Gpon Basics - Bsnl

Advanced regional Telecom Training Centre, Ranchi

Introduction • •

GPON - Gigabit Passive Optical Network.

•

PON utilizes Tree network topology

•

Offers data rates of up to 2.5 Gbps downstream and 1.2 Gbps upstream.

•

Provides extended reach (compared to xDSL)

The only active network elements are OLT (Optical Line Termination) and ONT / ONU (Optical Network Termination / Unit)


Overall GPON data transmission approach

ONU

Central office ONT

Fi be rl

in k

Upstream (ONTs OLT)

λ1 = 1310 nm

OLT GE/10GE

Fiber feeder

NT xDSL

Downstream (OLT ONTs)

λ2 = 1490 nm

MDU

xDSL

As far as one fiber feeder is used between OLT and ONUs for data transmission WDM is implemented to decouple Uplink and Downlink data paths.

NT ONT

Infrastructure – Fiber Optic

• OLT – Optical Line Termination

• Passive Optical splitter

• ONT – Optical Network Termination

• NT – Network Termination

• ONU – Optical Network Unit • DSL – Digital Subscriber Line


Downstream data transmission (TDM) • Every ONT receives each DS frame and picks up only that data addressed to it by the OLT

• OLT broadcasts data to every ONU using TDM approach

• FEC decoding and /or AES decryption algorithm applied if required

• TDM – continuous data stream divided into frames • Optionally FEC coding and AES encryption are applied to the user data

ONT 1

1

1

3

1

2

1

User #1

OLT

ONT 2

1

2

1

3

1

2

1

3

2

Passive Optical

User #2

Splitter 1: N (Distribution point) • DS signal is distributed by a passive splitter to all ONUs connected to it via separate fiber link

ONT 3

1

To broadcast data from the OLT to all ONTs TDM (Time Division Multiplexing) is used

2

1

3

3 User #3


Upstream data transmission (TDMA) • ONU receives data from the user ports and combines them into bursts • For synchronization purposes each ONU transmits its data in a strict

To provide multiple access to a single fiber link for all connected ONUs Time Division Multiple Access (TDMA) architecture is implemented for the Upstream channel

accordance with the Bandwidth Map generated by OLT • Using DBA mechanism OLT can rearrange US BW to provide more resources to those ONU tightly loaded with traffic. • For synchronous transmission within TDMA stream each ONU introduces equalization delay.

ONU 1

1

1

1 1

User #1

OLT 3

1

1

3

2

ONU 2

3

2

BW Allocation Map

2 User #2

Splitter

ONU 3

3 • OLT collects data from all ONUs and filters it with the use of ONU-IDs

3

3

• FEC is applied to data if required

3

3

User #3

3


Summary on T-CONTS

T-CONTs or Transmission Containers are used in GPON for provisioning of: traffic separation and bandwidth allocation on the level of CoS.

The main purpose of T-CONT is bundling of GEM traffic for management of US bandwidth allocation.

Every T-CONT may contain one GEM-port or bundle of several GEM-ports

T-CONTs are used only in US direction (from the ONTs to the OLT)

Every ONT in the GPON is allowed to use up to 8 T-CONTs. Every T-CONT may be assigned to a one of four different T-CONT types.

Every type of T-CONT is allocated US bandwidth differently.

T-CONT is identified by 12-bits of Alloc_ID (up to 4095 T-CONTs per GPON)


T-CONTs and Bandwidth allocation Static T-CONT

Dynamic T-CONT


Applications’ data prioritization

In US direction ONUs transmit their data in short packages called “bursts”

For OLT to understand from which ONU a burst is received the ONU-ID is added into the overhead field.

To separate the traffic of different user’s applications the so called traffic containers or T-CONTs are used.

Every T-CONT is associated with the traffic of certain Class (or Classes) of Service (CoS)

ONU #1

Upstream channel ONU #3 Burst

ONU #2 Burst

ONU #1 Burst

ONU #2 ONU #3 Overhead

ONU #2 T-CONT 4

T-CONT 3

T-CONT 2

ONU #1 T-CONT 1

Overhead

Overhead

ONU #3

Guard interval

Best Effort

High Priority Data (non-real time)

High Priority Data (real time)

Delay sensitive traffic (Voice, TDM.)

Guard interval


User’s data encapsulation

User Data frames

GEM payload partition

1.

User data represented by different types of traffic (Ethernet data, VoIP, TDM, OMCI) is adapted to the format suitable for transmission over GPON

2.

This task is accomplished by GTC Adaptation sublayer of GPON stack over GEM (GPON Encapsulation Method which uses GFP-like procedure).

3.

GEM frame contains encapsulated user data and overhead

4.

User frame may be partitioned prior to encapsulation into GEM-frame

5.

If there are no user data to transmit within an allocated resource then idle frames are inserted

PL#1

H#1 001

PL#2

PL#1

H#2 001

PL#3

PL#2

H#3A 000

PL#3A

PL#4

H#3B 001

H#4

PL#3B

001

PL#4

GEM frame GEM payload partition of GTC Frame #M

GEM Header

PLI

Port ID

PTI

HEC

12b

12b

3b

13b


Idle

GEM-ports. User data encapsulation TDM 1

GEM-port-ID1

T_CONT # 1

T-CONT is identified by Alloc_ID (1024 per GPON)

Type 1 TDM 2

GEM-port-ID2

(Fixed BW)

T_CONT # 2

POTS / VoIP

Leased Lines

GEM-port is identified by GEM_port_ID (4095 per GPON)

GEM-port-ID3 AGW

Burst is identified by ONT_ID (64 per GPON)

Type 1 (Fixed BW)

To the ODN

T_CONT # 3 ONT

Host µC

GEM-port-ID4

Data (rt)

Management GEM-port-ID5

Burst

T_CONT # 4 Type 2

GEM-port-IDn1 Data (best effort) Data (nrt)

Type 1 (Fixed BW)

GEM-port-IDn1+1

(Assured BW)

T_CONT # 5 Type 3

GEM-port-IDn2

(Assured + Non Assured BW)

GEM-port-IDn2+1

T_CONT # 6

GEM-port-IDn3

(Best Effort BW)

Type 4


Summary on GEM-ports

1.

From the point of view of GPON GEM-port defines logical bi-directional point-topoint connection with the following exceptions:

DASAN GPON uses the GEM-port ID 0xFFF(4095) for Multicast and Broadcast.

2.

GEM-port typically bundles payload per user per CoS.

3.

GEM-ports are used for encryption and performance monitoring

4.

GEM-port is identified by 12-bits of GEM-port-ID (up to 4095 GEM-ports per GPON)

5.

The allocation of GEM-ports per T-CONT and per physical ONU ports is dictated by OLT


Dynamic Bandwidth Allocation

• Upstream bandwidth (US BW) is granted to ONUs by OLT with the aid of US BW Map broadcasted to all PON participants

• BW assignment may be static or dynamic • Using DBA mechanism dynamic BW assignment is possible. DBA improves overall BW utilization

• Additional BW is granted by the OLT from the currently unused bandwidth pool • To know how much BW to grant for a certain ONU (T-CONT) OLT have to collect information on BW utilization

• BW utilization may be assessed using the following modes: •

Non Status Reporting DBA – OLT observes traffic patterns of ONUs (ONUs don’t report)

•

Status Reporting DBA – OLT receives information on queue fill level from ONUs (ONUs report to OLT)


GPON maximum reach User Side ONU 1

Outdoor Environment

Central Office

Outermost ONU

OLT ONU 2

a

ONU N

Splitter

b Nearest ONU

Maximum differential

Logical Reach

distance

Maximum distance between OLT

a – b ≤ 20 km

20 km

and outermost ONU supported by GTC

60 km 20 km

Physical Reach Typical maximum distance considering infrastructure losses

GPON Optical module specification CLASS of Laser B+ Specification


GPON reach influencing factors OLT

1) Class of Laser used Class of Laser

Type

Optical Budget

A

Fabry-Perot

5 – 20 dB

B

Fabry-Perot

10 – 25 dB

B+

Fabry-Perot

13 – 28 dB

C

Distributed

15 – 30 dB

GPON Release

Optical Budget

>

Total losses Connectors

Current version

Feedback Laser

2) Receiver susceptibility

Optical Budget

Splitter (1:32)

20 km

-----------28 dB

3) Forward Error Correction algorithm - Enabled / Disabled 4) Attenuation of the optical link - Depends on the fiber length (0.3 dB per 1 km) - Number of splices along the fiber channel - Number of connectors (0.3 dB per connector) 5) Attenuation of the passive optical splitter - Depends on the splitting ratio 1: 64 – 19.5 dB 1:16 – 14.4 dB 1:4 – 7.6dB

1:32 – 17.8 d 1:8 – 11 dB 1:2 – 4.2dB

ONT 1 Total losses Splitter – 17.8 dB Fiber - 0.3dB/km x 20km = 6 dB Connectors – 0.3dB x 4 = 1.2 dB

ONT 2

------------------------------------------25 dB


ONT N

Forward ForwardError ErrorCorrection Correction(FEC) (FEC) •

Forward Error Correction (FEC) is the mechanism to improve transmission quality of physical layer.

•

Reed Solomon code (RS) FEC is used by GPON. Up to 3 dB budget improvement (7% overhead)

•

Only payload part of frames is coded

•

OLT and ONUs negotiate the status of FEC using flags in the overhead fields for each ONU independently

ONT #1

OLT

FEC enabled for ONT#1

ONT #2

FEC disabled for ONT#2 PON

FEC enabled for ONT#3

ONT #3


Forward Error Correction (FEC) • For FEC purposes Reed Solomon code RS (255,239). • Codeword length is 255 B = 239 B data block + 16 B parity bytes

Downstream In DS direction OLT informs all ONTs of the FEC enabled / disabled status by installing FECi bit. FECi is valid for the whole GEM payload.

PCBd

Ident

FECi / 1b

Upstream ONTs that are capable of FEC support install their Ind bit # 6. Those ONTs not supporting FEC mechanism ignore FEC overhead.

PLOu

IND


Bit # 6

Encryption Encryption

User #1

•

Standardized AES (Advanced Encryption Standard) algorithm is used to protect individual users’ data from trespassing.

•

Encryption is applied to the data in the DS direction only

•

DS data is encrypted on the level of virtual connection.

•

AES Encryption machine operates in counter mode.

•

Unpredictable cipher block generation is done with the use of constantly changing counters and random cipher keys generated by each ONU independently

ONT #1 Encrypted connection

Encryption is enabled for User #1 of ONT#1

PON

Encryption is disabled for User #2 of ONT#1

User #2

ONT #N


OLT

Security on GPON Link The basic concern in PON is that the downstream data is being

broadcasted to all ONUs attached to the PON. AES is used for the encryption algorithm 128 (currently supported by Broadlight Chipset), 192, and 256 bit keys GEM frame payload is encrypted. PLOAM (Physical Layer OAM) for security Encrypted_Port-ID (downstream) : Enable/Disable Encryption Request_Key (downstream) : request ONU to generate new key Encryption_Key (upstream) : send new key to OLT Key_Switching_Time (downstream) : indicate when to begin using the new encryption key.

Rekeying time can be configurable ; minimum value is 10 seconds (Broadlight Implementation)


Security on GPON Link Authentication Mechanism in GPON

GPON supports two kinds of authentication mechanism. Serial Number Password The serial number is mandatory authentication during ONU activation. After registering the serial number, ONU ID is assigned Serial_Number_ONU (upstream) : 8 bytes serial number Password is optionally used for validating the ONU. Request_password (downstream) Password (upstream) : 10bytes password


ONU Failure Detection Time


ONU failure detection time ONU failure can be detected by LOSi and LOFi. LOSi (i : ONU ID) No valid optical signal from ONU when it was expected 4 consecutive non-contiguous allocations to that ONU

LOFi When 4 consecutive invalid delimiters from ONUi was received.

ONU Failure can be detected within 8 msec. (HW Detection Time)


OLT GPON port switch over mechanism


GPON Redundancy Type-A Protection in SIU_GPON4R ONT #1

SFU

GPON Port Redundancy within a GPIU

GPIU 12 x 1G

BL3458

Active ONT #2

GMAC#1 GMAC#2

Stand-by

GMAC#3

ONT #3

GMAC#4

• • • •

OLT can detect GPON link down by “LOS” event. After receive LOS, switch-over from Active to Standby Switch over in the same GMAC Fail over time is under 50msec


GPON Link LoS • OLT can detect GPON link down by “LOS” event. • LOS is defined as below in G984.3 – LOS : Los of Signal – The OLT did not receive any expected transmission in the upstream (complete PON failure) for 4 consecutive frames.

• Broadlight supports this as hardware triggering mechanism and GPON line card detect it as soon as it happens. • It takes 4 consecutive frames (125us * 4) to detect link down.


Gpon Basics - Bsnl

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