GEM Frame
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, as shown in Figure 2.
There are five types of T-CONT. T-CONT selection varies during the scheduling of different types of upstream service streams. Each T-CONT bandwidth type has its own quality of service (QoS) feature. QoS is mainly represented by the bandwidth guarantee, which can be classified into fixed, assured, non-assured, best-effort, and hybrid modes.
Type 1:
- 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.
T-CONT
Transmission container (T-CONT) is a service carrier in the upstream direction in the GPON system. All GEM ports are mapped to T-CONTs. Then service streams are transmitted upstream by means of OLT's dynamic bandwidth allocation (DBA) scheduling. T-CONT is the basic control unit of the upstream service stream in the GPON system. Each T-CONT is identified by Alloc-ID. The Alloc-ID is allocated by the GPON port of the OLT, and the T-CONTs used by ONUs connected to the same GPON port of OLT cannot have the same Alloc-IDs.kangyu.
There are five types of T-CONT. T-CONT selection varies during the scheduling of different types of upstream service streams. Each T-CONT bandwidth type has its own quality of service (QoS) feature. QoS is mainly represented by the bandwidth guarantee, which can be classified into fixed, assured, non-assured, best-effort, and hybrid modes.
Type 1:
- The fixed bandwidth is reserved for specific ONUs or specific services on ONUs. It cannot be used by other ONUs even if no upstream service streams are carried on the specific ONUs.
- It applies to services that are sensitive to service quality. The services can be TDM or VoIP services.
Type 2:
- The assured bandwidth is available at any time required by an ONU. When the bandwidth required by the service streams on the ONU is smaller than the assured bandwidth, the system can use the DBA mechanism to allocate the remaining bandwidth to services on other ONUs.
- Because DBA is required, this type provides a less real-time performance compared with the fixed bandwidth.
Type 3:
- This type is the combination of the assured bandwidth and maximum bandwidth. The system assures some bandwidth for subscribers and allows subscribers to preempt bandwidth. However, the total used bandwidth cannot exceed the maximum configured bandwidth.
- It applies to VoIP services.
Type 4:
- This type is the maximum bandwidth that can be used by an ONU, fully providing the bandwidth required by the ONU.
- It applies to IPTV and other high-speed Internet services.
Type 5:
This type is the combination of the fixed, assured, and maximum
bandwidth. It supports the following functions:
- Reserves bandwidth for subscribers and the bandwidth cannot be preempted by other subscribers.
- Provides the bandwidth to an ONU at any time when required
- Allow subscribers to preempt some bandwidth. (The total used bandwidth cannot exceed the maximum configured bandwidth.)
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