Hi there, Community friends!
This time, I will share with you GPON FEC. I hope you like
it!
In actual applications, the transmission of digital signals introduces bit errors and jitter, which degrade signal transmission quality.
To resolve the preceding issue, an error correction technology is required. Among the error correction technologies, the effective ones achieve transmission reliability by reducing bandwidth usages, which also increases telecom device complexity. The error correction technologies are used for controlling errors. The codes involved in these technologies are classified as error detection codes and error correction codes based on usage scenarios.
Error detection codes, such as parity check codes, are used for detecting error codes.
Error correction codes, such as BCH codes, Reed-Solomon (RS) codes, and Hamming codes, are used for automatically correcting errors.
The only difference between the error detection codes and error correction codes lies in performance parameters applied in different usage scenarios. FEC uses error correction codes.
FEC is a data coding technology, which enables the RX end to check error bits in transmission based on the coding data. FEC is unidirectional, not supporting error information feedback.
Redundant codes are added to signals on the TX end. Then, the RX end checks the signals for errors based on error-correcting code (ECC) and corrects errors is there is any. Common FEC codes include Hamming codes, RS codes, and convolutional codes. The following figure shows FEC principles.
Figure 1 FEC principles
In the GPON FEC algorithm, the most common RS code RS (255,239) is used, where the code word is 255 bytes long, consisting of 239 data bytes followed by 16 overhead redundant bytes. RS code RS (255,239) complies with ITU-T Recommendation G.984.3. The FEC algorithm drops the bit error rate (BER) of 10-3 to 10-12 for GPON lines. However, due to the overhead caused by multi-frame tail fragments, the bandwidth throughput of the GPON system with FEC enabled is about 90% of that with FEC disabled.
FEC characteristics are as follows:
Does not require data retransmission, thereby improving real-time efficiency.
Enables lines to tolerate louder noises on a basis of a higher bandwidth overhead. (In this case, users must balance between the transmission quality and the bandwidth usage based on site requirements.)
Based on the preceding characteristics, FEC applies to:
The services requiring error detection and correction at the RX end without retransmission.
Data transmission if the network is in a poor condition. For example, the transmission distance from the OLT to an ONT is long or the transmission line is of poor quality, which results in insufficient optical power budget or high BERs.
The services requiring no delays (a retransmission prolongs the delay).
FEC status can be configured in GPON systems based on GPON ports in the downstream direction (by running the port fec command) and based on ONUs in the upstream direction. To configure the FEC status in the upstream direction based on ONUs, run either of the following commands:
In profile mode, run the fec-upstream command.
In discrete mode, run the ont fec-upstream command.
That's all, I welcome everyone to leave a message and exchange in the comment area!
Thank you！
