Loop Start and Ground Start Signaling

The two most common methods for end-loop signaling are loop-start and ground-start signaling.

• Loop-Start Signaling—This is the simplest and least intelligent of the two signaling protocols. It also is the most common form of subscriber loop signaling. This protocol basically works in the same way as the telephone and the local end office, whereby the creation of a loop initiates a call and the closure of a loop terminates a call. Loop-start signaling is not common for PBX signaling and has one significant drawback, in that glare can occur. Glare occurs when two endpoints try to seize the line at the same time, and it often results in two people being connected unknowingly. The person picking up the phone thinks he has a dial tone, but unbeknownst to him he is connected to someone who called him.

• Ground-Start Signaling—This signaling protocol differs from loop-start signaling, in that it provides positive recognition of connects and disconnects. Current-detection mechanisms are used at each end of the trunk, enabling end office switches to agree on which end is seizing the trunk before it is seized. This form of signaling minimizes the effect of glare and costs the same as loop-start signaling. As such, it is the preferred signaling method for PBXs.

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