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

A method in a network switch for selectively outputting a data frame from a transmitting station to a destination station, comprising;

• setting a first queue threshold level for a first queue configured for storing free frame pointers, the free frame pointers specifying respective memory locations available for storing received data frames;
• setting a second queue threshold for a port queue configured for storing assigned frame pointers identifying stored data frames for the destination station;
• receiving a portion of a first data frame from the transmitting station;
• selectively halting transmission of the first data frame based on the first and second thresholds relative to a number of the stored free frame pointers and a number of the stored assigned frame pointers, respectively;
• wherein the selectively halting step comprises outputting an unrecognizable data frame to the transmitting station within a prescribed interval of receiving the portion of the first data frame;
• setting for the port queue a third queue threshold having a value greater than the second queue threshold, the selectivity halting step further comprising outputting the unrecognizable data frame based on at least one of;
• the number of stored assigned frame pointers exceeding the second and third queue threshold, and
• the number of the stored free frame pointers falling below the first queue threshold level and the number of assigned frame pointers exceeding the second queue threshold.

An apparatus for selectively outputting a data frame from a transmitting station to a destination station in a network, comprising:

• a first port for receiving at least a portion of the data frame from the transmitting station;
• a first queue for storing free frame pointers, the free frame pointers specifying respective memory locations available for storing received data frames,
• a first programmable threshold register for storing a first threshold value;
• a second port for outputting data frames identified for transmission to the destination station;
• a controller for selectively causing the first output port to induce a collision with the transmitting station during reception of the data frame from the transmitting station, based on a number of the free frame pointers stored in the first queue relative to the first threshold value;
• the second port comprises a second queue for storing assigned frame pointers specifying respective storage memory locations for the identified data frames;
• the apparatus further comprises a second programmable threshold register storing a second threshold value;
• the controller causes the first output port to induce the collision based on a number of the stored assigned frame pointers exceeding the second threshold value;
• and
• a third programmable threshold register storing a third threshold value greater than the second threshold value, the controller causing the first output port to induce the collision based on the number of stored assigned frame pointers exceeding the third threshold value and the number of free frame pointers falling below the first threshold value.