Output Port

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.