In the field of

telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, a call collision, commonly referred to as a glare, can occur in two situations: # Terminal and Data Circuit-Terminating Equipment ( DCE) Contention: This type of call

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 for ...

arises when both a

terminal Terminal may refer to: Computing Hardware * Computer terminal, a set of primary input and output devices for a computer * Terminal (electronics), a device for joining electrical circuits together ** Battery terminal, electrical contact used to ...

and a

data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...

circuit-terminating equipment (DCE) simultaneously designate the same

communication channel A communication 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 in telecommunications and computer networking. A channel is used for infor ...

to initiate a call request and handle an incoming call. In such cases, the DCE prioritizes the call request and proceeds with it, resulting in the cancellation of the incoming call. # Trunk or Channel Seizure at Both Ends: The second scenario involves a call collision when a

trunk Trunk may refer to: Biology * Trunk (anatomy), synonym for torso * Trunk (botany), a tree's central superstructure, and the stem of woody plants * Trunk of corpus callosum, in neuroanatomy * Elephant trunk, the proboscis of an elephant Comput ...

or channel is seized simultaneously at both ends. This situation can lead to conflicts and disruptions in the communication flow. Glare can sometimes be experienced when attempting to make an outgoing call on a private branch exchange ( PBX) system but getting connected to an incoming call instead. This occurrence can also happen in residential settings if an outgoing call is initiated at the precise moment when an incoming call is about to start ringing. To mitigate the risk of glare,

Multi-line hunting In telephony, line hunting (or hunt group) is the method of distributing phone calls from a single telephone number to a group of several phone lines. Specifically, it refers to the process or algorithm used to select which line will receive the ca ...

techniques are employed. These techniques involve selecting circuits in the opposite preference order, ensuring that the highest numbered line, which is typically the last choice for incoming calls, becomes the first choice for outgoing calls. The following example illustrates the sequence:

incoming -->1
            2
            3
            4
            5
            6
            7
            8<-- outgoing

With PRI circuits, the channel selection sequence is specified when the circuit is provisioned. Common practice is to have the PBX use descending channel selection, and the carrier to use ascending. Glare is not common on PRI circuits because the signaling is so fast, however it is not impossible (especially if there are subtle differences in the timers at either end, and the circuit is being used at near-capacity). The users will not experience a connection to an unexpected call (as would be the case with analog circuits), because glare causes protocol errors that generally prevent any sort of successful connection. Instead, one or both of the call attempts might fail, and ideally an error would appear in the logs (this depends on the logging capabilities of the systems at either end of the circuit). Glare is quite rare on PRI circuits, and can be difficult to troubleshoot. For old, analog PBX trunks, glare can be reduced by using

ground start {{refimprove, date=January 2008 In telephony, ground start is a method of signaling from a terminal of a subscriber local loop to a telephone exchange, where one side of a cable pair is temporarily grounded to request dial tone. Most middle 20th-c ...

signaling, which offers a better answer and disconnect supervision. IE: Nortel BSP discouraged using loop start trunks for this and other reasons. Long Distance exchanges in the 1950s and 60s incorporated Glare Detectors to alleviate the problem.

References

Teletraffic {{telecom-stub