Supervision Signaling

Supervision signals are continuously sent by endpoint exchanges indicating the state of the trunk. This is known as continuous two-state signaling. States can be different at each endpoint of the trunk. MF signaling is used to indicate on-hook and off-hook states, as listed in Table 3-2.

Table 3-2. Supervision Signals

Direction

Signal Type

Transition

Forward

Seizure

On-hook to off-hook

Forward

Clear-forward

Off-hook to on-hook

Backward

Answer

On-hook to off-hook

Backward

Clear-back

Off-hook to on-hook

Backward

Proceed-to-send (wink)

Off-hook pulse, 120-290 ms

Supervision signals operate slightly differently for analog and digital trunks. Analog Trunks

A Single Frequency 2600 Hz tone is used to indicate trunk state between exchanges over analog facilities. This tone is applied in-band over the trunk and is turned off when a call is in progress or established. Therefore, the state is on-hook or idle when the tone is present and off-hook or in use when the tone is absent. The supervision signals for the Single Frequency method are illustrated in Figure 3-6.

Figure 3-6. Forward and Backward Supervision Signals for a Call

Figure 3-6. Forward and Backward Supervision Signals for a Call

In Figure 3-6, assume that Switch A sends the forward signals and Switch B sends the backward signals. Switch A sends a forward seizure or off-hook signal to Switch B on a chosen trunk. Then, Switch B sends a backward wink or proceed-to-send to Switch A and waits for address signaling or dialed digits. After the digits are sent and the call is answered, Switch B sends a backward answer or off-hook to Switch A, enabling an end-to-end voice path.

In this case, the calling party hangs up first and a clear-forward is sent from Switch A to Switch B. When the called party hangs up, a clear-back signal is sent by Switch B.

Two important aspects of this signaling method need to be discussed:

• First, Bell System MF does not have backward signaling for connections that fail during setup.

Therefore, the exchange where the call failed must connect an announcement server indicating to the calling party that a problem occurred.

The signaling system then relies on the calling party to release or drop the call so that clear-forward procedures can be initiated.

• Second, no release guard-type signal exists, and timers are used after trunks are released. Therefore, after an exchange releases a trunk, it initiates a timer for approximately 1 second. After this timer expires, the exchange assumes that the trunk was released at the other end and is available for use.

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