Dear 802.3dj TF Participants,
During the Ad Hoc call, Mike Dudek presented an important proposal for PMD reuse.
There was broad recognition of the benefits of reducing latency by bypassing the inner FEC. However, there was also concern about autonomously switching modes.
It may benefit to look back at how modes are switched in communications and Ethernet. The first widespread digital communication devices were voiceband modems which initiate at low rate guaranteed to close the link, and increase the rate as progressively more powerful equalization enables faster and higher order modulation, i.e. same basic idea as proposed by Mike. The drawback is that one doesn't know until after the highest rate is established, how fast the link will be. In contrast, Ethernet has always guaranteed a fixed rate. Even though many optical modules ship as multirate, each rate is a separate PDM spec. The datacenter operator decides when configuring their network which rate to operate at based on what performance the PDM specs guarantee.
This suggests that the desired behavior for selecting whether the inner FEC is bypassed or not, is by the datacenter operator when configuring their network, not autonomously in real time after the module is plugged in. If a shorter latency is not guaranteed, then the link must be assumed to operate at a longer latency for the purposes of defining the network configuration. What the operator needs is information on capability, which means two independent PMD specs, one with inner FEC turned on, and one with inner FEC turned off, available before the network is configured. The BER rate can be one of the many real time diagnostics provided to the operating system, just like optical power and temperature. It is unlikely that the operator would want the module to configure the network.
It would be helpful if any datacenter operator on the reflector chimed in and commented whether there is a benefit for optical modules to configure a network autonomously. If yes, sharing the application scenario would further help understand the need.
Thank you
Chris