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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