gpon basics - bsnl

Advanced regional Telecom Training Centre, Ranchi

Introduction

• GPON - Gigabit Passive Optical Network.

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

• 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)


Overall GPON data transmission approach

GE/10GE

OLT

ONU

ONT

MDU

ONT

NT

xDSL

NT

xDSL

Upstream (ONTs �� OLT)λλλλ1 = 1310 nm

Downstream (OLT �� ONTs)λλλλ2 = 1490 nm

Fiber feeder

Central office

Infrastructure – Fiber Optic

• Passive Optical splitter

• NT – Network Termination

• OLT – Optical Line Termination

• ONT – Optical Network Termination

• ONU – Optical Network Unit

• DSL – Digital Subscriber Line

Fiber link

�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.


Downstream data transmission (TDM)

• OLT broadcasts data to every ONU using TDM approach

• TDM – continuous data stream divided into frames

• Optionally FEC coding and AES encryption are applied to the

user data

• Every ONT receives each DS frame and picks up only that data

addressed to it by the OLT

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

• DS signal is distributed by a passive splitter to all

ONUs connected to it via separate fiber link

User #1

User #3

User #2

2 1 31

21

3

1

2 1 31

21

3

1

3

2

1 1

ONT 1

ONT 2

ONT 3

OLT

Passive Optical

Splitter 1: N

(Distribution point)

�To broadcast data from the OLT to

all ONTs TDM (Time Division

Multiplexing) is used


Upstream data transmission (TDMA)

• OLT collects data from all ONUs and filters it

with the use of ONU-IDs

• FEC is applied to data if required

• ONU receives data from the user ports and combines them into

bursts

• For synchronization purposes each ONU transmits its data in a strict

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.

User #1

User #3

User #2

2

1 1ONU 1

ONU 2

ONU 3

OLT

Splitter

33 3

2

1

1

3

3

3

3 1 1 3 2 3

BW Allocation Map

�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


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

ONU #1

ONU #2

ONU #3

ONU #3 ONU #1ONU #2

Overhead OverheadOverheadT-CONT 1T-CONT 2T-CONT 3T-CONT 4

Guard

intervalGuard

interval

Upstream channel

ONU #3 Burst ONU #1 BurstONU #2 Burst

Best EffortDelay sensitive traffic

(Voice, TDM.)

High Priority Data

(non-real time)

High Priority Data

(real time)

� 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)


User’s data encapsulation

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

PL#1H#1

001

PL#2

PL#2H#2

001

PL#3

PL#3AH#3A

000PL#3B

H#3B

001

PL#4

PL#4H#4

001Idle

PLI

12b

Port ID

12b

PTI

3b

HEC

13b

GEM frame

GEM payload partition of GTC Frame #M

User Data

frames

GEM payload

partition

GEM

Header


GEM-ports. User data encapsulation

AGW

To the ODN

Burst is identified by

ONT_ID (64 per GPON)

T-CONT is identified by

Alloc_ID (1024 per GPON)

GEM-port is identified by

GEM_port_ID (4095 per GPON)

Leased

Lin

es

PO

TS

/ V

oIP

Data

(rt

)D

ata

(n

rt)

Data

(b

est

eff

ort

)

ONT

Management

T_CONT # 1

Type 1

(Fixed BW)

T_CONT # 2

Type 1

(Fixed BW)

T_CONT # 3

Type 1

(Fixed BW)

T_CONT # 4

Type 2

(Assured BW)

T_CONT # 5

Type 3

(Assured + Non Assured BW)

T_CONT # 6

Type 4

(Best Effort BW)

Burst

GEM-port-ID1

GEM-port-ID2

GEM-port-ID3

GEM-port-ID4

GEM-port-ID5

GEM-port-IDn1

GEM-port-IDn1+1

GEM-port-IDn2

GEM-port-IDn2+1

GEM-port-IDn3

Host µC

TDM 2

TDM 1


Summary on GEM-ports

1. From the point of view of GPON GEM-port defines logical bi-directional point-to-

point 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

ONU 2 Splitter

ONU N

ONU 1

OLT

Outermost ONU

Nearest ONU

60 km

Logical Reach

Maximum distance between OLT

and outermost ONU supported

by GTC

20 km

Physical Reach

Typical maximum

distance considering

infrastructure losses

a

b

a – b ≤≤≤≤ 20 km

20 km

Maximum

differential

distance

User Side Outdoor Environment Central Office

GPON Optical module specification

CLASS of Laser B+ Specification


GPON reach influencing factors

Class of Laser Type Optical Budget GPON Release

A Fabry-Perot 5 – 20 dB

B Fabry-Perot 10 – 25 dB

B+ Fabry-Perot 13 – 28 dB Current version

C Distributed

Feedback Laser

15 – 30 dB

1) Class of Laser used

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 1:32 – 17.8 d

1:16 – 14.4 dB 1:8 – 11 dB

1:4 – 7.6dB 1:2 – 4.2dB

3) Forward Error Correction algorithm- Enabled / Disabled

2) Receiver susceptibility

Optical Budget

Total losses

Optical Budget

Total losses

>

OLT

Splitter

(1:32)

ONT NONT 2

ONT 1

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

Splitter – 17.8 dB

Fiber - 0.3dB/km x 20km = 6 dB

Connectors – 0.3dB x 4 = 1.2 dB

20 km

-------------------------------------------

25 dB

Connectors


OLT

ONT #1

PONONT #2

ONT #3

FEC enabled for ONT#1

FEC enabled for ONT#3

FEC disabled for ONT#2

• 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

Forward Error Correction (FEC)Forward Error Correction (FEC)


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

PCBd Ident FECi / 1b

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

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


OLT

ONT #1

PON

ONT #N

Encryption is enabled for User #1 of ONT#1

User #2

User #1

Encryption is disabled for User #2 of ONT#1

Encrypted connection

• 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

EncryptionEncryption


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 LinkAuthentication 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 RedundancyType-A Protection in SIU_GPON4R

BL3458

GMAC#4

GMAC#1

GMAC#2

GMAC#3

GPIU

12 x 1GSFU

ONT #1

ONT #2

ONT #3

Active

Stand-by

GPON Port Redundancy within a GPIU

• 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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