2 Wire Presented
SSAC13 is an automatic 1VF signalling system for the link by link transmission of supervisory signals and 10pps pulses between PBXs. It uses a single frequency of 2280 Hz in each direction of the transmission path. The system is designed to be inserted in the 2 wire section of the transmission Path. Signals cannot therefore be sent simultaneously in both directions

Provision of SSAC13 in the 4 wire Path is available on some PBXs (e.g. IDX)

1. The answer and clear-back signals are repeated at intervals of 275 plus or minus 75ms until acknowledgements are received.
2. The clear-forward signal is repeated at intervals of 300 to 600ms until a release guard signal is received.
3. If NU tone is received if would prevent the VF receiver from recognising a clear-forward signal. To overcome this, the transmission path is chopped for 35ms every second prior to the receipt of an answer signal. The effect of this chopping is perceptible when listening to supervisory tones.

Transmission Requirements
2 Wire Presented
The circuit is provided to EPS 3G. The circuit is provided at the 0dBr reference point and the signalling code is injected at this point at - 10 dBm0.
SSAC13 was originally designed to operate over an EPS2A circuit, that is a circuit with a maximum nominal insertion loss of 10 dB at 800 Hz. The SSA send level was adjusted to ensure that the receive level was - 23dBm or higher (max - 26 dBm). 4 Wire Presented
SSAC13 may be provided between a modern type PBX that supports 4 wire SSAC13 and an older type PBX that only supports 2 wire SSAC13.

The 'Answer' and 'Clear-back' signals are repeated at intervals of 275msecs (plus or minus 75 msecs) until acknowledgements are received. If after a few minutes the acknowledgement is not received an alarm is generated at Distant PBX.
The 'Clear-Forward' signal is repeated at intervals of 300-600 msecs until a 'Release Guard' is received. If after a few minutes the release guard is not received an alarm is generated at Local PBX.

The time taken to generate an alarm depends on the product used. When an alarm is generated if does not mean that the Circuit is faulty at the PBX that is showing the alarm.

Methods of Testing Circuit
The testing of SSAC13 circuits is not as easy as SSAC15 as there is no 'tone on idle' on the circuit. It is however possible to busy out a circuit at the local PBX, which produces a 2280 Hz tone on the transmit pair. This tone can be monitored by listening on the transmit pair with a high impedance telephone or any other suitable listening device.

Busying out the circuit at the distant PBX will produce a 2280 Hz tone on the receive Pair.

To prove the level of these tones you will need a Level Measuring Set.

Cross Connection of Circuits
If you have two circuits at the local PBX it is possible to cross connect for testing purposes.
Using this method you can make local end calls in either direction.

Testing with Circuit Control
It is normally possible to obtain assistance with the local testing centre who will 'patch' in a SSAC19 relay set. Calls can then be made and conditions monitored.

SSAC13 Circuits which Terminate on Relay Sets
Circuits which terminate on relay sets (normally associated with Strowger PBXs) are prone to timing faults. Particular attention should be paid to the adjustment of High Speed Relays.

SSAC13 is not in use today as it was effectively superseded by SSAC15. One of the reasons for this is that as the system was `tone off' there was no indication of a line fault and users could therefore seize a faulty line. On SSAC15 however, a dead line would automatically seize the circuit thus busying it out and therefore preventing user difficulties.


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Last revised: December 18, 2010