Medium Access Control: Stochastic Methods - Optimizing CSMA/CD

Removing Acknowledgements

On the wired networks where CSMA/CD is used, collisions are almost the only cause of transmission errors that affect frames. Transmission errors that only affect a few bits inside a frame seldom occur in these wired networks. For this reason, the designers of CSMA/CD chose to completely remove the acknowledgment frames in the data link layer.

• When a host transmits a frame, it verifies whether its transmission has been affected by a collision.

• If not, given the negligible Bit Error Ratio of the underlying network, it assumes that the frame was received correctly by its destination. So, the bit errors might be detected or corrected using a checksum field. If not, we can rely on the layers above to implement retransmission.

• Otherwise, the frame is retransmitted after some delay.

Edge Case: Short Frames

Removing acknowledgments reduces the number of frames that are exchanged on the network and the number of frames that need to be processed by the hosts. However, to use this optimization, we must ensure that all hosts will be able to detect all the collisions that affect their frames. The problem is important for short frames.

Let us consider two hosts, A and B, that are sending a small frame to host C as illustrated in the slides below. If the frames sent by A and B are very short, the situation illustrated below may occur.

1. Hosts A and B send their frame and stop transmitting (first slide). Since until the end of the transmission, no collision was detected, hosts A and B are content that they were successfully able to use the channel to transmit the frame.

2. When the two short frames arrive at the location of host C, they collide and host C cannot decode them (second slide).

3. The two frames are absorbed by the ends of the wire. Neither host A nor host B has detected the collision. They both consider their frame to have been received correctly by its destination.