Network Access Methods, CSMA/CD, CSMA/CA and Token Passing, Difference between CSMA/CD CSMA/CA and Token Passing
CSMA/CD (Carrier Sense Multiple Access/Collision Detection)
In CSMA/CD (Carrier Sense Multiple Access/Collision Detection) Access Method, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. When a host want to place data on the wire, it will “sense” the wire to find whether there is a signal already on the wire. If there is traffic already in the medium, the host will wait and if there is no traffic, it will place the data in the medium. But, if two systems place data on the medium at the same instance, they will collide with each other, destroying the data. If the data is destroyed during transmission, the data will need to be retransmitted. After collision, each host will wait for a small interval of time and again the data will be retransmitted, to avoid collision again.
CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)
In CSMA/CA, before a host sends real data on the wire it will “sense” the wire to check if the wire is free. If the wire is free, it will send a piece of “dummy” data on the wire to see whether it collides with any other data. If it does not collide, the host will assume that the real data also will not collide.
In CSMA/CD and CSMA/CA the chances of collisions are there. As the number of hosts in the network increases, the chances of collisions also will become more. In token passing, when a host want to transmit data, it should hold the token, which is an empty packet. The token is circling the network in a very high speed. If any workstation wants to send data, it should wait for the token. When the token has reached the workstation, the workstation can take the token from the network, fill it with data, mark the token as being used and place the token back to the network.