Hello, everyone!
Today's topic is GEM frame and T-CONT. Let's meet them!
GEM frame
GEM stands for Gigabit Passive Optical Network Encapsulation Mode. It is the smallest service unit and the basic encapsulation structure. All services in the GPON system are encapsulated in GEM frames. Service streams are identified via GEM Ports. One GEM port can have one or more services. The GEM port is a uniquely tagged port ID by OLT. There cannot be two identical port IDs on the same PON port. But different PON ports may have the same port IDs. The GEM port identifies the virtual service channel that is established between OLTs and ONUs.
In the next figure, we can see the GEM frame.
It is composed of a GEM header and payload. A GEM header has four fields:
PLI. This field indicates the length of the payload data. The length of the field is 12 bits.
Port ID. This field is used to identify the GEM port I was talking about. The length of the field is 12 bits.
PTI. This field identifies the payload type. The length of the field is 3 bits.
HEC. This is an abbreviation of Header Error Correction. This enables FEC (Forward Error Correction) and thus increases transmission reliability. It is important for sensitive services because retransmission is not required. The length of the field is 13 bits.
T-CONT
The T-CONT or Transmission container carries services in the upstream direction. T-CONT is a uniquely determined AlloC-ID by OLT. There may not be two or more of the same Alloc-IDs on the same PON port, but there may be the same Alloc-IDs on different PON ports.
There are five types of T-CONT:
Type 1 (Fixed bandwidth). This type is usually used for sensitive services or specific ONUs. They can be TDM or VoIP services.
Type 2 (Assured bandwidth). This type can be used for VoIP, MNG, and the same services.
Type 3 (Non-assured bandwidth). This is the combination of assured and maximum bandwidth. It is usually used for VoIP services.
Type 4 (Best-effort). It is maximum bandwidth, it is used for IPTV and HSI (High Service Internet).
Type 5 (Hybrid mode). This type is a combination of all types.
The following figure shows us the mapping.
One or more GEM Ports are mapped to one T-CONT. One or more services are mapped to one GEM Port. The recommendation is one T-CONT, one GEM Port, one service.
This is the end of this article. I hope you learned something new!
Thank you!
