Available Bit Rate(ABR) is one of five classes of service defined by the ATM Forum. It supports variable rate data traffic in flow control, minimum guaranteed data rate, and special performance parameters.
An ABR connection can be divided into several ABR segments, which are controlled separately. Each ABR segment, except the first, has a virtual source( VS, Virtual Source); where the virtual source emulates the behavior of an ABR source. Backward RM cells received from a virtual source are removed from the link. Terminated ABR segments, except for the last segment, are terminated by virtual terminators (VD, Virtual Destination), which emulate the behavior of ABR terminators.