Tuesday, July 9, 2013
- CSMA with collision detection is abbreviated as CSMA/CD.
- CSMA in itself makes use of the LBT technology i.e., listen or sense before talk.
- But when incorporated with the ability of collision detection, it gets much better.
- If the channel is sensed to be idle the data packets or frames are transmitted immediately but if not, the transmitter is bound to wait for some time before it can re-transmit.
- Sensing the channels prior to transmission is absolutely necessary if the collisions are to be avoided.
- Sensing the channel is the most effective way of avoiding the collisions.
- There are two types of CSMA protocols namely persistent and the non-persistent CSMA.
- In CSMA/CD protocol all the hosts have freedom for transmitting and receiving the data frames on one and the same channel.
- Also, the size of the packets is variable.
CSMA/CD comprises of two processes:
Carrier Sense: In this process the transmitter or the host checks if the channel or the line is not occupied before starting the transmission.
Collision Detection: CSMA/CD tries to detect the collisions in the shortest possible time. If it happens to detect a collision, it stops the transmission then and there and waits for a random amount of time which is equal to the binary exponential back-off. It then again senses the channel.
- For ensuring there occurs no collision during the transmission of a packet, a host must have the capability of detecting the collision before the transmission process is complete.
- What happens is that the host A sensing the line to be idle starts transmitting a frame.
- Just before the first unit of this frame reaches host B, it also senses the line to be idle and starts its transmission.
- Now the host B receives data while its transmission is still in progress and so it detects that a collision is about to occur.
- A collision occurs close to the host B. the host A also receives data in midst of its transmission and therefore detects the collision.
- For making the hosts detect collision before transmission, a minimum length has to be decided for the packets that are transmitted via CSMA/CD networks.
There are 3 states for a CSMA/ CD channel namely:
- Ethernet is the most popular example of the CSMA/CD networks.
- A minimum slot time is required for collision detection between the stations.
- This slot time must equal twice the maximum value of the propagation delay. - The host acquires the channel on the basis of the 1 – persistence.
- Also, a jam signal is transmitted if a case of collision detection occurs.
- CSMA/CD make use of the binary exponential back-off algorithm.
- It is obvious that the idle time of the channel will be small if the load is heavy.
- It normalizes all the packets with respect to the time of the packet transmission.
- CSMA/CD represents a very effective method for media access control.
- There are different methods available for detecting the collisions.
- Which method is to be followed depends largely on the transmission medium that exists between the two stations.
- For example, if the two stations are connected via electrical buses, the collision can be detected by making comparison between the transmitted and the received data.
- Some other way involves recognition of a signal of higher amplitude than the normal one.
- The jam signal used in the CSMA/CD networks is constituted of 32 bit binary pattern.