Analog and Digital Voice Connections

Analog Voice Fundamentals

Interfacing Cisco Systems equipment with traditional analog telephony devices requires an understanding of the various interfaces used in the industry. This section introduces analog interfaces that you can select from, including Foreign Exchange Station (FXS), Foreign Exchange Office (FXO), and ear and mouth (E&M).


Local-Loop Connections

A subscriber home telephone connects to the telephone company central office (CO) via an electrical communication path called a local loop, as illustrated in Figure 2-1. The loop consists of a pair of twisted wires. One is called tip; the other is called ring, as shown in Figure 2-2.


The names tip and ring come from the plug used by the operators of yesteryear to interconnect calls. As you can see in Figure 2-2, the plug used by these operators resembles the plug you might use to connect your headphones to your home stereo equipment. There are three conductors on this plug. The conductor (that is, wire) connected to the tip of the plug is called the tip wire, and the conductor connected to the ring in the middle of the plug is called the ring wire.

In most arrangements, the ring wire ties to the negative side of a power source, called the battery, while the tip wire connects to the ground. When you take your telephone off hook, current flows around the loop, allowing dial tone to reach your handset. Your local loop, along with all others in your neighborhood, connects to the CO in a cable bundle, either buried underground or strung on poles.

Note
Your home telephone service is provided to you from your service provider by way of two wires. Your home telephone controls whether the service on these wires is activated via the switch hook inside the telephone.


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.


Supervisory Signaling

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.

Address Signaling

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.


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

1. When a subscriber calls someone by dialing a digit on the rotary dial, a spring winds.

2. the dial is released, the spring rotates the dial back to its original position.


3. 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 (that is, breaks and makes) represents the dialed digit.


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.