Contention

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In packet mode communication networks, contention is a media access method that is used to share a broadcast medium.^[1]

Collision detection and recovery

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. Capturing data from the medium and performing error detection can also identify when a collision has occurred. For recovery, some systems use simple algorithms such as merely retransmitting the previously-collided data. Others may use backing-off algorithms, where a sender gradually reduces their attempts to transmit if collisions keep occurring. Error correction techniques can also be used for recovery, such as FEC.

Collision avoidance

An alternative method to handle collisions in a contention based system is to minimize the probability of collisions, thereby attempting 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.

Common examples

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. A contention based channel access (multiple access) protocol is a protocol where data packet collisions may occur. Examples of such protocols are:

• The Aloha protocol
• Carrier Sense Multiple Access (CSMA)
• Multiple Access with Collision Avoidance

Other examples

In telecommunication, the term contention also has the following less usual meanings:

• Competition by users of a system for use of the same facility at the same time. This may also be known as oversubscription. The term contention ratio applies specifically to the number of people connected to an ISP who share a set amount of bandwidth. Example values would be 50:1, for home users (that is to say that 50 people or lines will vie for the same bandwidth) and 20:1 for business users. It is for this reason that internet access speeds fall during periods of peak use.
• A contention can occur in data communications when no station is designated a master station. In contention, each station must monitor the signals and wait for a quiescent condition before initiating a bid for master status.

See also

• Contention free pollable

References