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Also known as Signalling System Alternating Current No. 15
(SSAC15) is a 4-wire connection between PBXs of unlimited
distance. It has now replaced AC13 as the long distance inter
PBX signalling system.
Signalling is via a signalling frequency of 2280Hz. One pair
is used for transmission and the other for reception together
with incoming and outgoing speech. Signals are mostly applied
(tone on or tone off) continually.
The table below shows the signalling conditions relevant to
the call status on the transmit and receive pairs for AC15
A the most common variant of this signalling system.
| Link Status |
Transmission Pair |
Reception Pair |
| Idle |
On |
On |
| Seize |
Off |
- |
| Delay Dial |
- |
Off |
| Proceed To Dial |
- |
On |
| Dialling |
On / Off Pulses |
- |
| Answer |
- |
Off |
| Forward Hold |
Off |
- |
| Backward Hold |
- |
Off |
| Clear Forward On |
>300 ms |
- |
| Clear Backward On |
- |
>300 ms |
| Forward Aux On |
45 - 135 ms |
- |
| Backward Aux On |
- |
45 - 135 ms |
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Table 4 - AC15 Signals
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As with DC10 the Delay Dial, Proceed Dial and Aux signals
are optional and are not always used. The meaning of a signal
will be determined by the tone state and timing in the call
set-up process.
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