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Concept
Contention (telecommunications)
Summary
In statistical time division multiplexing, contention is a media access method that is used to share a broadcast medium. In contention, any computer in the network can transmit data at any time (first come-first served).
This system breaks down when two computers attempt to transmit at the same time. This is known as a collision. To avoid collisions, a carrier sensing mechanism is used. Here each computer listens to the network before attempting to transmit. If the network is busy, it waits until network quiets down. In carrier detection, computers continue to listen to the network as they transmit. If computer detects another signal that interferes with the signal it is sending, it stops transmitting. Both computers then wait for a random amount of time and attempt to transmit. Contention methods are most popular media access control method on LANs.
One method to handle collisions in a contention based system is to optimize collision detection and subsequent recovery.
A collision can be detected by listening to the shared medium immediately after transmitting and identifying collision characteristics ; or by capturing data from the medium and performing error detection.
For recovery, some systems simply cause senders to re-transmit collided data (perhaps with backing-off algorithms which reduce the sender's re-transmit rate when collisions keep occurring) or use Error Correction techniques such as FEC.
An alternative method to handle collisions in a contention-based system is to attempt to avoid them. Some systems may utilize a strict scheduling guideline to identify who may use which resources when. Other systems may have the senders listen to the channel immediately prior to transmitting and determine suitable times to transmit.
Collisions are a condition that arises when two or more data stations attempt to transmit at the same time over a shared channel, or when two data stations attempt to transmit at the same time in a half duplex communication link.
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