In
statistical time division multiplexing
Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation (DBA). In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bitrate digital channels or ...
, 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
In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...
. 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.
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 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.
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
In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...
stations attempt to transmit at the same
time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...
over a shared
channel, or when two data stations attempt to transmit at the same time in a
half duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
communication link. A contention-based
channel access
In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical med ...
(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 (CSMA) is a medium access control (MAC) 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 ...
(CSMA)
*
Multiple Access with Collision Avoidance
Other examples
In
telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, 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. 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
Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
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
Contention-free pollable (CF-Pollable) is a state of operation for wireless networking nodes. The condition is saying that the node is able to use the Point Coordination Function, as opposed to the Distributed Coordination Function, within a wire ...
References
{{Channel access methods
Telecommunications techniques
Media access control