Call Signaling

Call signaling, in its most basic form, is the ability of a device to communicate a need for service to a network. The call-signaling process requires the network to detect a request for service and termination of service, send addressing information, and provide progress reports to the initiating party. This functionality corresponds to the three call-signaling types:

• Supervisory signaling

• Address signaling

• Informational signaling

A basic call setup, as illustrated in Figure 1-6, includes supervisory, address, and information signaling components. The supervisory signaling is used, for example, to detect that a phone went off hook. Address signaling occurs when a caller dials digits, and information signaling is represented by the dial tone heard by the caller.

This call setup can be broken down into three major steps. These steps include:

1. Local signaling: originating side The user signals the switch by going off hook and sending dialed digits through the local loop.

2. Network signaling The switch makes a routing decision and signals the next, or terminating, switch through the use of setup messages sent across a trunk.

3. Local signaling: terminating side The terminating switch signals the call recipient by sending ringing voltage through the local loop to the recipient telephone.


Supervisory Signaling

A subscriber and telephone company notify each other of call status with audible tones and an exchange of electrical current. This exchange of information is called supervisory signaling, as shown in Figure 1-7.


There are three different types of supervisory signaling:

• On hook When the handset rests on the cradle, the circuit is on hook. The switch prevents current from flowing through the telephone. 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. This prevents the current from flowing through the telephone. Only the ringer is active when the telephone is in this position.

• Off hook When the handset is removed from the telephone cradle, the circuit is off hook. The switch hook toggles to a closed state, causing circuit current to flow through the electrical loop. The current notifies the telephone company equipment 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 a dial tone to indicate that it is ready.

• Ringing When a subscriber makes a call, the telephone sends voltage to the ringer to notify the other subscriber of an inbound call. The telephone company also sends a ringback tone to the caller, alerting the caller that it is sending ringing voltage to the recipient telephone. Although the ringback tone sounds similar to ringing, it is a call-progress tone and not part of supervisory signaling.

The ringing pattern in the United States is 2 seconds of ringing tone followed by 4 seconds of silence. Europe uses a double ring followed by 2 seconds of silence.


Address Signaling

There are two types of telephones, as shown in Figure 1-8: a push-button (tone) telephone and a rotary-dial telephone.


These telephones use two different types of address signaling to notify the telephone company where a subscriber is calling:

• Dual-tone multifrequency (DTMF) Each button on the keypad of a touch-tone pad or push-button telephone is associated with a pair of high and low frequencies. On the keypad, each row of keys is identified by a low-frequency tone and each column is associated with a high-frequency tone. The combination of both tones notifies the telephone company of the number being called, thus the term dual-tone multifrequency (DTMF).

• Pulse The large numeric dial-wheel on a rotary-dial telephone spins to send digits to place a call. 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," which are achieved by opening and closing the local loop circuit. The break segment is the time during which the circuit is open. The make segment is the time during which the circuit is closed. The break-and-make cycle must correspond to a ratio of 60 percent break to 40 percent make.

A governor inside the dial controls the rate at which the digits are pulsed. For example, when a subscriber calls someone by dialing a digit on the rotary dial, a spring winds. When the dial is released, the spring rotates the dial back to its original position. While the spring rotates the dial back to its original position, a cam-driven switch opens and closes the connection to the telephone company. The number of consecutive opens and closes, or breaks and makes, represents the dialed digit.


Information Signaling

Tone combinations indicate call progress and are used to notify subscribers of call status. Each combination of tones represents a different event in the call process. These events, whose frequencies and patterns are listed in Table 1-2, include the following:

• Dial tone Indicates that the telephone company is ready to receive digits from the user telephone.

• Busy Indicates that a call cannot be completed because the telephone at the remote end is already in use.

• Ringback (line or PBX) Indicates that the telephone company is attempting to complete a call on behalf of a subscriber.

• Congestion Indicates that congestion in the long-distance telephone network is preventing a telephone call from being processed.

• Reorder tone Indicates that all the local telephone circuits are busy, thus preventing a telephone call from being processed.

• Receiver off hook Indicates that a receiver has been off hook for an extended period of time without placing a call.

• No such number Indicates that a subscriber has placed a call to a nonexistent number.

A call placed from your residential telephone uses all three types of call signaling. When you lift the handset, a switch in your telephone closes to start current flow and notifies the telephone company that you want to make a call (supervisory signaling). The telephone company then sends dial tone to indicate that it is ready to receive your dialed digits (informational signaling). You then dial your digits by pressing numbers on the keypad (address signaling).


Digital versus Analog Connections

Supervisory, address, and informational signaling must be carried across both analog and digital connections. Depending on your connection to the network, you must configure specific signaling to match the type of signaling required by the service provider. Figure 1-9 illustrates digital and analog connections coexisting in the same network.

Digital PBX connections to the network are common in many countries. They may be T1 or E1 lines carrying channel associated signaling (CAS) or PRI lines using common channel signaling (CCS).

CAS is a signaling method that allows passing on-hook or off-hook status by setting bits that are associated with each specific voice channel. These bits are carried in band for T1 and out of band for E1.

An ISDN connection uses the D channel as the common channel to carry signaling messages for all other channels. CCS carries the signaling out of band, meaning that the signaling and the voice path do not share the same channel.

Analog interfaces require configuration of a specific signaling type to match the provider requirement. For interfaces that connect to the PSTN or to a telephone or similar edge device, the signaling is configured for either loop start or ground start, the functions of which are discussed in later. For analog trunk interfaces that connect two PBXs to each other (that is, E&M interfaces), or a PBX to a CO switch, the signaling is either wink- start, immediate-start, or delay-start, with the signaling type set to 1, 2, 3, 4, or 5.