The access capacity of the CO switch used determines how soon a dial tone is sent to the caller phone. If all the circuits are used, the customer could have to wait for a dial tone. There is no guarantee that the customer hears a dial tone right away. This dial tone signals the customer can begin to dial. The CO switch detects this current flow and transmits a dial tone (350- and 440-hertz tones played continuously) to the telephone set. The switch hook closes the loop between the CO switch and the telephone set and allows current to flow. The off-hook phase occurs when the telephone customer decides to make a phone call and lifts the handset from the telephone cradle. Only the ringer is active when the telephone is in this position. The power supply located at the CO switch prevents a loss of telephone service when the power goes out at the location of the telephone set. The CO switch contains the power supply for this DC circuit. In this state, the 48-VDC circuit from the telephone set to the CO switch is open. In other words, before a phone call is initiated, the telephone set is in a ready condition waiting for a caller to pick up its handset. When the handset rests on the cradle, the circuit is on-hook. The progress of a telephone call with loop-start signaling in place can be divided into five phases on-hook, off-hook, dialing, switching, ringing, and talking. Refer to Cisco Technical Tips Conventions for more information on document conventions. This document is not restricted to specific software and hardware versions. There are no specific requirements for this document. In this document a discussion of specific signaling types within each category precedes an examination of basic call progress from call origination to termination. A phone call cannot take place without all of these signaling techniques. Alerting provides audible tones to the user, which indicate certain conditions such as an incoming call or a busy phone. These dialed digits provide the switch with a connection path to another phone or customer premises equipment (CPE). Addressing involves passing dialed digits (pulsed or tone) to a private branch exchange (PBX) or central office (CO). A circuit (loop) can close to connect a call, for example. Once these changes are detected, the supervisory circuit generates a predetermined response. Supervision involves the detection of changes to the status of a loop or trunk. These signaling techniques can be placed into one of three categories supervision, addressing, or alerting. This paper discusses the signaling techniques required to control voice transmission.
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