Figure 1-8 shows the GPON frame structure. The GPON downstream frame is invariably 125 µs long and it comprises Physical Control Block downstream (PCBd) and Payload. PCBd mainly consists of the GTC header and the upstream bandwidth map (BWmap). The GTC header is mainly used for performing frame delimitation, clock synchronization, and FEC. The BWmap is mainly used for notifying every ONU of its upstream bandwidth allocation, thereby
determining the start and end timeslots of the T-CONT corresponding to every ONU in the upstream transmission process. In this way, all ONUs send data according to timeslots uniformly specified by the OLT and data collision is avoided. In the upstream direction, service scheduling is performed in the TDMA mode according to T-CONT. All ONUs connected to a GPON port share the upstream bandwidth and send their data upstream at their own timeslots according to the BWmap requirements. At the same time, every ONU reports its status of data to be sent to the OLT through the upstream frame. Then, the OLT uses DBA to allocate upstream timeslots to ONUs and sends updates in every frame.
Module
Feature Description 1 GPON
Figure 1-8 GPON frame structure
T-CONT1 (ONT 1)
T-CONT 2 (ONT 2)
257 100 200 258 300 500
AllodID Start End AllodID Start End Payload
125 µs Downstream framing
PLOu PLOAMu PLSu DBRu Payload x DBRu Y Payload y
Upstream framing Upstream Bandwidth Map Physical Control Block
Downstream (PCBd)
l PLOu: Physical Layer Overhead upstream l PLOAM: Physical Layer OAM
l PLOAMu: PLOAM upstream
l PLSu: Power Levelling Sequence upstream l DBRu: Dynamic Bandwidth Report upstream l The current application is:
l GPBC: Alloc-ID = T-CONT ID x 256 + ONU ID
l GPBD: When T-CONT ID < 8, Alloc-ID = T-CONT ID x 256 + ONU ID. When T-CONT ID ≥ 8, Alloc-IDs are assigned automatically from the first idle Alloc-ID.
The lengths of the upstream frame and downstream frame at each GPON rate are the same. Every upstream frame contains the content carried by one or more T-CONTs. The BWmap in each downstream frame identifies the start time and end time of each T-CONT transmission. An ONU must send a PLOu each time before the ONU receives the media access right to PON from another ONU. If an ONU is allocated two consecutive Alloc-IDs (the end time of one is smaller by 1 than the start time of the other), the ONU must not send the PLOu of the second Alloc-ID.
The payload of an upstream frame may contain three types of content: the ATM cell, the GEM frame, and the DBA report.
Figure 1-9 shows the GPON upstream frame structure.
Module
Feature Description 1 GPON
Figure 1-9 GPON upstream frame structure
PLOu DB
PLSu RuX Payload X DB
RuYPayload Y
ONU A ONU B
Upstream Frame
PLO
AMu PLOu DB
RuZ Payload Z
The GPON upstream frame consists of the PLOu, PLOAMu, PLSu, DBRu, and Payload fields and the meanings of these fields are described as follows:
l PLOu: physical control header, mainly used for frame delimitation, synchronization, and indication of which ONU the current frame targets at.
l PLOAMu: PLOAM message of upstream data, mainly used for reporting management information such as ONU maintenance and management status. (Not every frame has such a field. This field may not be sent but needs to be negotiated.)
l PLSu: Power Levelling Sequence upstream. It is a 120-byte field and is used for power control measurements by the ONU.
l DBRu: mainly used for reporting the T-CONT status for applying for bandwidth and completing dynamic bandwidth allocation for ONUs next time. (Not every frame has such a field. This field may not be sent but needs to be negotiated.)
l Payload: DBA status report or data frame. The data frame may be GEM header or frame.
l GEM header: mainly used for differentiating data of different GEM ports. The GEM port is the smallest unit for data transmission in the GPON system, which is similar to the PVC of ATM. Every type of upstream service stream must be mapped to the GEM port and then to the T-CONT for transmission. The GEM header field consists of PLI, Port ID, PTI, and HEC.
– PLI: Indicates the length of data payload.
– Port ID: Uniquely identifies a GEM port.
– PTI: Identifies the payload type. It is mainly used for identifying the status and type of data that is being transmitted (for example, whether the OAM message is being transmitted and whether data transmission is complete).
– HEC: Provides the FEC function and transmission quality.
GPON supports a downstream transmission rate of 2.488 Gbit/s, a frame length of 38880 bytes, and a frequency of one frame every 125 µs, as shown in Figure 1-10 and Figure 1-11.
Module
Feature Description 1 GPON
Figure 1-10 GPON downstream frame structure
TDM & Data Fragments over GEM Section N * 53
bytes
"Pure" ATM cells Section
T 125us
Figure 1-11 PCBd structure
US BW Map
Coverage of next BIP Coverage of this BIP
PLOAMd
The OLT broadcasts PCBd to all ONUs. Every ONU receives the entire PCBd and then acts upon the relevant information contained therein.
A PCBd contains information such as frame synchronization information, physical layer OAM information, and BIP check field. US BWMap (upstream bandwidth map) is the upstream transmission bandwidth map sent to each T-CONT by the OLT. The bandwidth map is transmitted through the US BW Map field in the PCBd of the downstream frame. In this way, MAC control is implemented.
GPON uses TDM for the upstream transmission. Therefore, when multiple ONUs transmit data upstream concurrently, transmission conflicts occur. The avoidance mechanism for such a conflict is that the OLT sends a notification through the downstream frame, informing each ONU of its corresponding timeslot for upstream transmission.
1.7 Key GPON Technologies
Key GPON technologies include the burst optical/electrical technology, ranging, forward error correction (FEC), line encryption, and dynamic bandwidth allocation (DBA).