FOT Lesson: GPON system protocol stack

“GTC: GPON Transmission Convergence; GEM: GPON Encapsulation Method; OMCI: ONU management and control interface; PLOAM: Physical layer OAM; DBA: Dynamic Bandwidth Allocation; GPM/PMD: GPON Physical Medium Dependent; BW: Bandwidth”

What is a GPON system protocol stack?

  The GPON system is usually consists of the OLT on the central office side, the ONU / ONT and the ODN on the user side, usually adopting a point-to-multipoint network structure. The ODN is composed of passive optical components such as indifference optical fiber, optical splitter and optical connector, and is an OLT. The physical connection to the ONU provides an optical transmission medium.
  The ONU / ONT of the GPON system can be placed in different locations such as users, buildings and communities to form different network structures such as FTTH, FTTB and FTTC. The ONT is an ONU for FTTH and has a user port function. The system protocol stack of GPON is shown in the figure below.

GPON system protocol stack (FOT Telecom blog)

  The GPON physical medium attachment sublayer mainly completes the transmission of GPON service signals on the physical layer, and the GPON physical layer rate supports the following types.

  • Downstream: 1243.16 Mbps, uplink: 1243.16 Mbps
  • Uplink: 2488.32 Mbps, uplink: 1243.16 Mbps

  The GPON downlink transmission uses a wavelength of 1490 nm and the uplink uses a wavelength of 1310 nm.
The GTC transmission aggregation layer is mainly responsible for adapting and de-binding the upper layer service data and the management data into GPON frames, and is divided into an adaptation sublayer and a framing sublayer. The service or management data at the adaptation sublayer enters different adaptation units depending on its type. Service data such as Ethernet, IP, and TDM enters the GEM (GPON encapsulation mode) adaptation unit, and the GPON management control data is first adapted to the OMCI adaptation unit, and then enters the GEM adaptation unit for adaptation. The data and management data finally enters the GTC framing sublayer for framing.
  The GTC adaptation sublayer also has a DBA dynamic bandwidth control function, and its bandwidth control data also enters the GTC framing sublayer for the frame.
  The control and management plane of the GTC system includes embedded OAM, PLOAM and OMCI. Embedded OAM and PLOAM channels manage PMD and GTC layer functions. OMCI provides a unified system for managing upper layers (business definitions).
  The embedded OAM channel is provided by the domain information formatted in the GTC frame header. Because each piece of information is mapped directly to a specific area in the GTC frame header, the OAM channel provides a low latency channel for time sensitive control information. Features that use this channel include bandwidth authorization, key exchange, and dynamic bandwidth allocation indication.

How does the GPON service flow working?

  The PLOAM channel is a formatted information system carried by a specified location within the GTC frame and is used to carry all other PMD and GTC management information that is not sent through the embedded OAM channel.
  The OMCI channel is used to manage the service definition of the upper layer of the GTC. The specific provisions of the OMCI are outside the scope of this standard. The GTC must provide a transport interface for the OMCI stream. The GTC function provides a way to configure optional channels based on device capabilities, including defining a transport protocol flow identifier (Port-ID).
  The service flow is identified by the service type (GEM mode) and Port-ID. The protocol stack is shown in the following figure. The downlink block or uplink allocation ID (Alloc-ID) bearer data indicates the stream type. The 12-bit Port-ID is used to identify the GEM service flow. The T-CONT is identified by the Alloc-ID and is a set of service flows. Bandwidth allocation and QoS control are done in each T-CONT by allocating BWs(Bandwidth), which are implemented by controlling different numbers of time slots. Different service types must be mapped to different T-CONTs and identified by different Alloc-IDs.

GPON service flow protocol stack (FOT Telecom blog)

GEM service stream

The GEM stream operations in GTC are summarized as follows:

  1. In the downstream direction, GEM frames are carried by GEM blocks and sent to all ONUs. The ONU framing sublayer extracts frames, and the GEM TC adapter filters the cells according to the 12-bit Port-ID. Only frames carrying the correct Port-ID are allowed to reach the GEM client block.
  2. In the upstream direction, the GEM stream is carried by one or more T-CONTs. After the OLT receives the stream associated with the T-CONT, it forwards the frame to the GEM TC adapter and sends it to the GEM client.

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