Carrier Sense with Multiple Access with Collision Detection (CSMA/CD)

Carrier sense multiple access collision detect (CSMA/CD) describes the Ethernet access method. CSMA/CD follows rules similar to those in a meeting. In a meeting, all individuals have the right to speak. The unspoken rule that all follows, though, is "Only one person can talk at a time." If you have something to say, you need to listen to see if someone is speaking. If someone is already speaking, you must wait until they are finished. When you start to speak, you need to continue to listen in case someone else decides to speak at the same time. If this happens, both parties must stop talking and wait a random amount of time. Only then do they have the right to start the process again. If individuals fail to observe the protocol of only one speaker at a time, the meeting quickly degenerates and no effective communication occurs. (Unfortunately, this happens all too often.)

In Ethernet, multiple access is the terminology for many stations attaching to the same cable and having the opportunity to transmit. No station has any priority over any other station. However, they do need to take turns per the access algorithm.

Carrier sense refers to the process of listening before speaking. The Ethernet device wishing to communicate looks for energy on the media (an electrical carrier). If a carrier exists, the cable is in use and the device must wait to transmit. Many Ethernet devices maintain a counter of how often they need to wait before they can transmit. Some devices call the counter a deferral or back-off counter. If the deferral counter exceeds a threshold value of 15 retries, the device attempting to transmit assumes that it will never get access to the cable to transmit the packet. In this situation, the source device discards the frame. This might happen if there are too many devices on the network, implying that there is not enough bandwidth available. When this situation becomes chronic, you should segment the network into smaller segments. Chapter 2, "Segmenting LANs," discusses various approaches to segmentation. If the power level exceeds a certain threshold, that implies to the system that a collision occurred. When stations detect that a collision occurs, the participants generate a collision enforcement signal . The enforcement signal lasts as long as the smallest frame size. In the case of Ethernet, that equates to 64 bytes. This ensures that all stations know about the collision and that no other station attempts to transmit during the collision event. If a station experiences too many consecutive collisions, the station stops transmitting the frame. Some workstations display an error message stating Media not available . The exact message differs from implementation to implementation, but every workstation attempts to convey to the user that it was unable to send data for one reason or another.