Supervision Signaling

Supervision signaling is accomplished by two frequencies, sent either individually or in combination. CCITT No. 5 uses compelled supervision signaling, whereby the signaling tone is left on until an acknowledgment is received.

The two in-band frequencies are f1, which equals 2400 Hz, and f2, which equals 2600 Hz. The combination of f1 and f2 produces a composite signal; these signals and frequencies are listed in Table 3-5.

Table 3-5. CCITT No. 5 Supervision Signals

Direction

Signal Type

Frequency

Forward

Seizure

f1

Backward

Proceed-to-send

f2

Backward

Answer

f1

Forward

Acknowledgment

f1

Backward

Clear-back

f2

Forward

Acknowledgment

f1

Forward

Clear-forward

f1 and f2

Backward

Release-guard

f1 and f2

Backward

Busy-flash

f2

Forward

Acknowledgment

f1

Forward

Forward-transfer

f2

Three new signals are introduced in Table 3-5: Release-guard, Busy-flash, and Forward-transfer.

• Release-guard—This signal is used by the incoming exchange to acknowledge a clear-forward from the outgoing exchange. It also indicates to the outgoing exchange that the trunk is now available for an incoming call.

• Busy-flash—This signal is used by the incoming exchange to indicate to the outgoing exchange that call setup cannot be extended toward the destination.

• Forward-transfer—This signal is used on calls for operator services.

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