTTP : Hypertext Transfer Protocol TELNET : Remote Terminal Protocol NNTP : Net News Transfer Protocol MIME: Multipurpose Internet Mail Extensions

Link layer LAN-Ethernet WAN-PPP X.25, ISDN, Frame Relay ..

Figure 11: IP network model and services PPP was designed as a standard method of transmitting IP datagrams over point-topoint links. Initial deployment intended to govern point-to-point link over short local lines, leased lines, ISDN, and old telephone service POTS using standard dial-up modems. The second layer is the Internet Layer (IP-v4/v6 RTP, RSVP...) which provides routing and relaying between LANs and sub-networks. The next layer, TCP-UDP transport layer, controls end-to-end communication links between different Internet systems. TCP is used to provide a reliable transportation using end-to-end control. It is also includes error control detection and recovery for the transmitted packets within the Internet network. For short message applications another protocol, the User Datagram Protocol (UDP), is used. UDP is also connectionless oriented but without any overhead for establishing end-to-end control. Finally, application layer provides Internet applications like e-mail, WWW, FTP, Telenet.

3.2.2

IP protocols: IP v4/v6, RTP and RSVP IPv4 defines datagram format, addressing routing and error reporting. It supports 32 bit address space. IPv4 can only offer connectionless, Best Effort packet delivery and does not support end-to-end QoS. IPv6 introduces a new modular datagram format with the following advantage :

•

Flow label: the header includes a flow label. Flow values can be assigned to particular streams of traffic with special QoS requirements.

• •

Large network addressing: IPv6 supports 128 bit network addresses. Security: IPv6 requires and supports both authentication and confidentiality.

Real Time Protocol RTP is designed especially to support a new type of real time traffic. The RTP architecture, called application layer framing, replaces TCP with a simple framework for a direct application. For example, audio algorithms can tolerate missing data much better than can lengthy delays. Instead of introducing delays with re-transmissions, RTP applications prefer the transport layer to simply forget about missing data. Resource reservation protocol RSVP is a signalling protocol for IP allowing applications to make network resource reservations for data flow. It can be used for multi-cast application in IP network. The deployment of RSVP in IP network can support end-to-end QoS.


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3.3

Deliverable 1

Network configurations required by ATM/IP client signals First, the main Optical network functionalities considered are: -Optical Multiplexing / De-multiplexing OMux/Demux -Optical Cross Connect OXC -Optical Add/Drop Multiplexing OADM -Optical Protection Switching OPS

IP v4/v6

•Routin TM VC/VP

• Switchin • XC WDM

SDH LOP/HOP

• XC • ADM • APS

•OXC •OADM •OPS Figure 12: IP / ATM / SDH & WDM network functionalities

Figure 12 illustrates network functionalities for WDM optical transport network as well as for SDH, ATM and IP network. It is clear from this figure that the main routing and switching functions are for IP and ATM respectively, but crossconnecting; Add/Drop and protection functions can be obtained using both SDH and WDM technologies. The main difference between SDH and WDM remains the processing of SDH in time domain and for WDM in frequency domain. Taking into account the network functionalities of the different client signals the following layering structure is proposed for the 2003-2005 period (Figure 13). Different configurations of network architecture are proposed in the next part of the document. Inter-working of IP, ATM, SDH and WDM will be discussed later.

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IP v4/v6 PPP(HDLC) ATM VC / VP (Cell Based) ?

SDH HOP/LOP

PDH

WDM Optical Network OMS/OCH Figure 13: IP/ATM/SDH and WDM network mapping for 2003-2005

Based on the above network functionalities, 5 configurations were selected to study the impact of an ATM or IP client signal on the planning of an optical network. The configurations are the following :

• • • • •

ATM over SDH over WDM ATM over WDM IP over ATM over SDH over WDM IP over SDH over WDM IP over WDM

For each configuration 2 cases are presented, according to the implementation of protection and routing in the WDM optical network.

3.4

Impact of non SDH client signal on the planning of an optical network

3.4.1

ATM over SDH over WDM : SDH protection vs. WDM protection Under normal operation only two degradation sources are active: the propagation delay and the statistical physical errors. In what concerns the propagation delay, this is constant and will not vary unless one fault occurs. The statistical physical errors are directly related to the physical medium and technology used. In the WDM case, lower values for the BER (less than 10E-10) are expected. Another case is the situation when a connection problem occurs. The problems could be grouped into three main areas:

• • •

connection degradation (problems in the transmission cable) connection breaks (cable cut-off) terminal equipment problems (faults).

In the case of the connection degradation and soft terminal equipment problems, several ATM parameters will be affected, and in that way the several services


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supported by the connection. Depending on the degradation factor the services could be smoothly or strongly affected. This depends on the CLR and CER values and their impact on the services. There are services where low CLR and CER are required and others where higher values, of the order of 10E-3, can be tolerated for some period of time. One example of each is the video and the voice service, respectively. Connection breaks and strong terminal equipment problems will severely affect the connections and render the normal way of signal transmission impossible. In these situations the ATM services supported will be affected. The connections will be lost. Within a fault condition several situations could occur. Normally the re-routing and protection schemes of the network will be activated in order to establish one alternative connection between affected end points. The services and the ATM performance parameters will be affected. Several ATM cells are lost and others will be delayed. Another MCTD is established. After the establishment of the alternative connection we will have a situation similar to the normal operation. If the fault affects one end of the system, in what concerns the ATM service, the service will be affected permanently until the fault is cleared. So, taking into account what was expressed before, the re-routing or protection scheme used in the transmission technology, such as the WDM technology, will have a strong impact on the ATM performance. Several protection schemes should be studied in order to minimise the impact on the ATM performance, in case of WDM link failure. This configuration study should be performed taking into account two different scenarios. The first one considers the WDM as a means of transmission and all the protection is provided by the SDH structure. The second considers that all the protection is provided by the WDM. The SDH is used as traffic encapsulation. The influence of such scenarios should be analysed taking into account the ATM signal characteristics. For that, the most critical ATM performance parameters CER, CLR, CTD and CDV are considered. In order to perform this study higher level signals such as video and audio signals in interactive applications, are considered. From the user point of view these are the most critical ones. Data applications, such as file transfer, can be affected by the degradation of the transmission signal for some milliseconds. The user does not sense this degradation or problem most of the time. One of the most critical points is the switching time or protection action when a

failure occurs. As a reference the «APS protocols for OMS SP Ring network» document issued by the EURESCOM P615 Project is used. The main results are summarised in table 10. Example

MS-SPRing

OMS-SPRing

Node failure

4.325 ms

7.325 ms

Bi-directional signal fail

5.190 ms

8.190 ms

Unidirectional signal fail

5.810 ms

8.810 ms

Unidirectional signal degrade

7.655 ms

13.655 ms

Table 10: Protection switching times for 7-node rings with 51 km-long spans

In the above table the worst cases are used. This means that for SDH and WDM times of 8ms and 14ms will be used respectively as reference times for protection actions. Considering higher level signals, such as video signals, their QoS requirements can be mapped into ATM performance parameters as shown in table 11.

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ATM Performance Parameters

Value for High video quality

Value for Low video quality

CTD

4 ms (99% of time)

400 ms

CDV

500 µs (99% of time)

130 ms

CLR

< 1.7 E-9

< 3 E-7

CER

---

< 4 E-6

Table 11: Video performance parameters Also for video signals one question to be considered is the number of transmitted frames. For normal transmission rates one frame is transmitted each 33 ms. 3.4.1.1

Protection at SDH level According to table 10 the protection scheme takes about 8 ms to reroute the traffic. Each VC-4 takes 125 µs at 155 Mbps (STM-1). So dividing 8 ms (the rerouting time)

by 125 µs (the time necessary to transfer one VC-4) we get 64. In each VC-4 we can transfer 44151 cells. So, multiplying 44151 by 64 we get 2826 cells. Concerning ATM at full SDH link speed, 2826 cells are lost. 3.4.1.2

Protection at WDM level

According to Table 10 the protection scheme takes about 14ms to re-route the traffic. This means that with STM-1 link 112 VC-4 are lost. Concerning ATM at full SDH link speed, 4945cells are lost. The several ATM services supported by the SDH link will be affected. The impact on the higher level service (such as video-conference) depends on fail recovery mechanisms or applications used, as well as on the amount of information lost. As an example, for a video-conference signal transmitted at 30 frames/s a fail will affect one or two frames. The loss of one or two frames does not impact video services. In conclusion, the impact of SDH on the performance is not critical for this service. 3.4.2

Configuration ATM over WDM

The recent standardisation of ATM cell based interface leads to a direct connection between the ATM VP/VC and the Optical Channel OCH of the WDM optical network. At present, ATM cell based is proposed for UNI interfaces with 155, 622 Mbit/s bit rate for the ITU standard and up to 2.5 Gbit/s for the ATM Forum. A first scenario of introducing WDM switching layer in an ATM network is the use of WDM ring architectures (Figure 14). In this configuration, the VP cross-connects are directly interfaced with OADM nodes. When a failure occurs in an ATM network, the VP connections are re-routed over the OCH network without the need for any electrical back up VPs. The vacant bandwidth can be used for other services. The use of WDM ring architectures provides a high capacity self-healing network for the VP services. A second scenario is also proposed. The WDM switching layer is composed of OXC nodes connected to VP cross-connect. The OXCs create a flexible and highly secure system needed for CBR paths in an ATM backbone network with a data rate from 155 Mbit/s to 2.5 Gbit/s.


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_ Figure 14: WDM/ATM network scenarios Considering the values of table 10 for the worst case protection switching time, the protection scheme takes about 14ms to perform the traffic protection.

3.4.3

Configuration IP over ATM [9] Different approaches to running IP over ATM are considered for performing connection-less IP and connection oriented ATM technologies. Classical IP defines IP to ATM address resolution and IP packet encapsulation. It allows the use of direct ATM connections inside a sub-network, but shares the VC connection with all other applications running between the same transmitter and receiver.

•

IP encapsulation over ATM Adaptation Layer 5 (ALL5): two methods for carrying IP over AAL5 are proposed. The first method, called Logical Link Control (LLC) Encapsulation, allows multiplexing of multiple protocols over a single VC, whereas the second method, called VC based multiplexing, assumes that each protocol is carried on a separate VC.

•

IP Switching: IP switching provides high-speed IP routing. It combines the highspeed layer 2 switching of ATM with standard IP routing, establishing direct VC connection whenever a long IP datagram flow is detected by the router (Figure 15). The advantage of an ATM switch is that the QoS and multicast are available. The disadvantage of an IP switch is that it is cell-oriented, not packet-oriented, and connection-oriented, unlike the connection-less IP network protocols . IP Flow control

IP Router ATM VC Switch

Figure 15: IP switching

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3.4.4


Configuration IP over SDH [10, 11, 12] SDH is a serious candidate to interconnect high capacity IP routers. IP over SDH is an efficient approach compared to IP over ATM. The additional overheads required for IP over SDH is about 3% compared to 18-25% for IP over ATM. IP The PPP encapsulated IP datagrams are framed into SDH Virtual Container VC (Figure 16). The HDLC provides the delineation of PPP encapsulated IP datagrams [13]. IP datagram size can be can be in theory up to 64 Kbytes, but for most individual Internet users 1500 bytes is the maximum length which is determined by the Ethernet access network. Scrambling of HDLC framed signals is required to provide security against emulation of the SDH set-rest scramble pattern and replication of the STM-N frame alignment word.

•

IP Client datagrams, IPv4, IPv6..

•

PPP IP multiprotocol encapsulation Error checking Link initialisation control

•

HDLC PPP packet delineation

•

SDH Path STM-N, VC4-Nc

• •

Figure 16: IP-PPP over SDH layers IP datagram can be directly encapsulated over SDH, without PPP interface, using HDLC frame switches. This configuration provides multiple access, broadcast/multicast-capable switched LAN and WAN environment using essentially point-to-point SDH lines transmission media.

3.4.5

Configuration IP over WDM The importance of running IP over WDM is a new issue for the all-optical network. At the moment there is no standard for direct working of IP over the Optical Channels of a WDM network. To solve this problem two items are identified:

•

encapsulation mechanism to carry IP payloads over OCH, overheads, error control...

•

procedures and protocols between IP routers and WDM network elements OXC, OADM.., for overall network routing and protection mechanism.

For the IP over OCH configuration, various all-optical network architectures, presented in section 2, such as point-to-point, interconnected rings and ring-mesh configurations, should be considered. A typical example of inter-working between the IP and the WDM layer is the signalling protocol for the optical protection mechanism. The signalling protocol must be implemented between the IP and WDM layers to provide a fast reconfiguration of the optical protection layer or switches (Fig.17). The most important parameter of the signalling protocol is the connection set-up time, the time from when the host driver sends a request, until the configuration from the switch is received. Connection set-up


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time depends on signalling transport network. In WDM network the signalling protocol could be transported over the optical supervisory channel . TCP ,

UDP

IP Signalling Control OPTICAL SWITCHES

Fig17: Signalling protocol layers for IP/WDM using optical protection mechanism

3.5

Conclusion This last section has been focused on analysing an alternative solution to SDH/WDM based optical networks. First, an overview of ATM and IP client signals (network functions, services and performance) has been presented, focusing on IP, ATM, SDH and WDM optical network functionalities. The following multi-layer network architectures have been selected to study the impact of ATM and IP client signals on the planning of optical network. The following configurations have been identified:

• • • • •


It was concluded that ATM and IP client signals requirements on the performance and the management of WDM optical network are not the same as SDH client signal. WDM protection mechanism, reconfiguration time and network architectures should be reconsidered for ATM or IP client signals. For the IP over WDM scenario the following topics are identified for a future EURESCOM Project:

• • •

Encapsulation of IP over the Optical Channel Signalling protocol for the layer inter-working Management and OAM issues.

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Deliverable 1

4


Conclusion In this Deliverable, results from Task 2 ‘Considerations on Optical Network Architectures’ activities are presented. Most of the important optical functions used for simple network topologies are considered to be available by 1998. A list of desirable functions enabling or enhancing optical networking, have been identified as follows:

• •

Flexible OADMs and OXCs

• • •

Optical 3R regeneration

Optical signal monitoring functions (QoS, optical spectrum, and Failure detection)

Network survivability (protection, restoration) Management functions

From the assessment of optical network architectures, four complex network architectures have been selected as reference configurations. The selected configurations are listed as follows:

• • • •

two-level CS-Ring architecture two-level OMS-SP Ring architecture two-level mesh-ring architecture two-level ring- mesh architecture

Characteristic parameters dealing with architectures, topologies, demands and physical limitation have been identified and ranges of value have been proposed. The selected configurations will be used for network planning studies in Tasks 3 and 4 activities of the Project. In the last Section of the Deliverable, the potential of optical transport network for multi-client applications is discussed. Multi-layer network configurations are proposed using IP, ATM, SDH and WDM network functionalities. The following configurations are proposed:

• • • • •


The cumulative configurations using many layers like ATM over SDH over WDM and IP over ATM over SDH over WDM are considered to be an inefficient solution for their resource consuming overhead requirements. Unfortunately, directly based ATM over WDM and IP over WDM approaches require the definition of a new network interface and corresponding management system.


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References [1]

Katsusuke Tajima, «Low loss optical fibres realised by reduction of Raleigh scattering loss», OFC ’98 Technical Digest, pp 305-306.

[2]

Yadlowsky, M.J., Deliso, E.M., Da Silva, V.L., «Optical fibres and Amplifiers for WDM Systems», Proc. of the IEEE, Vol. 85, No. 11, Nov. 1997

[3]

D.R.Hjelme, A.Royset, D.J.Slagsvold, ‘How many wavelengths does it take to build a wavelength-routed optical network ’, ECOC ’96, Sept.1996

[4]

Demeester,P. et al, “Photonic transport networks : Why, How and When ? ”, NOC’96, Vol.I pp. 158 –165.

[5]

Schiess, M. et al, “ Guidelines for the introduction of Optical Networking Functions”, EURESCOM Project P615, D3, May 98.

[6]

Peter Newman et al, ‘IP switching and Gigabit routers’, IEEE communication magazine January 1997, pp 64-69.

[7]

Gurdeep Singh Hura, ‘The Internet: global information superhighway for the future’, Computer Communication 2,1998, pp 1412-1430.

[8]

Reuven Cohen et al, ‘Using proxies to enhance TCP performance over hybrid coaxial networks’, Computer Communication 2,1998, pp 1502-1518.

[9]

RFC 1483, ‘Multiprotocol http://www.ietf.org/

[10]

C.J. den Hollander, ‘The mapping of HDLC payloads into SDH Virtual Containers’, ITU-T, COM 15-61, December 1997.

[11]

RFC 1619, ‘PPP over SONET/SDH’, http://www.ietf.org/

[12]

RFC 2171, ‘MAPOS’, http://www.ietf.org/

[13]

RFC 1661, ‘Point-to-point Protocol PPP’, http://www.ietf.org/

page 40 (42)

encapsulation

over

ATM

ALL

5’,


Deliverable 1


Appendix 1: Recent Progress in the Performance of Optical Transmission System Components Introduction Even though the state-of-the-art of available optical functions was reviewed at the beginning of the Project the progress achieved in the performance of some of the functions has been considerable since then and it seems appropriate to take that into account.

EDFA properties The absolute values of EDFA key properties power gain, saturated output power and noise figure of EDFAs have not changed dramatically. The variations of these properties as a function of optical carrier frequency seriously impacts the performance of WDM transmission systems. These limit e.g. the maximum number of channels and the total number of cascaded amplifiers in a WDM link. In recent work it has been demonstrated that the variation of these parameters as a function of optical carrier frequency can be reduced considerably and corresponding improvements in system performance can be achieved. Within the entire gain bandwidth of an EDFA the deviation of actual gain from the nominal gain is specified e.g. in terms of a dB value as it is often undesirable to take into account the more detailed information of the optical gain spectrum. The diagram in Fig. a-1 represents a gain spectrum of a conventional type EDFA. In contrast to that the gain spectrum, Fig. a-2 shows the considerably enlarged gain range and correspondingly lower gain variation within a certain wavelength band that has been achieved in [2].

Figure a-1 : Gain spectrum of a conventional EDFA Figure a-2: Wide band EDFA gain spectrum [2]

Temporal behaviour of EDFAs is characterised by very slow gain relaxation effects [1], [3], [4]. Usually these relaxations do not interfere with the very rapid power variations of the transmitted optical signals. Under special circumstances e.g. if some of the WDM channels fail, it is, however, possible that distinct power transients in the surviving channels are produced. In WDM links with a large number of cascaded EDFAs these gain relaxations will be even more pronounced. It has been shown [4] that in a cascade of conventional EDFAs these gain relaxations can induce power transients of nearly 0dBm in the surviving channel under extraordinary conditions. Under the same conditions in a cascade of EDFAs with an advanced gain control scheme, however, the power transient was suppressed by 10dB.


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DWDM Planning

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