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 ...

s, a call collision (commonly known as glare) is one of two things: #The

contention Contention or contentious may refer to: * Resource contention, in computer science, a conflict over access to a shared resource * Contention (telecommunications), a media access method to share a broadcast medium * Bus contention, an undesirable ...

that occurs when a

terminal Terminal may refer to: Computing Hardware * Terminal (electronics), a device for joining electrical circuits together * Terminal (telecommunication), a device communicating over a line * Computer terminal, a set of primary input and output dev ...

and

data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted ...

circuit-terminating equipment ( DCE) specify 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 informa ...

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, to ...

transfer Transfer may refer to: Arts and media * ''Transfer'' (2010 film), a German science-fiction movie directed by Damir Lukacevic and starring Zana Marjanović * ''Transfer'' (1966 film), a short film * ''Transfer'' (journal), in management studies ...

call Call or Calls may refer to: Arts, entertainment, and media Games * Call, a type of betting in poker * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage Music and dance * Call (band), from Lahore, Paki ...

request and handle an incoming call. When 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 fo ...

occurs, the DCE proceeds with the call request and cancels the incoming call. #The condition that occurs when a trunk or channel is seized at both ends simultaneously. Glare can sometimes be experienced as trying to make a call out on a PBX, and instead being connected to an incoming call. This can happen at home too, if a call out is made at the exact second that a call is about to start ringing in. Multi-line hunting attempts to avoid glare by selecting circuits in opposite preference order so the highest numbered line, which is last choice for incoming calls, is first choice for outgoing calls, like so:

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 signalling 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-ce ...

signalling, which offers better answer and disconnect supervision. IE: Nortel BSP discouraged using

loop start Loop start is a telecommunications supervisory protocol between a central office or private branch exchange (PBX) and a subscriber telephone or other terminal for the purpose of starting and terminating a telephone call. It is the simplest of the ...

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