Flow control in a high-speed bus-based ATM switching hub

This paper studies flow control in a high-speed bus-based ATM switching hub for premises switching. The switching fabric is a dual-bus with slots for diverse port cards that interface to the external world. Due to the potentially large switching capacity needed, there can be a significant discrepancy in the switching fabric speed and the port card speed. This can result in buffer overflows at the receiving port buffer and consequently high losses in the switching fabric. Adequate flow control mechanisms are necessary in order to prevent buffer overflows and consequently losses. This paper first examines two different flow control strategies and highlights their strengths and weaknesses. Then we consider a third hybrid strategy which combines the strengths of the first two strategies. In the first flow control scheme, which We refer to as physical flow control, all streams destined to a receiver with buffer problems such as high occupancy and loss are shut down till the congestion is cleared. This scheme has the advantage that loss is severely limited but has the disadvantage that high-rate streams arriving at this buffer can in some circumstances starve lower-rate streams. In a second strategy, referred to as logical flow control, streams are implicitly selected based on their rates and shut down. This scheme has the advantage that the higher-rate streams will eventually be shut down more often and hence cannot overwhelm the lower-rate streams. However, loss is not easily controlled in this scheme. Finally, in a hybrid strategy, we combine physical and logical flow control with logical control activated first and physical control activated later when the logical control is unable to limit the buffer occupancy and loss. We show that this hybrid strategy has the desirable properties of the physical and logical control schemes and hence is the recommended choice for flow control in the setting of interest.