CAS T1 RBS E1 R2 Protocol Stacks -- C Source Code
T1 lines carry 24 voice channels at almost 64K bits per second. The "almost" is because the least significant bit in one out of every six bits is "robbed" to provide signalling via a protocol known as "Robbed Bit Signalling" or "RVS". This information is used to convey on-hook/off-hook stgatus, dialed digits and otgher information.
R2 is similar, but is used on an E1. Unlike RBS, R2 is transmitted out of band--all 30 voice channels on an E1 are sent over a 31st 64Kbps channel.