What's the GPON Frame Structure

GPON frame structure

Upstream GPON Frame

An upstream GPON frame has a fixed length of 125 µs. Each upstream frame contains the content carried by one or more T-CONTs. All ONUs connected to a GPON port share the upstream bandwidth

• All ONUs connected to a GPON port send their data upstream at their own timeslots according to bandwidth map (BWmap) requirements.
• Each ONU reports the status of data to be sent to the OLT using upstream frames. Then, the OLT uses DBA to allocate upstream timeslots to ONUs and sends updates in each frame.

In Figure an upstream GPON frame consists of the physical layer overhead upstream (PLOu), PLOAM upstream (PLOAMu), power level sequence upstream (PLSu), dynamic bandwidth report upstream (DBRu), and payload fields, as described below,

Field	Description	Function
PLOu	Upstream physical layer overhead	Used for frame alignment, synchronization, and identification for an ONU.
PLOAMu	PLOAM messages of upstream data	Used for reporting ONU management messages, including maintenance and management status. This field may not be contained in a frame but must be negotiated.
PLSu	Upstream power level sequence	Used by ONUs for adjusting optical port power. This field may not be contained in a frame but must be negotiated.
DBRu	Upstream dynamic bandwidth report	Used for reporting the T-CONT status to apply for bandwidth next time and for allocating dynamic bandwidths. This field may not be contained in a frame but must be negotiated.
Payload	Payload user data	Can be a DBA status report or data frame. If this field is a data frame, this field consists of a GEM header and frames.

 

Downstream GPON Frame

A downstream GPON frame has a fixed length of 125 µs and comprises physical control block downstream (PCBd) and payload. PCBd mainly consists of the GTC header and BWmap. The OLT broadcasts PCBd to all ONUs. Then, the ONUs receive the PCBd and perform operations based on the information contained in PCBd.

• The GTC header is used for frame delimitation, synchronization, and forward error correction (FEC).
• The BWMap field notifies every ONU of upstream bandwidth allocation. It specifies the start and end upstream timeslots for the T-CONTs of each ONU, ensuring that all ONUs send data using the timeslots specified by the OLT to prevent data conflict.

PCBd contains PSync, Ident, PLOAMd, BIP, PLend, and US BW Map fields, where US BW Map is the upstream bandwidth mapping sent by the OLT for each T-CONT.

Field	Description	Function
PSync	Physical synchronization domain, frame synchronization information	Used by ONUs to specify the start of each frame.
Ident	Identification domain	Used for sorting a frame in the frames of the same type in length sequence.
Downstream PLOAM (PLOAMd)	PLOAM messages of downstream data	Used for reporting ONU management messages, including maintenance and management status. This field may not be contained in a frame but must be negotiated.
BIP	Bit interleaved parity	Used for performing a parity check for all bytes between two BIP fields (excluding the preamble and delimit) to monitor error codes.
PLend	Length of downstream payloads	Used for specifying the length of the BWmap field.
Upstream bandwidth map (US BW Map)	Upstream bandwidth mapping	Used by the OLT for sending the upstream bandwidth mapping to each T-CONT. The BWmap specifies the start and end times for each T-CONT in transmitting data.

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GPON Protocol Stacks

ITU-T Recommendation G.984.3 defines a new set of frame structures, which consider traditional voice, video, and Ethernet packets as payloads of GPON (GPFD) frames. 

GPON protocol stacks involve the physical medium dependent (PMD) layer and GPON transmission convergence (GTC) layer.

PMD Layer
The GPON PMD layer corresponds to the GPON interfaces between OLTs and ONUs. Parameter values of the GPON interfaces specify the maximum reach and split ratio for a GPON system.

GTC Layer
The GTA layer is used to encapsulate payloads using ATM cells or GEM frames, and GEM frames are commonly used in GPON systems. GEM frames can carry Ethernet, POTS, E1, and T1 cells.
GTC is the core GPON layer, where media access is controlled for upstream service flows and ONUs are registered. Ethernet frame payloads are encapsulated into GEM frames and then packetized as GTC frames. These GTC frames are converted to binary codes for transmission based on interface parameters configured at the physical layer. The process is reversal on the receive end. Specifically, the receive end decapsulates the data to obtain GTC frames, GEM frames, and then payloads for data transmission.

The GTC layer is classified as TC adaptation sub-layer and GTC framing sub-layer by structure.

• The TC adaptation sub-layer involves the ATM, GEM TC, and optical network terminal management and control interface (OMCI) adapters and dynamic bandwidth assignment (DBA) control module. ATM and GEM TC adapters identify OMCI channels by virtual path identifier (VPI)/virtual channel identifier (VCI) or GEM port ID. OMCI adapters can exchange OMCI channel data with the ATM and GEM TC adapters and send the OMCI channel data to OMCI entities. The DBA control module is a common functional module, which generates ONU reports and controls DBA allocation.
• On the GTC framing sub-layer, GTC frames include GEM blocks, PLOAM blocks, and embedded OAM blocks. The GTC framing sub-layer supports the following functions:
  • Multiplexes and demultiplexes data. Specifically, the GTC framing sub-layer multiplexes PLOAM and GEM data into downstream TC frames based on the boundary information specified in the frame header. In addition, the GTC framing sub-layer demultiplexes PLOAM and GEM data from upstream TC frames based on frame header instructions.
  • Generates frame headers and decodes data. The GTC framing sub-layer generates the TC header of downstream frames in a specified format and decodes the frame header of upstream frames. In addition, the GTC framing sub-layer terminates the embedded OAM data encapsulated into the GTC header and uses the OAM data to control this sub-layer.
  • Routes data internally based on alloc-IDs. The GTC framing sub-layer routes the data sent by or to the GEM TC adapters based on internal alloc-IDs.

The GTC layer consists of plane C/M and plane U based on functions.

• The protocol stacks of plane C/M include embedded OAM, PLOAM, and OMCI. Embedded OAM and PLOAM channels are used for managing PMD and GTC sub-layer functions. OMCI provides a unified system for upper-layer sub-layer management.
  • Embedded OAM channels are defined in GTC frame headers for determining bandwidths, exchanging data, and dynamically allocating bandwidths.
  • Dedicated space is reserved in GTC frames for format-based PLOAM channels. The PLOAM channels carry the PMD and GTC management information that does not pass through the embedded OAM block.
  • OMCI channels are used for managing services.
• Service flows on plane U are identified based on service flow types (ATM or GEM) and port ID/VPI. Port IDs identify GEM service flows and VPIs identify ATM service flows. In T-CONTs, bandwidths are allocated and QoS is controlled using the timeslots that can be adjusted.

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