The function of the port is take the packets that have been stored in the out put port memory and transmits them over the out going link. The queuing and buffer management functionality required when the switch fabric delivers packets to the output ports at a rate that exceeds the output link rate.
Figure output port processing
Output port queuing is that a packet scheduler at the output port must over packet among those queued for transmission. This selection might be done on a simple basis. Such as first come first served scheduling. Or a more sophisticated scheduling such a weighted fair queuing which shares the outgoing link fairly among the different end to end connections that have packet queued for transmission.
Figure output port queuing
Figure shows the output port queuing. At time a packet has arrived at each of the incoming input ports each destined for the outgoing port. Assuming identical line speeds and a switch operating at three times the line speed, one time unit latter that is in the time needed to receive or send a packet all three original packets have been transferred to the outgoing out and are queued awaiting transmission. In the next time unit one of these three packets will have been transmitted over the outgoing link. By the example two new packet shave arrived at the incoming side of the switch one of these packets is destined for output port.