In the gigabit-capable passive optical network (GPON) system, a GPON encapsulation mode (GEM) frame is the smallest service-carrying unit and the basic encapsulation structure. All service streams are encapsulated into the GEM frame and transmitted over GPON lines. The service streams are identified by GEM ports and each GEM port is identified by a unique port ID. The port ID is globally allocated by the OLT. Therefore, the ONUs connected to the same OLT cannot use GEM ports that have the same port ID. A GEM port is used to identify the virtual service channel that carries the service stream between the OLT and the ONU. It is similar to the virtual path identifier (VPI)/virtual channel identifier (VCI) of the asynchronous transfer mode (ATM) virtual connection.
A GEM header consists of PLI, Port ID, PTI, and header error check (HEC) and is used for differentiating data of different GEM ports.
PLI: indicates the length of data payload.
Port ID: uniquely identifies a GEM port.
PTI: indicates the payload type. It is used for identifying the status and type of data that is being transmitted, for example, whether the operation, administration and maintenance (OAM) message is being transmitted and whether data transmission is complete.
HEC: ensures the forward error correction (FEC) function and transmission quality.
Fragment payload: indicates the frame fragment.
The following section describes the GEM frame structure based on the mapping of the Ethernet service in GPON mode
The GPON system parses Ethernet frames and maps data into GEM payloads for transmission.
Header information is automatically encapsulated into GEM frames.
The mapping format is clear and has good compatibility.