collision detection

[kə′lizh·ən di‚tek·shən]

(computer science)

A procedure in which a computer network senses a situation where two computer devices attempt to access the network at the same time and blocks the messages, requiring each device to resubmit its message at a randomly selected time.

collision detection

(networking)

A class of methods for sharing a data transmission medium in which hosts transmit as soon as they have data to send and then check to see whether their transmission has suffered a collision with another host's.

If a collision is detected then the data must be resent. The resending algorithm should try to minimise the chance that two hosts's data will repeatedly collide. For example, the CSMA/CD protocol used on Ethernet specifies that they should then wait for a random time before re-transmitting.

See also backoff.

This contrasts with slotted protocols and token passing.

This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

collision avoidance system

(1) See adaptive cruise control, semiautonomous vehicle and self-driving car.

(2) An in-vehicle safety system that applies the brakes when it detects a possible forward collision. Based on the current driving speed and the distance to the vehicle ahead (determined by radar), a collision avoidance system warns the driver first and intervenes if no human action is taken. Advanced systems can apply the brakes when heading into an oncoming car while turning the corner.

Cross Traffic Avoidance
A cross traffic system alerts the driver that a car or cyclist is about to cross in front. It may also stop the vehicle.

Collision Avoidance in Reverse
Safety devices when backing up a vehicle have matured. The most elementary "rear blind zone assist" is a set of sensors that cause audible beeps when getting close to an object. That evolved to a rear camera that enables drivers to see objects on screen that are directly behind the car. The camera was later augmented with a "cross traffic" or "cross path alert" system that warns the driver when a car is moving perpendicular to the rear. Advanced systems can also sense pedestrians and cyclists and may even be able to apply the brakes as in forward collision detection. See automotive safety systems.

Rear Cross Path Alert

When backing up in this 2017 Honda, cars coming into the driver's path are alerted by audio and visual alerts. In this example, beeps were heard along with flashing orange arrows (right side) as a car was approaching from behind on the passenger side.

CSMA/CD

(Carrier Sense Multiple Access/Collision Detection) The transmission method used in Ethernet networks. When Ethernet was designed in the 1970s, it was a shared medium. At any moment, only one frame from one station was transmitting in one direction (half duplex). See 10Base5 and 10Base2.

With CSMA/CD, if the network is busy when a station wants to transmit (carrier sense), the station waits a random number of microseconds before trying again. However, if two stations coincidentally transmit their frames at exactly the same time, their signals will collide. Both stations detect the collision and back off a random duration before retrying.

Today, collisions have been mostly eliminated, because shared Ethernet gave way to full-duplex, point-to-point channels between sender and receiver (see switched Ethernet). However, CSMA/CD provides compatibility for older shared Ethernet hubs that may still be in place. Ethernet is a data link protocol, and CSMA/CD is a MAC layer protocol (see MAC layer). See data link protocol, Ethernet and CSMA/CA.

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