V.35 is the interface used by most routers and DSUs that connect to T-1 carriers.  Although V.35 is commonly used to support speeds ranging anywhere from 48 to 64 Kbps, much higher rates are possible [ ISDN (64 or 128Kbps), Factional T1@ 128 Kbps to 1.544Mbps T1, ATM and Frame Relay]. 

V.35 Cables are high-speed serial assemblies designed to support both higher data rates and connectivity between DTE's (Data Terminal Equipment) or DCE's (Data Communication Equipment) over digital lines.

V.35 cable distances theoretically can range up to 4000 feet (1200 m) at speeds up to 100 Kbps. Actual distances will depend on your equipment and the quality of the cable. 

To achieve such high speeds and great distances, V.35 combines both balanced and unbalanced voltage signals on the same interface. Transmission is usually a synchronous protocol (note clocking pins i.e. receive and transmit clock). 

       V.35 Male                  V.35 Female