GPON Framing Structure:Downlink Frame

GPON Framing Structure

Downlink Frame Structure

GPON Transmission Convergence frame

GPON Transmission Convergence frame


Each GTC (GPON Transmission Convergence) frame consists of PCBd (Physical Control Block downstream) and payload.

  1. PCBd contains sync, OAM, DBA info, etc.
  2. payload may have ATM and GEM partitions (either one or both); 

Downlink frame’s fixed 125us frequency is 8000Hz,and it’s size is 38,880 bytes, when speed rate reaching 2.488Gbps.

GTC payload potentially has 2 sections:

  • ATM partition (Alen * 53 bytes in length).
  • GEM partition (now preferred method);

Because GTC payload definitely is the “payload” of GEM Frame in downstream. So firstly let’s focus our attention on PCBd,the downlink control information frame’s header.

Downstream Framing structure

Downstream Framing structure

For better showing the detail structures inside GPON downstream frame,we need a very clear diagram,just like the above one.



Synchronization for olt and onu.(4 bytes)


MSB indicates if FEC is used, 30 LSBs are superframe counter.

1bit FEC(Forward Error Correction)Ind + 1bit  reserve + 30bits Super-frame Counter. (4 bytes)


carries OAM, ranging, alerts, activation messages, etc. GPON uses PLOAMd as control channel.

1Byte (8bits) ONUid + 1Byte (8bits) message segment + 10Bytes Content body + 1Byte (8bits) CRC checksum bits. (13 Bytes)


SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP.(1 Byte)

PLend: (4 Bytes)

Transmitted twice for robustness, including three parts:

  • Blen – 12 MSB are length of BW map in units of 8 Bytes
  • Alen – Next 12 bits are length of ATM partition in cells
  • CRC – final 8 bits are CRC over Blen and Alen

US BW map:

The array of Blen 8Bytes structures granting BW to US flow, telling in which time segment the ONT should transmit the data.

Each access 8Bytes defines a time message.More detail description is sharing in our blog “Gpon-Epon’s key technologies:DBA & T-CONT ”. (N*8Bytes)

BWmap is sent by OLT to ONUs ,including a list of these fields:

  • Alloc-id: ONU allocation IDs
  • Flags: Used to indicate the behavior of ONU, such as, next uplink data transmission(PLOAMu,PLSu,DBRu etc).
  • S-start,S-stop:start and stop times (16bits fields, in Bytes from beginning of US frame)
  • CRC:CRC(cyclic redundancy check) Checksum.

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