Barney Waiting to Send, When Fred Sends to Barney

With Barney's CSMA/CD logic enabled, he must wait before sending a frame. However, physically, there is a single cable between Barney and the switch. No collisions can occur on that cable because Barney sends on the pair using pins 1 and 2, and the switch sends to Barney on the pair using pins 3 and 6. And you already know that the switch will buffer any frames, rather than cause a collision, so there is truly no danger of a collision.

In this particular case, Barney chooses not to send the frame to Fred because his CSMA/CD logic tells Barney to wait. But if Barney sent the frame, he wouldn't cause a collision. The solution: Barney's NIC must suspend its CSMA/CD logic. By disabling CSCMA/CD logic, Barney can send and receive at the same instant in time as receiving the frame from Fred. The ability to send and receive at the same time is called full duplex. (The CSMA/CD imposed restriction of only sending or only receiving at one point in time is called half duplex.) Figure 6-7 shows the result with full duplex enabled on both Barney and Fred.