Hello, everyone!
Today, I want to continue to explain the XG(S)-PON technology. This is the second article about basic concepts. Now, I will explain XG(S)-PON service mapping in the upstream and in the downstream and XGEM frame.
Figure 1. Service mapping
In the upstream direction, XG(S)-PON technology uses TDMA mode, as I said in a previous article. Mapping services are done on different XGEM ports on the ONU. Then, XGEM ports are mapped to T-CONTs. Finally, it is uploaded to the OLT. OLT demodulates the T-CONT in the GEM port unit. It sends the XGEM port payload to the XG(S)-PON MAC chip. This chip demodulated the XGEM port. And the final step, this chip emits the demodulated XGEM port payload to the unit for service processing.
In the downstream direction, XG(S)-PON technology uses broadcast mode. OLT encapsulates services in the XGEM port of the service processing unit. Then, OLT sends a downstream frame to all ONUs. ONUs have XGEM port ID, thanks to that, ONUs can extract their data.
In the XG(S)-PON system, virtual channels between OLT and ONUs are identified via XGEM ports. Each XGEM has a unique XGEM port ID. It is allocated by the OLT. One or more types of services are carried by that GEM port.
T-CONT (Transmission Container) is the basic control unit and is used to transfer data upstream. One or more XGEM ports can be mapped to one T-CONT. It depends on the configuration. T-CONT is uniquely identified with Alloc-ID, which is assigned by OLT.
Figure 2. XGEM
XGEMis the main and basic unit in XG(S)-PON technology. For transmission, all services are encapsulated into XGEM frames. It is composed of an XGEM header and XGEM payload. We can see an XGEM in figure 2. XGEM header consists of:
PLI (Payload Length Indication). It indicates the length of the data payload. This field is 14 bits,
Key Index. This field indicates the key for encryption. It has 2 bits,
XGEM Port-ID. It describes the XGEM port and this field has 16 bits,
Options. This field has 18 bits, and it is reserved for the future,
LF (Last Fragment). This field has only one bit, and it indicates whether the fragment is the last fragment of an XGEM frame, and
HEC (Header Error Correction). This is the last field, it has 13 bits and it is used for error correction.
This is the end of this article. I will continue to write about this topic in the next article >> XG(S)-PON (4): Key technologies.
Thank you!
