If the distance between the two sites is below 150 miles or 200 KM we can go ahead with SRDF Synchronous.
If its more than 150miles then SRDF Asynchronous is preferred for Replication
Though the theoretical limit is 200Km but in real life scenario the distance limitation is less.The distance actually depends on many parameters like-Data Volume,Rate of change of Data,Desired latency time in the Production site.
Light in Fiber need to pass through from source to target and again back to source for the acknowledgement of IO in the Symmetrix.Its the round trip time which is in the range of 8 ms .If huge amount of data need to be replicated over a considerable distance then IO accelerator card should be used in the fabric configurations.
Years ago I had a couple of DMX2 and DMX3s doing SRDF/S over 370 km ! Ok, we used true FC from DMX to the switches and FCIP between the Cisco 9216is on each site, but still: the distance on both paths was around 370 km (230 miles). The latency is the key factor here and we had 6 ms between the two sites if I remember correctly. The bandwidth we were using was about 1Gbps on each link and the throughput I actually measured was 400MBps for a new empty META (yes, MegaBytes, not Megabits! --> the Cisco 92xx series are doing a great job with their data compression !!)