Method and apparatus providing programmable thresholds for half-duplex flow control in a network switch

Abstract

A network switch having a shared memory architecture for storing data frames has a set of programmable thresholds that specify when flow control should be initiated on a selected network port. The network switch includes a queue for storing free frame pointers, each specifying available memory locations in an external memory for storing data frames received from a network station. The network switch takes a frame pointer from a free buffer queue for each received data frame, and stores the received data frame in the location in external memory specified by the frame pointer while a decision making engine within the switch determines the appropriate destination ports. Flow control is initiated on a half-duplex network port by transmitting a phantom packet to a transmitting network station if the output buffer of a destination port exceeds a programmed high threshold, or if the output buffer of the destination port exceeds a low programmed threshold and the queue of free frame pointers falls below a low programmable threshold. The switch thus provides flexibility in providing selective collisions with transmitting stations to minimize the possibility of network congestion without wasting network bandwidth with unnecessary collisions.