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GPON Unlink Frame Structure
A PON’s upstream channel behaves in a model of point to point communication. Here, each ONT transmits content to the OLT at 1310nm. It is therefore necessary to use arbitration transmission channel models by use of TDMA (Time Division Multiple Access) for each ONT send the information in different times. This control is done from the OLT. At the same time, all users are synchronized through a process known as “Ranging.”
US (Up Stream) GPON format
As shown in the picture below, there are 4 different US overhead types:
PLOu: Physical Layer Overhead upstream – always sent by ONU when taking over from another ONU – contains preamble and delimiter (lengths set by OLT in PLOAMd) BIP (1B), ONU-ID (1B), and Indication of real-time status (1B)
PLOAMu: PLOAM upstream (13B) – messaging with PLOAMd
PLSu: Power Levelling Sequence upstream (120B) – used during power-set and power-change to help set ONU power so that OLT sees similar power from all ONUs
DBRu: Dynamic Bandwidth Report upstream – sends traffic status to OLT in order to enable DBA computation
Payload: Payload data, may be a data frame or DBA status report，Payload=（DBA Report + Pad）/（Gem Header + Gem Frame)
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Different US overhead types
GEM introduction
Unlike ATM cells, GEM（GPON Encapsulation Mode）delineated frames may have any length, any number of GEM frames may be contained in the GEM partition. ONUs accept GEM frames based on 12b Port-ID in GEM header
- GEM is generic – any packet type (and even TDM) supported
- GEM supports fragmentation and reassembly
GEM（GPON Encapsulation Mode） is based on GFP, and the header contains the following fields:
- PLI: Payload Length Indicator – payload length in Bytes
- Port ID : Identifies the target ONU
- PTI: Payload Type Indicator (GEM OAM, congestion/fragmentation indication)
- HEC: Header Error Correction field (BCH(39,12,2) code+ 1b even parity)
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