  The Available Bit Rate (ABR) service in ATM networks has been specified to
allow fair and efficient support of data applications over ATM utilizing
capacity left over after servicing higher priority classes. One of the
architectural features in the ABR specification [tm4] is the Virtual
Source/Virtual Destination (VS/VD) option. This option allows a switch to
divide an end-to-end ABR connection into separately controlled ABR segments by
acting like a destination on one segment, and like a source on the other. The
coupling in the VS/VD switch between the two ABR control segments is
implementation specific. In this paper, we model a VS/VD ATM switch and study
the issues in designing coupling between ABR segments. We identify a number of
implementation options for the coupling. A good choice significantly improves
the stability and transient performance of the system and reduces the buffer
requirements at the switches.
