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

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 ; or by capturing data from the medium and performing error detection.
• For recovery, some systems simply cause senders to retransmit collided data (perhaps with backing-off algorithms which reduce the sender's retransmit rate when collisions keep occurring) or use Error Correction techniques such as FEC
  Forward error correction
  In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
  
  .

Collision avoidance

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.

Common examples

Collisions are a condition that arises when two or more data

Data

The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...

 stations attempt to transmit at the same time

Time

Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 over a shared channel

Channel (communications)

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio 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
  Carrier Sense Multiple Access
  Carrier Sense Multiple Access is a probabilistic Media Access Control protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum."Carrier Sense" describes the fact that a...
  
   (CSMA)
• Multiple Access with Collision Avoidance
  Multiple Access with Collision Avoidance
  Multiple Access with Collision Avoidance is a slotted media access control protocol used in wireless LAN data transmission to avoid collisions caused by the hidden station problem and to simplify exposed station problem....
  
  

Other examples

In telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

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

• Competition by users of a system
  System
  System is a set of interacting or interdependent components forming an integrated whole....
  
   for use of the same facility
  Building
  In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...
  
   at the same time. This may also be known as oversubscription
  Shared resource
  In computing, a shared resource or network share is a device or piece of information on a computer that can be remotely accessed from another computer, typically via a local area network or an enterprise Intranet, transparently as if it were a resource in the local machine.Examples are shared file...
  
  . The term contention ratio
  Contention ratio
  In computer networking, the contention ratio is the ratio of the potential maximum demand to the actual bandwidth. The higher the contention ratio, the greater the number of users that may be trying to use the actual bandwidth at any one time and, therefore, the...
  
   applies specifically to the number of people connected to an ISP who share a set amount of bandwidth. Typical 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 the shortfall between supplier-claimed access speeds and those experienced by the consumer once signed-up to a contract is particularly bad at just those times when most consumers actually want to use the service.
• A contention can occur in data communications
  Telecommunication
  Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
  
   when no station is designated a master station
  Master station
  In telecommunication, the term master station has the following meanings:1. In a data network, the station that is designated by the control station to ensure data transfer to one or more slave stations....
  
  . In contention, each station must monitor the signals and wait for a quiescent condition before initiating a bid for master status.