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Also known as Signalling System Alternating Current No. 13
(SSAC13) this connection type has been superseded by AC15.
AC13 is a 2 wire connection used for the connection of PBXs
over distances of miles introduced in 1968.
Signalling is via transmitted and received signals known as
handshaking using a signal tone of 2280 Hz frequency. Signals
can only be sent in one direction at a time. Signals must
be applied for a definite period followed by a silence to
allow acknowledgement or return signals to be received; if
a reply signal is not received then the original signal is
repeated until the correct response is received.
The table below shows the signals corresponding to the call
states on the wires.
| Link Status |
Outgoing |
Incoming |
| Seize |
53 - 61ms |
- |
| Dialling |
53 - 61ms Pulses |
- |
| Clear Forward |
>1200ms |
- |
| Clear Forward Repeat Period |
300 - 600 ms |
- |
| Operator Recall |
100 - 150 ms |
- |
| Answer |
- |
200 - 330 ms |
| Answer Acknowledge |
- |
53 - 61 ms |
| Meter Signal Period |
- |
200 - 350 ms |
| Clear Backwards |
- |
200 - 330 ms |
| Clear Back Repeat Period |
- |
200 - 350 ms |
| Clear Back Acknowledge |
- |
53 - 61 ms |
| Release Guard Time |
- |
>650 ms |
| Busy |
- |
Continuous |
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Table 3 - AC13 Signalling
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The Meter signal is the same as the Answer signal in duration.
Which end sends them and the prevailing status determine the
meaning of signals sharing the same duration.
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