Contention (telecommunications)

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In packet mode communication networks, contention is 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

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.