Medium Access Control: Stochastic Methods - CSMA/CD

Carrier Sense Multiple Access with Collision Detection

CSMA improves channel utilization compared to ALOHA. However, the performance can still be improved further, especially in wired networks.

Consider the situation of two terminals that are connected to the same cable. This cable could, for example, be a coaxial cable, or it could also be built with twisted pairs. Before extending CSMA, it’s useful to understand more intuitively, how frames are transmitted in such a network and how collisions can occur.

The slides below illustrate the physical transmission of a frame on such a cable. To transmit its frame, host A must send an electrical signal on the shared medium.

1. The first step is thus to begin the transmission of the electrical signal. This is depicted in the first slide below.

2. This electrical signal will travel along the cable. Although electrical signals travel fast, we know that information cannot travel faster than the speed of light (i.e. $300,000$ kilometers/second). On a coaxial cable, an electrical signal is slightly slower than the speed of light in a vacuum which is at about $200,000$ kilometers/second.

   • This implies that if the cable has a length of one kilometer, the electrical signal will need 5 microseconds to travel from one end of the cable to the other.

3. The ends of coaxial cables are equipped with termination points that ensure that the electrical signal is not reflected back to its source. This is illustrated in the third slide below, where the electrical signal has reached the left endpoint and host B.

4. At this point, B starts to receive the frame being transmitted by A. Notice that there is a delay between the transmission of a bit on host A and its reception by host B. If there were other hosts attached to the cable, they would receive the first bit of the frame at slightly different times. As we will see later, this timing difference is a key problem for MAC algorithms.

5. In slide 4, the electrical signal has reached both ends of the cable and occupies it completely. Host A continues to transmit the electrical signal until the end of the frame.

6. As shown in slide 5, when the sending host stops its transmission, the electrical signal corresponding to the end of the frame leaves the coaxial cable.

7. The channel becomes empty again once the entire electrical signal has been removed from the cable.