| Thread Links | Date Links | ||||
|---|---|---|---|---|---|
| Thread Prev | Thread Next | Thread Index | Date Prev | Date Next | Date Index |
All, Andy Bach asked me to forward his reply to Vipul’s email to the reflector. Regards, John Per Andy That would have been me In the actual hardware that we have tested and used and depending on the vendor the FEC – no FEC time can easily be upwards of 10microseconds To the second point the BER given the quality of the fiber and the short distances in the metro region is very low From: Vipul Bhatt [mailto:Vipul_Bhatt@xxxxxxxx] Dear colleagues, In the last BWA conference call, if I recall correctly, someone stated that for applications like financial transactions, FEC is often turned off to reduce latency -- even for traffic between New York and Chicago. Please help me understand that. Light propagation delay in fiber is ~5 microseconds per kilometer. In contrast, FEC latency would be sub-microsecond. Even within a data center of 300 meter span, a round-trip through just one switch can add up to several microseconds. On the other hand, turning off FEC may increase BER and make TCP retransmissions more likely, thus adding to latency. Or is it that in such links, the BER is so low that FEC ends up being all latency and no benefit? Thank you. Regards, Vipul Vipul Bhatt (408) 461-8521 [Re-sent, with apologies for inadvertently including a restrictive notice in the previous message.] |