Hi, everybody.
This article will introduce concepts related to GPON, such as the GEM frame and T-CONT.
The GEM (GPON Encapsulation Mode) frame is the smallest service bearer unit in the GPON technology. It is the basic data structure. All services are encapsulated in GEM frames and transmitted on the GPON line, which is identified by GEM Port.
Each GEM Port is identified by a unique Port ID and is globally allocated by the OLT. That is, each ONU under each GPON port cannot use a GEM Port with a repetitive Port ID.
The GEM Port identifies the service virtual channel between the OLT and the ONU, that is, the channel that carries the service flow, which is similar to the VPI (Virtual Path Identifier) /VCI (Virtual Channel Identifier) identifier in the ATM virtual connection.
PLI, Port ID, PTI, and HEC (Header Error Check) constitute a GEM frame header, which is mainly used to distinguish data in different GEM Ports. The meanings of the fields are as follows:
· PLI: Indicates the length of the data payload.
· Port ID: Indicates the unique GEM Port
· PTI: Payload type ID, which is used to identify the status and type of the currently transmitted data, for example, whether the message is an OAM (Operation, Administration and Maintenance) message or whether the data has been transferred.
· HEC: Provides the forward error correction coding function to ensure transmission quality.
· Fragment Payload: Indicates the frame fragment of the user data.
The GPON system parses the Ethernet frames and maps the part of data to the GEM Payload for transmission.
The GEM frame can automatically encapsulate the header information, and the mapping format is clear and compatible well.
The T-CONT (Transmission Container) is the carrier that carries services in the upstream direction of the GPON. All the GEM Ports will be mapped to the T-CONT.
T-CONT is the basis of DBA(Dynamic Bandwidth Allocation) implementation. The DBA of upstream service flows of the entire GPON system is implemented by applying for T-CONT bandwidth on ONU and authorizing to T-CONT on OLT.
The T-CONT is the most basic control unit of the upstream bandwidth in the GPON system. Each T-CONT is identified by a unique Alloc-ID. The Alloc-ID is allocated by each GPON port of the OLT. That is, the ONUs connected to the same GPON port of the OLT do not have the T-CONT with the same Alloc-ID.
T-CONTs(Transmission Containers): Dynamically receives the authorization issued by the OLT, which is used to manage the upstream bandwidth allocation at the transmission convergence layer and improves the upstream bandwidth in the PON system.
T-CONTs is a pipeline that carries ATM PVC and GEM Port. A single T-CONT can carry multiple GEM Ports. An ONU can contain multiple T-CONTs. The uplink bandwidth is divided based on T-CONT, the different services use different T-CONTs.
A GEM port is used as a service channel and can carry one or more service flows.
The number of T-CONTs supported by the OLT and ONU is limited by hardware implementation and is one of the important system specifications.
Thanks.
