CSMA/CD Protocol

Q. Explain in detail CSMA/CD Protocol in detail. How it detects collision.


The persistent and the non-persistent CSMA protocols are clearly a development over ALOHA because they ensure that no station starts to transmit when it senses the channel is busy. Advancement is for stations to abort their transmissions as soon as they sense a collision. In other words, if the two stations sense the channel to be idle and starts transmitting simultaneously, they will both detect the collision almost instantly. Rather than complete transmitting their frames, which are irretrievably garbled anyway, they should suddenly stop transmitting as soon as the collision is detected. Quickly terminating damaged frames saves the time and the bandwidth.

This protocol, called as CSMA/CD (CSMA with Collision Detection) is extensively used on LANs in the MAC sub layer. In particular, it is the base of the popular Ethernet LAN, so it is worth giving some time for looking at it in detail. CSMA/CD, as well as number of other LAN protocols, makes use of the conceptual model of Fig.5. At the point marked t0, a station has complete transmitting its frame. Any other station having a frame to transmit may now try to do so. If two or more stations decide to transmit together, there will be a collision. Collisions can be detected by seeing at the power or pulse width of the received signal and then comparing it to the transmitted signal.


After the station detects a collision, it stops its transmission, waits for a random period of time, and then tries again, supposing that no other station has started transmitting in the meantime. Hence our model for CSMA/CD will comprise of alternating contention and transmission periods, with idle time period occurring when all the stations are quiet (e.g., lack of work occurs).

Now lets us have a close look at the details of the contention algorithm. Suppose that two stations both start transmitting at exact time t0. How much time it would take them to realize that there has been a condition of collision? The answer to this question is vital to determining the length of the contention period and therefore what the delay and throughput will be. The minimum time to sense the collision is then just the time it takes the signal to propagate from one station to the other station.

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