Switching  via memory

Traditional  computers with switching  between  input and output  being  doen under  direct  control  of the CPU input and output  ports  functioned as traditional I/O devices in a traditional  operating systems. Aninput port with an arriving  packet first signalled the routing  processor. The packet  was then copied  from the  input   port into processor  memory. The routing  processor  then extract the destination address from the heard looked  up the appropriate output port in  the forwarding table and the packet to the output  buffers. Note  that if memory  bandwidth  is B packets per second  can be written into  or read from memory them the overall forwarding throughput ( the  total  rate at which  packets are transferred   from input  ports to output  ports ) must be  less than B/2.