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...

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

1. The contention
   Contention
   Contention may refer to:* The main contention, in rhetoric, the main point being argued* Resource contention, a general concept in communications and computing, is competition by users of a system for the facility at the same time:...
   
    that occurs when a terminal
   Terminal (telecommunication)
   In the context of telecommunications, a terminal is a device which is capable of communicating over a line. Examples of terminals are telephones, fax machines, and network devices - printers and workstations....
   
    and 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...
   
    circuit
   Telecommunication circuit
   A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
   
   -terminating equipment (DCE
   DCE
   DCE may refer to:* Delhi College of Engineering, University of Delhi, India* Dalian Commodity Exchange* Data circuit-terminating equipment, also called Data Communications Equipment or Data Carrier Equipment* Cisco's DCE product portfolio...
   
   ) specify the same 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...
   
    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....
   
    to transfer
   Call transfer
   A call transfer is a telecommunications mechanism that enables a user to relocate an existing call to another phone or attendant console by using the transfer button and dialing the required location. The transferred call is either announced or unannounced....
   
    a call
   Telephone call
   A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
   
    request and handle an incoming call. When call collision
   Collision
   A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
   
    occurs, the DCE proceeds with the call request and cancels the incoming call.
2. The condition that occurs when a trunk
   Trunking
   In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...
   
    or channel is seized at both ends simultaneously.

If you have ever tried to make a call out on a PBX, and been accidentally connected to an incoming call, you have experienced glare. This can sometimes happen at home too, if you pick up your phone to make a call out at the exact second that a call is about to start ringing in.

Multi-line hunting generally avoids glare by picking trunks 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

For PBX trunks, glare can further be reduced by using ground start

Ground start

In telephony, a ground start or GST is a method of signaling from a terminal or subscriber local loop to a telephone exchange, in which method a cable pair is temporarily grounded to request dial tone...

 trunking. IE: Nortel BSP discourages using loop start

Loop start

In telecommunications, a loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit, commonly referred to as 'off-hook'. When idle, or 'on-hook', the loop is at 48V DC...

trunks for this and other reasons. Long Distance exchanges in the 1950s and 60s incorporated Glare Detectors to alleviate the problem.