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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.
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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
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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
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trunks for this and other reasons. Long
Distance exchanges in the 1950s and 60s incorporated Glare Detectors to alleviate the problem.
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incoming -->1 2 3 4 5 6 7 8<-- outgoing
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or channel is seized simultaneously at both ends. This situation can lead to conflicts and disruptions in the communication flow.
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Glare can sometimes be experienced when attempting to make an outgoing call on a private branch exchange (
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prioritizes the call request and proceeds with it, resulting in the cancellation of the incoming call.
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signaling, which offers a better answer and disconnect supervision. IE: Nortel BSP discouraged using
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Trunk or
Channel Seizure at Both Ends: The second scenario involves a call collision when a
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to initiate a call request and handle an incoming call. In such cases, the
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For old, analog PBX trunks, glare can be reduced by using
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139:but its sources remain unclear because it lacks
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