Mapping of Ethernet in GPON

GEM Format

PTI Type

Transmission convergence layer

Types of PONs

T-CONT:

A traffic bearing object within an ONT that represents a group of logical connections, and is treated as a single entity for the purpose of upstream bandwidth assignment on the PON. In the upstream direction, it is used to bear the service traffic. Each T-CONT corresponds to service traffic of one bandwidth type. Each bandwidth type has its own QoS feature.

ALLOC_ID:

Each T-CONT is identified by the ALLOC_ID uniquely. The ALLOC_ID ranges from 0 to 4095. It is allocated by OLT i.e. a T-CONT, and can only be used by one ONT per PON interface on the OLT.

GEM Port:

A GPON Encapsulation Method (GEM) port is a virtual port for performing GEM encapsulation for transmitting frames between the OLT and the ONT. Each different traffic-class (TC) per UNI is assigned a different GEM Port. Each T-CONT consists of one or more GEM Ports. Each GEM port bears one kind of service traffic i.e. a T-CONT type.

GEM Port ID:

Each GEM Port is identified by a port ID uniquely. The Port ID ranges from 0 to 4095. It is allocated by the OLT, i.e. a GEM port, and can only be used by a single ONT per PON interface on the OLT.

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)

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

  PCBd contains sync, OAM, DBA info, etc.
  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.

Psync:

Synchronization for olt and onu.(4 bytes)

Ident:

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)

PLOAMd:

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)

Bip:

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.