Using Full Duplex: Making the Streets Two Way

Using Full Duplex: Making the Streets Two Way

The network interface cards (NICs) in the computers in this chapter still use CSMA/CD logic. In most cases, because the switch forwards frames only out the right destination port, most of the PCs simply weren't receiving frames the majority of the time. With CSMA/CD, if you are not receiving a frame, it's okay for you to go ahead and send. The switch avoids collisions by buffering the frame if the output port is busy.

However, there is still one case for which a computer NIC, using CSMA/CD, believes it should not send. That's when the NIC is actually receiving a frame. Imagine that Fred is sending another frame to Barney, as in Figure 6-6. Barney wants to send a frame. (In this case, Barney wants to send a frame to Fred, but the destination does not really matter in this scenario.)