We are participating in the IEEE 802.3bj standardization effort, which has the objective to standardize a low-latency transcoding (TC) and forward error correction (FEC) sublayer for 100 Gb/s ethernet transmission over backplane and twin-axial copper cable channels. The architecture of the TC/FEC sublayer that is being standardized in 802.3bj must allow an implementation that exhibits a total TC/FEC latency (sum of the latencies at the transmitter and the receiver) of less than 100 ns. Both TC and FEC are performed at an aggregate rate of about 100 Gb/s in order to minimize total TC/FEC latency. However, signal transmission over backplane and twin-axial copper cable channels is over four separate physical links operating at rates slightly above 25 Gb/s. Reed-Solomon (RS) FEC coding is employed in order to correct burst errors at the output of decision feedback equalizers and minimize total FEC latency. Two different signaling schemes, NRZ and four-level PAM are considered for standardization to serve different markets with different requirements of insertion loss and power dissipation. Conventional high-rate transcoding schemes reshuffle the order of the 66-bit data or control blocks that are compressed.
We are active in the areas of FEC coding design, transcoding, and overall error protection assessment. For instance, we have designed a novel 256b/257b low-latency transcoding scheme without reshuffling for IEEE 802.3bj that reduces the overall TC/FEC latency, the total TC/FEC gate complexity and the total TC/FEC power dissipation considerably.
We are also investigating the error mechanisms of all relevant TC/FEC coding schemes and the resulting performance measure mean time to false packet acceptance (MTTFPA) to propose appropriate error protection strategies. In a well-designed ethernet transmission system, undetected errors in medium access control (MAC) layer packets, which are protected by a 32-bit cyclic redundancy check (CRC) code, are extremely rare. Although the required bit error rate for data transmission in 802.3bj is 10-12 or lower, a link is only shut down if the bit error rate is higher than 10-4. Therefore, MTTFPA should be sufficiently large for all bit error rates lower than 10-4. Assuming that the FEC decoder is turned on, several error detection schemes must fail before an undetected MAC packet error occurs. First, a miscorrection event must occur at the output of the FEC decoder. Second, the inverse transcoder has to fail to detect an error event due to FEC miscorrection. Finally, MAC CRC must fail to detect a packet error. Analysis of the resulting MTTFPA suggests that FEC encoding should be mandatory without the possibility to bypass FEC if the sublayers above and below the TC/FEC sublayer shall remain unchanged.