# SS5 signaling

The SS5 TCP implements the CCITT Recommendations Q.140-Q.164, Specifications of Signaling System No. 5, CCITT Red Book, Volume VI, Fascicle VI.2, Geneva 1985.

SS5 line signaling is performed by in-band compelled single or dual-frequency tones. Two frequencies are used, either alone or in combination:

- f1 = 2400 Hz

- f2 = 2600 Hz

These frequencies are sufficiently far apart from the register signaling tones not to cause any false detection, and are in a frequency band where the likelihood of a false detection due to voice is minimal.

Since no signaling bits are involved, this is not strictly a digital CAS protocol, but can be used on analog trunks as well. However, the NMS implementation is for digital boards.

Note: The SS5 protocol can be used to set up calls on both T1 and E1 trunks.

This topic describes:

- Signaling states

- Signal exchange

## Signaling states

The following table describes line signaling for a typical call:

State | Forward tone | Direction | Backward tone
--- | --- | --- | ---
Idle | None |  | None
Seizure | f1 (2400 Hz) |  |
Proceed to send |  |  | f2 (2600 Hz)
The outbound side starts to send the address information using in-band MF tones. Call setup continues with the inbound side playing a ring-back tone if the call is accepted, and then answering the call.
Ringing |  |  | Ring tone
Answer |  |  | f1 (2400 Hz)
Answer acknowledge | f1 (2400 Hz) |  |
If the inbound side rejects the call, the tone played is different.
Busy-flash |  |  | f2 (2600 Hz)
Busy-flash acknowledge | f1 (2400 Hz) |  |
Idle | None |  | None
A busy-flash tone may not be played at this time to reject a call. Instead, the busy tone of the target network is played. Depending on which side hangs up the call first, a clear back signal or a clear forward signal is generated. A clear back signal must be acknowledged by the outbound side, then a clear forward signal is sent, acknowledged by a release guard signal. Idle follows.
Inbound hangs up first: Clear back |  |  | f2 (2600 Hz)
Clear back acknowledge | f1 (2400 Hz) |  |
Pause: Two consecutive signals in the same direction | 100 ms minimum
Clear forward | f1 + f2 (2400+2600 Hz) |  |
Release guard |  |  | f1 + f2 (2400+2600 Hz)
Idle | none |  | none
Outbound hangs up first: Clear forward | f1 + f2 (2400+2600 Hz) |  |
Release guard |  |  | f1 + f2 (2400+2600 Hz)
Idle | None |  | None

## Signal exchange

The signals are exchanged following a compelled scheme. The exchange is as follows:

1. Side A starts playing a tone.

2. When side B detects the tone, it starts playing its tone.

3. When side A detects side B's tone, it stops playing its tone.

4. When side B detects silence, it stops playing the tone it was playing.

Thus, there is no fixed timing, but the whole cycle proceeds at the maximum speed allowed by the tone detection and generation equipment.

Register signaling is implemented by in-band MF tones that the outbound equipment sends to the inbound equipment. These tones are unacknowledged. The relevant address is preceded by a KP tone (start-of-pulsing) and ended by an ST tone (end-of-pulsing).