Anatomy of a Telephone Call


Mrs. Smith is a typical telephone customer. She lives on the south side of Colorado Springs and has a home telephone number of 634-9000. Today, she has decided to call her friend Mrs. Jones, who lives on the north side of town. Mrs. Smith has a single telephone, and a pair of wires run from her home to the central office, a facility of the local telephone company. This is central office number 634 (the first three digits of Mrs. Smith's telephone number). We shall call it "central office A."

The wires that go from the central office to Mrs. Smith's home are used only by that one telephone and are not shared with any other customers. This dedicated pair of wires is called a "line circuit" or "loop." The central office places a voltage of about 48 volts across the loop to monitor activity and power the telephone.

Mrs. Smith initiates a call by lifting the handset off the telephone. This closes a switch in the telephone and permits current to flow through the loop. This signals the central office that Mrs. Smith would like to place a call.

Upon detecting loop current, the central office searches for an unused dial pulse register to store the dialed digits as they are received. (We are assuming that Mrs.Smith is using old-fashioned pulse dialing. Later we shall consider tone dialing.) The register is connected and dial tone is sent down the line. This last part is the handshake and is an important part of signaling. By taking the handset "off hook," Mrs. Smith has made a request for access to the network. The central office

[Waveform of Pulses]

acknowledges this request by sending back a dial tone. The handshake is an integral part of all signaling systems. You hear handshake signals after your modem connects with a server.

Upon hearing the dial tone, Mrs. Smith begins to dial the telephone number of Mrs. Jones. The telephone set rapidly opens and closes the loop at a rate of about 10 pulses per second. (You can hear the pulses as clicks if you set your tone/pulse phone to PULSE.) The number of pulses corresponds to the digit dialed.

This continues until all of the digits of Mrs. Jones' telephone number have been dialed. The dial pulse register at the central office is now full and contains the number 599-1234. The central office recognizes this number as belonging to central office number 599 (central office B). Since this number cannot be directly served by central office A, it looks for a trunk (interoffice line) that will connect it with central office B.

Mrs. Smith Calls Mrs. Jones

A trunk circuit provides a signal path between two central offices. Unlike a line circuit, or loop, a trunk circuit is shared by many different subscribers, although only one uses a trunk circuit at any given time. There may be 100 trunk circuits between these central offices, and as one telephone call ends, a trunk circuit is released and made available for another call.

Central office A seizes the first available trunk it finds. This trunk is a dedicated path to central office B, so only the last four digits of Mrs. Jones' telephone number need to be sent. These digits are received by central office B, which proceeds to make a connection with the appropriate loop.

Although a path has been established, Mrs. Jones still needs to be alerted to the fact that there is an incoming call. Central office B therefore sends an 88-volt, 20-hertz signal down the loop to drive a bell inside the telephone. This signal is repeated - 2 seconds on, 4 seconds off - to create the familiar telephone ringing sound. When Mrs. Jones lifts the handset, current flows in the telephone loop, telling the central office to disconnect the ringing and establish the audio connection.

This is the basic anatomy of a typical telephone call. Although there are many more details that could be discussed, many of the basic elements of signaling have been illustrated.

One method of address signaling, dial pulsing, was discussed. A more common method is tone dialing. Each digit is formed by selecting two out of seven possible frequencies. The frequencies were carefully chosen to minimize the possibility of accidental duplication by voice.

As an example of the frequencies, the digit "5" would be indicated by sending a 770-hertz tone and 1336-hertz tone simultaneously. This is sometimes referred to as Dual-Tone Multi-Frequency (DTMF) signaling.

Ever hear on radio talk shows people sometimes dialing out a phone numbers ? wouldn't be nice to know what it is if they were dialing in to someone famous like a basketball player or Bill Gates ? What you hear are DTMF tones. To find out what the phone number is, is simple. This can be done on the fly (meaning live) "requires a special device" or with any personal computer and a simple tape recorder. Record the radio show on tape, then plug the tape recorder into the computer, transfer the sound file into digital format like .wav and run the file thought a DTMF decoder. Everything is ignored but the DTMF tones and as you hear the radio talk show and when they enter the numbers, you will see them display on the screen. Neat ?

My point is that... never use a cordless phone or even a cellular for important financial calls, like accessing your bank statements, calling your stock broker or voice mail, trust me :- ) using one of those cheap $10 land line phones is the best way to go, or encrypt it with 128-bit PGP voice keys.

[Keypad and Tones]

If you want to convince yourself that this is how tone signaling really works, try a little experiment. Lift the handset and listen for a dial tone. Now get rid of the dial tone by pressing any button ("3," for example). Now, while listening to the handset, press the digits "4" and "5" simultaneously. You will hear a pure 770-hertz tone. By pressing "2" and "5" simultaneously, you will hear a pure 1336-hertz tone. Now, notice that the digit "5" sounds like something in between these tones _ like a combination of them.

In fact, by pressing any two keys in a row or column, you can hear the tone for that row or column. By pressing a single digit, you hear the combination of tones corresponding to the intersecting row and column.

Besides address signaling (tone and pulse), there are supervisory signaling and informational signaling. Supervisory signaling includes Multi-Frequency (MF) signaling, which is used only on trunk lines. It uses six different tones, giving the ten digits (0-9) plus five control functions.

sounds simple, no ?