For best performance of FC, ensure correct number of buffer credits in switch, (speed of FC/2 * km), and use native extension. Native extension is particularly relevant if sync disk replication is used, as you need to be absolutely sure the sent data is written at the far storage, and not spoofed.
If distance is beyond 50-100km (it depends on what the application is) or so native synchronous replication will cause the application to run too slowly (block write time is extended by 1ms at 100km), in really chatty applications like SAP this can be critical. OSA is very low latency DWDM equipment so the additional latency of the DWDM is negligible compared to the fibre at 5 microseconds per km. If any other carrier (Geo, Vtesse, Colt etc) have fibre, then they can do similar to BT. Hope this helps Dave David Whitaker ADVA Optical Networking SE
