Local-Loop Signaling

Local-Loop Signaling

A subscriber and telephone company notify each other of the call status through audible tones and an exchange of electrical current. This exchange of information is called local-loop signaling. Local-loop signaling consists of supervisory signaling, address signaling, and informational signaling, each of which has its own characteristics and purpose. The three types of local-loop signaling appear on the local loop and serve to prompt the subscriber and the switch into a certain action.

Resting the handset on the telephone cradle opens the switch hook and prevents the circuit current from flowing through the telephone, as seen in Figure 2-3. Regardless of the signaling type, a circuit goes on hook when the handset is placed on the telephone cradle and the switch hook is toggled to an open state. When the telephone is in this position, only the ringer is active.

To place a call, a subscriber must lift the handset from the telephone cradle. Removing the handset from the cradle places the circuit off hook, as shown in Figure 2-4. The switch hook is then toggled to a closed state, causing circuit current to flow through the electrical loop. The current notifies the telephone company that someone is requesting to place a telephone call. When the telephone network senses the off-hook connection by the flow of current, it provides a signal in the form of the dial tone to indicate that it is ready.

When a subscriber makes a call, the telephone switch sends voltage to the ringer to notify the other subscriber of an inbound call, as illustrated in Figure 2-5. The telephone company also sends a ringback tone to the caller, alerting the caller that it is sending ringing voltage to the recipient telephone.

The pattern of the ring signal, or ring cadence, varies around the world. As depicted in Figure 2-6, the ring cadence (that is, ringing pattern) in the United States is 2 seconds of ringing followed by 4 seconds of silence. The United Kingdom uses a double ring of 0.4 seconds separated by 0.2 seconds of silence, followed by 2 seconds of silence.

Although somewhat outdated, rotary-dial telephones are still in use and easily recognized by their large numeric dial-wheel. When placing a call, the subscriber spins the large numeric dial-wheel to send digits. These digits must be produced at a specific rate and within a certain level of tolerance. Each pulse consists of a "break" and a "make," as detailed in Figure 2-7. The break segment is the time that the circuit is open. The make segment is the time during which the circuit is closed. In the United States, the break-and-make cycle must correspond to a ratio of 60 percent break to 40 percent make.

Figure 2-7. Pulse Dialing

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A governor inside the dial controls the rate at which the digits are pulsed. The dial pulse signaling process occurs as follows:

A more modern approach to address signaling is touch-tone dialing. Users who have a touch-tone pad or a push-button telephone must push the keypad buttons to place a call, rather than rotating a dial as they did with pulse dialing. Each button on the keypad is associated with a set of high and low frequencies. Each row of keys on the keypad is identified by a low-frequency tone; each column of keys on the keypad is identified by a high-frequency tone. The combination of both tones notifies the telephone company of the number being called, hence the term dual-tone multifrequency (DTMF). Figure 2-8 illustrates the combination of tones generated for each button on the keypad.