PMD Layer
The GPON PMD layer corresponds to the GPON interfaces between Huawei OLTs and Huawei ONUs. Parameter values of the GPON interfaces specify the maximum reach and split ratio for a GPON system.
GTC Layer
The GTA layer is used to encapsulate payloads using ATM cells or GEM frames, and GEM frames are commonly used in GPON systems. GEM frames can carry Ethernet, POTS, E1, and T1 cells.
GTC is the core GPON layer, where media access is controlled for upstream service flows and ONUs are registered. Ethernet frame payloads are encapsulated into GEM frames and then packetized as GTC frames. These GTC frames are converted to binary codes for transmission based on interface parameters configured at the physical layer. The process is reversal on the receive end. Specifically, the receive end decapsulates the data to obtain GTC frames, GEM frames, and then payloads for data transmission.
The GTC layer is classified as TC adaptation sub-layer and GTC framing sub-layer by structure.
The TC adaptation sub-layer involves the ATM, GEM TC, and optical network terminal management and control interface (OMCI) adapters and dynamic bandwidth assignment (DBA) control module. ATM and GEM TC adapters identify OMCI channels by virtual path identifier (VPI)/virtual channel identifier (VCI) or GEM port ID. OMCI adapters
can exchange OMCI channel data with the ATM and GEM TC adapters and send the OMCI channel data to OMCI entities. The DBA control module is a common functional module, which generates ONU reports and controls DBA allocation.
On the GTC framing sub-layer, GTC frames include GEM blocks, PLOAM blocks, and embedded OAM blocks. The GTC framing sub-layer supports the following functions:
− Multiplexes and demultiplexes data. Specifically, the GTC framing sub-layer multiplexes PLOAM and GEM data into downstream TC frames based on the boundary information specified in the frame header. In addition, the GTC framing sub-layer demultiplexes PLOAM and GEM data from upstream TC frames based on frame header instructions.
− Generates frame headers and decodes data. The GTC framing sub-layer generates the TC header of downstream frames in a specified format and decodes the frame header of upstream frames. In addition, the GTC framing sub-layer terminates the embedded OAM data encapsulated into the GTC header and uses the OAM data to control this sub-layer.
− Routes data internally based on alloc-IDs. The GTC framing sub-layer routes the data sent by or to the GEM TC adapters based on internal alloc-IDs.
The GTC layer consists of plane C/M and plane U based on functions.
The protocol stacks of plane C/M include embedded OAM, PLOAM, and OMCI. Embedded OAM and PLOAM channels are used for managing PMD and GTC sub-layer functions. OMCI provides a unified system for upper-layer sub-layer management.
− Embedded OAM channels are defined in GTC frame headers for determining bandwidths, exchanging data, and dynamically allocating bandwidths.
− Dedicated space is reserved in GTC frames for format-based PLOAM channels. The PLOAM channels carry the PMD and GTC management information that does not pass through the embedded OAM block.
− OMCI channels are used for managing services.
Service flows on plane U are identified based on service flow types (ATM or GEM) and port ID/VPI. Port IDs identify GEM service flows and VPIs identify ATM service flows. In T-CONTs, bandwidths are allocated and QoS is controlled using the timeslots that can be adjusted.
