V.35 - the original serial interface
V.35 is the ITU (formerly CCITT) standard termed “Data Transmission at 48 kbps Using 60–108 KHz Group-Band Circuits.
Basically, V.35 is a high-speed serial interface designed to support both higher data rates and connectivity between DTEs (data-terminal equipment) or DCEs (data-communication equipment) over digital lines.
V.35 combines the bandwidth of several telephone circuits to provide the high-speed interface between a DTE or DCE and an NTU (Network Terminating Unit).
Although it’s commonly used to support speeds ranging anywhere from 48 to 64 kbps, much higher rates are possible. For instance, maximum V.35 cable distances can theoretically range up to 610m at speeds up to 64 kbps. Actual distances will depend on your equipment and cable. To achieve such high speeds and great distances, V.35 combines both balanced and unbalanced voltage signals on the same interface.
RS-232 - A group of specifications for interfaces