glare
A call collision; the situation where an incoming call occurs at the same time as an outgoing call.
glare: simultaneous incoming and outgoing call collision
In telephone switching systems, glare is the condition that arises when an incoming call arrives at an exchange at the same instant an outgoing call departs from the same line or trunk. Both the exchange and the distant exchange attempt to seize the same circuit resource simultaneously, creating a conflict that must be resolved by the switching logic to prevent call failure or crosstalk.
Glare occurs most commonly on two-way trunk circuits connecting exchanges, where traffic flows in both directions over the same physical path. In older electromechanical systems, the switching apparatus would detect the collision through off-hook conditions appearing at both ends within microseconds. Modern digital systems detect glare through signaling protocols that reveal the collision state before voice or data paths are established.
Resolution and prevention
Telephone exchanges handle glare through predetermined priority rules. In most systems, one direction is granted priority: the outgoing call proceeds while the incoming call receives a busy tone or is routed to an alternative circuit. Some systems use odd-even rules based on trunk numbering or time-division multiplexing to allocate the circuit deterministically. The switching software must resolve the collision within tens of milliseconds to avoid perceptible delays or dropped calls.
Glare becomes more frequent during peak traffic periods when trunk circuits approach saturation. Systems engineered with adequate trunk capacity experience glare rates below 1 percent; heavily loaded networks may see rates of 2 to 5 percent. The problem is virtually eliminated in modern digital exchanges with large trunk groups and advanced call routing algorithms, but legacy networks and single-trunk circuits remain vulnerable.
The term originated in electromechanical switching where the competing electrical signals would create visible arcing or sparking at the relay contacts, producing a literal glare. Although modern systems use electronic switching without visible light, the terminology persisted as the logical concept remained central to trunk circuit design and network engineering.