SIGNALING IN THE SUBSCRIBER LOOP
Background and Purpose
In Section 5.4 we described loop-start signaling, although we did not call it that. When a
subscriber takes a telephone off-hook (out of its cradle), there is a switch closure at the
subset (see the hook-switch in Figure 5.3), current flows in the loop alerting the serving
exchange that service is desired on that telephone. As a result, dial tone is returned to the
subscriber. This is basic supervisory signaling on the subscriber loop.
A problem can arise from this form of signaling. It is called glare. Glare is the result of
attempting to seize a particular subscriber loop from each direction. In this case it would
be an outgoing call and an incoming call nearly simultaneously. There is a much greater
probability of glare with a PABX than with an individual subscriber.
Ground-start signaling is the preferred signaling system when lines terminate in a
switching system such as a PABX. It operates as follows: When a call is from the local
serving switch to the PABX, the local switch immediately grounds the conductor tip to
seize the line. With some several seconds delay, ringing voltage is applied to the line
(where required). The PABX immediately detects the grounded tip conductor and will not
allow an outgoing call from the PABX to use this circuit, thus avoiding glare.
In a similar fashion, if a call originates at the PABX and is outgoing to the local
serving exchange, the PABX grounds the ring conductor to seize the line. The serving
switch recognizes this condition and prevents other calls from attempting to terminate the
circuit. The switch now grounds the tip conductor and returns dial tone after it connects
a digit receiver. There can be a rare situation when double seizure occurs, causing glare.
Usually one or the other end of the circuit is programmed to back down and allow the
other call to proceed. A ground start interface is shown in Figure 7.8.
Terminology in signaling often refers back to manual switchboards or, specifically, to
the plug used with these boards and its corresponding jack as illustrated in Figure 7.9.
Thus we have tip (T), ring (R), and sleeve (S). Often only the tip and ring are used, and
the sleeve is grounded and has no real electrical function.
Ground-start interface block diagram. (From Figure 2-7 of Ref. 8, reprinted with permission.)