CSMA/CD:
Carrier sense multiple access with collision detection (CSMA/CD) is one of the most popular access methods, with CSMA/CD, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. If a host wishes to place data on the wire, it will “sense” the wire and determine whether there is a signal already on the wire. If there is, the host will wait to transmit the data; if the wire is free, the host will send the data, as shown in Figure 1.
Carrier sense multiple access with collision detection (CSMA/CD) is one of the most popular access methods, with CSMA/CD, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. If a host wishes to place data on the wire, it will “sense” the wire and determine whether there is a signal already on the wire. If there is, the host will wait to transmit the data; if the wire is free, the host will send the data, as shown in Figure 1.
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Figure 1: A host "sensing" the wire to see if it is free of traffic. |
The problem with the process just described is that, if there are two systems on the wire that “sense” the wire at the same time to see if the wire is free, they will both send data out at the same time if the wire is free. When the two pieces of data are sent out on the wire at the same time, they will collide with one another, and the data will be destroyed. If the data is destroyed in transit, the data will need to be retransmitted. Consequently, after a collision, each host will wait a variable length of time before retransmitting the data (they don’t want the data to collide again), thereby preventing a collision the second time. When a system determines that the data has collided and then retransmits the data, that is known as collision detection.
To summarize, CSMA/CD provides that before a host sends data on the network, it will “sense” (CS) the wire to ensure that the wire is free of traffic. Multiple systems have equal access to the wire (MA), and if there is a collision, a host will detect that collision (CD) and retransmit the data.
CSMA/CA:
CSMA/CA:
Carrier sense multiple access with collision avoidance (CSMA/CA) is not as popular as CSMA/CD and for good reason. With CSMA/CA, before a host sends data on the wire, it will “sense” the wire as well to see if the wire is free of signals. If the wire is free, it will try to “avoid” a collision by sending a piece of “dummy” data on the wire first to see whether it collides with any other data. If it does not collide, the host in effect assumes “If my dummy data did not collide, then the real data will not collide,” and it submits the real data on the wire.
