ITU-T G.989.3：NGPON2核心技术规范解析

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"NGPON2 标准G.989.3是2021年更新的传输汇聚层（TC）规范，是光网络接入的关键技术之一，特别适用于北美地区的运营商如Verizon的网络部署。该标准属于ITU-T系列推荐的数字化部分和数字线路系统，特别是40-Gigabit-capable Passive Optical Networks (NG-PON2) 的一部分，旨在定义传输汇聚层的具体要求。" NGPON2（Next Generation Passive Optical Network 2）是一种高速光接入网络技术，旨在提供比第一代GPON更高的带宽能力，支持高达40Gbps的数据传输速率。G.989.3是NGPON2标准的一部分，专注于传输汇聚层（Transmission Convergence Layer, TCL），这个层负责将多种业务和数据流整合到一个单一的物理媒介上，以实现更高效、灵活的网络服务。 TCL在NGPON2中的作用至关重要，它定义了如何在光网络的不同部分之间进行数据包的封装、复用和解复用，以及错误管理和流量控制。这一层的工作确保了不同类型的业务，如语音、数据和视频，能够在同一物理光纤链路上并行传输，同时保持服务质量（QoS）和网络效率。 G.989.3标准详细规定了TC层的功能、协议和接口，包括如何处理多路光通道（WDM，Wavelength Division Multiplexing）的信号，这是NGPON2的一个关键特性。通过WDM，多个不同波长的光信号可以在同一根光纤上传输，进一步增加了网络容量。标准还涵盖了同步、安全性、管理及故障检测等方面，这些都是构建可靠且高性能光接入网络的基础。此外，G.989.3与其他ITU-T系列推荐如G.600至G.699（关于数字终端设备）、G.700至G.799（关于数字网络）和G.800等紧密关联，这些推荐共同构成了完整的光通信系统和网络架构。它们一起定义了从底层物理层到高层应用层的完整通信协议栈，确保了全球范围内电信网络的互操作性和一致性。 NGPON2标准G.989.3是光接入网络技术发展的重要里程碑，它推动了超高速宽带服务的普及，为电信运营商提供了更高效、更灵活的网络解决方案，同时也为用户带来了更快的互联网速度和更丰富的多媒体体验。对于北美地区的运营商来说，如Verizon，遵循这一标准进行网络部署，可以有效提升网络性能和用户体验。

8 Rec. ITU-T G.989.3 (05/2021)

6.1.3.2 TWDM TC framing sublayer

The TWDM TC framing sublayer is responsible for the construction and parsing of the overhead

fields that support the necessary PON management functionality. The TWDM TC framing sublayer

formats are devised so that the frames, bursts and their elements are aligned to 4-byte word

boundaries, whenever possible.

On the transmitter side, the TWDM TC framing sublayer accepts multiple series of XGEM frames

forming the FS payload from the TWDM TC service adaptation sublayer, and constructs the

downstream FS frame or upstream FS burst by providing the overhead fields for the embedded

operation, administration and maintenance (OAM) and the physical layer operation, administration

and maintenance (PLOAM) messaging channel. The size of each downstream FS frame payload is

obtained by subtracting the variable size of the upstream bandwidth management overhead and the

PLOAM channel load from the fixed size of the downstream FS frame. In the upstream direction, a

FS burst multiplexes FS payloads associated with multiple Alloc-IDs, the size of each payload being

determined based on the incoming bandwidth management information.

On the receiver side, the TWDM TC framing sublayer accepts the FS frames or FS bursts, parses the

FS overhead fields, extracts the incoming embedded management and PLOAM messaging flows, and

delivers the FS payloads to the TWDM TC service adaptation sublayer. The incoming PLOAM

messages are delivered to the PLOAM processor. The embedded OAM information to the extent

pertaining to upstream bandwidth management (BWmap parsing) and dynamic bandwidth

assignment (DBA) signalling is processed within the framing sublayer itself, providing partial control

over the PHY adaptation sublayer (upstream PHY burst timing and profile control). The rest of the

embedded OAM information is delivered to the appropriate TWDM TC functional entities outside of

the framing sublayer, such as ONU electrical power management and performance monitoring

blocks.

See clause 8.1.1 for the details of downstream FS frame format specification, including BWmap

parsing, and clause 8.1.2 for the details of upstream FS burst format specification, including DBA

signalling.

6.1.3.3 TWDM TC PHY adaptation sublayer

The TWDM TC PHY adaptation sublayer encompasses the functions that modify the bitstream

modulating the optical transmitter with the goal to improve the detection, reception and delineation

properties of the signal transmitted over the optical medium.

On the transmitter side, the TWDM TC PHY adaptation sublayer accepts the FS frames (in the

downstream direction) or FS bursts (in the upstream direction) from the framing sublayer, optionally

performs forward error correction (FEC) encoding, performs scrambling of the content, prepends the

physical synchronization block appropriate for downstream (PSBd) or upstream (PSBu) transmission

and provides timing alignment of the resulting bitstream.

On the receiver side, the TWDM TC PHY adaptation sublayer performs physical synchronization

and delineation of the incoming bitstream, descrambles the content of the PHY frame or PHY burst,

optionally performs FEC decoding, delivering the resulting FS frames (in the downstream direction)

or FS bursts (in the upstream direction) to the TWDM TC framing sublayer.

The details of the PSBd and PSBu overhead fields are specified in clauses 10.1.1.1 and 10.1.2.1,

respectively.

The use of FEC improves the effective sensitivity and overload characteristics of the optical receiver

by introducing redundancy in the transmitted bitstream and allowing the receiver to operate at a higher

bit error ratio (BER) level. FEC is specified in detail in clause 10.1.3.

Bitstream scrambling randomizes the transmission and helps to meet the specified consecutive

identical digits (CID) tolerance. The TWDM PON scrambling method is specified in clause 10.1.4.

Rec. ITU-T G.989.3 (05/2021) 9

The downstream and upstream line codes employed in TWDM PON are specified in clause 11.1.2 of

[ITU-T G.989.2].

6.1.4 Management of a TWDM PON system

The ONU control, operation and management information in a TWDM PON system is carried over

three channels: embedded OAM, PLOAM and OMCC (see Figure 6-4). The embedded OAM and

PLOAM channels manage the functions of the PMD and TWDM TC layers. The OMCC carries the

messages of the OMCI protocol, which provides a uniform system for managing higher (service-

defining) layers. For the PLOAM channel, two transportation options are available: the in-band

transportation option and the auxiliary management and control channel (AMCC) transportation

option. The AMCC transportation option is restricted to the upstream transmission of the

Serial_Number_ONU and Tuning_Response PLOAM messages (see Table 11-3).

Figure 6-4 – ONU management channels and options

In addition, the inter-CT communication channel, which carries the inter-channel-termination

protocol (ICTP) primitives, supports the multi-wavelength aspect of the NG-PON2 system operation.

The detailed ICTP specification is out of scope of this Recommendation. An informative discussion

of ICTP use cases and functional primitives can be found in Appendix VI. The ICTP specification

can be found in [b-BBF TR-352].

6.1.4.1 Embedded OAM

The embedded OAM channel is provided by well-defined header fields and embedded structures of

the downstream FS frame and the upstream FS burst. The embedded OAM channel offers a low-

latency path for the time-urgent control information because each information piece is directly

mapped into a specific field. The functions that use the embedded OAM channel include upstream

PHY burst timing and profile control, bandwidth allocation, dynamic bandwidth assignment

signalling, forced wake-up and dying gasp indication. The detailed description of the header fields

and structures involved in support of these functions is provided in clause 8.1 as a part of the TWDM

TC framing sublayer specification.

6.1.4.2 PLOAM channel

The PLOAM channel is message based and is used for all PMD and FS management information that

is not sent via the embedded OAM channel. The PLOAM message structure, message types and

detailed format specifications are provided in clause 11.

There are two transportation options for the PLOAM channel. The primary transportation option is

in-band: the PLOAM messages are carried in a designated partition of the downstream FS frame and

the upstream FS burst.

10 Rec. ITU-T G.989.3 (05/2021)

As an alternative, which is applicable only to the upstream Serial_Number_ONU and

Tuning_Response messages, the AMCC transportation option can be used. The AMCC transportation

option is a low data rate means to allow the discovery of ONUs which are insufficiently calibrated,

while eliminating the risk of harmful interference to the operational TWDM channels.

In a System_Profile PLOAM message, the OLT CT indicates the availability of the AMCC

transportation option and communicates the calibration accuracy constraints for the ONUs to use the

in-band transportation option for ONU discovery. If an ONU does not meet the calibration constraints

(i.e., is insufficiently calibrated), it must use the AMCC transportation option to transmit the

Serial_Number_ONU PLOAM message or remain silent if the AMCC transportation option is

unavailable. After the OLT CT receives the ONU's transmission, it instructs the ONU to proceed with

activation on a specific TWDM channel.

Out of the two PLOAM channel transportation options, the support of in-band is mandatory, whereas

the support of AMCC is optional for ONUs meeting the calibration constraints.

6.1.4.3 ONU management and control channel (OMCC)

The ONU management and control channel (OMCC) uses the OMCI messages to manage the service-

defining layers residing above the TWDM TC layer. The TWDM TC layer must provide a XGEM-

based transport interface for this management traffic, including configuration of appropriate transport

protocol flow identifiers (XGEM Port-IDs). This Recommendation specifies a format and transfer

mechanism for the OMCC channel. The detailed OMCI specification can be found in [ITU-T G.988].

The OMCI adapter at the ONU is responsible for filtering and de-encapsulating OMCI-carrying

XGEM frames in the downstream direction, and for encapsulating OMCI SDUs in the upstream

direction. OMCI SDUs are handed off to the logic that implements the OMCI functions.

The OMCI adapter at the OLT CT is responsible for filtering and de-encapsulating OMCI-carrying

XGEM frames in the upstream direction, and for encapsulating the OMCI SDUs from the OMCI

control logic into XGEM frames for transport to the ONU.

6.1.4.4 ICTP

In order to support such functionalities as channel profile configuration and status sharing, ONU

activation, ONU wavelength channel handover and rogue ONU mitigation, the OLT channel

terminations in an NG-PON2 system need to interact with each other. This interaction between CTs

takes the form of exchanging ICTP functional primitives over the abstract ICTP transportation

channel. The ICTP transportation channel abstraction allows a variety of physical implementations

depending on the relative location of the interacting CTs. The discussion of the ICTP use cases and

functional primitives can be found in Appendix VI. The detailed ICTP specification is out of scope

of this Recommendation.

6.1.5 Time and wavelength division multiplexing architecture

6.1.5.1 Overview

The wavelength division multiplexing aspect of a TWDM PON system is represented by multiple

OLT TWDM channel terminations (CTs), each OLT CT being associated with a specific TWDM

channel. Within the design constraints and the limits of the protocol, the ONUs can switch (be handed

over) between the TWDM channels. Within each individual TWDM channel, the principles of time

division multiplexing (TDM) and time division multiple access (TDMA) apply.

In the downstream direction of a TWDM channel, the traffic multiplexing functionality is centralized

as shown in Figure 6-5. The OLT CT multiplexes XGEM frames onto the transmission medium using

XGEM Port-ID as a key to identify XGEM frames that belong to different downstream logical

connections. Each ONU filters the downstream XGEM frames based on their XGEM Port-IDs and

processes only the XGEM frames that belong to that ONU. A multicast XGEM port can be used to

carry XGEM frames to more than one ONU.

12 Rec. ITU-T G.989.3 (05/2021)

6.1.5.2 NG-PON2 system identifier

A next generation passive optical network 2 (NG-PON2) system identifier (NG2SYS ID) is a 20-bit

number that identifies a specific NG-PON2 system among multiple NG-PON2 systems under

common administration. The NG2SYS ID may be coded to include data to support administration

such as an operator name, geographical location, service profile, and whether the system is for

protection. It is supplied by an element management system/ operations support system (EMS/OSS)

to the OLT CT and is identical for all TWDM channels and PtP WDM channels within the NG-PON2

system. The OLT CT communicates the NG2SYS ID to all subtending ONUs in the System_Profile

PLOAM message. An ONU stores the NG2SYS ID and uses it as a reference.

6.1.5.3 PON-ID

A PON-ID is a 32-bit structured number that uniquely identifies a TWDM or PtP WDM channel

termination (CT) entity within a domain.

In a TWDM PON system, PON-ID consists of a 28-bit administrative label and a 4-bit downstream

wavelength channel ID (DWLCH ID). The administrative label is supplied by an EMS/OSS to the

OLT NE. It is expected to follow some consistent physical or logical equipment numbering plan, and

is treated transparently by the OLT.

The PON-ID of the specific TWDM channel termination is carried downstream within the operation

control (OC) structure of the PSBd field. The PON-IDs of all TWDM channel terminations forming

a TWDM system are included into the set of Channel_Profile PLOAM messages.

In a PtP WDM PON system, PON-ID consists of a 22-bit administrative label and a 10-bit ONU ID.

The administrative label is supplied by an EMS/OSS to the OLT NE. It is expected to follow some

consistent physical or logical equipment numbering plan, and is treated transparently by the OLT.

The PON-ID of the specific PtP WDM channel termination is carried downstream within the

operation control (OC) structure of the PSBd field. The PON-IDs of other PtP WDM channel

terminations forming a PtP WDM system may be included into the set of Channel_Profile PLOAM

messages.

6.1.5.4 Downstream wavelength channel identifier

In a TWDM PON system, downstream wavelength channel ID (DWLCH ID) is a 4-bit number that

identifies a downstream wavelength channel and is equal to the ordinal number of the channel defined

in Table 11-2 of [ITU-T G.989.2], converted to the range from 0 to 7, as shown in Table 6-3.

Table 6-3 – DWLCH-ID to Channel Mapping

Channel

Frequency (THz)

Wavelength (nm)

DWLCH ID

187.8

1596.34

0000

187.7

1597.19

0001

187.6

1598.04

0010

187.5

1598.89

0011

187.4

1599.75

0100

187.3

1600.60

0101

187.2

1601.46

0110

187.1

1602.31

0111

DWLCH ID is a part of PON-ID (see clause 6.1.5.3), which is transmitted downstream within the OC

structure of the PSBd field.

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