Introduction to Switching Systems Lecture 1 Course Instructor: Marzia Alam

Evolution of Telecommunication Historically, transmission of telegraphic signals over wires was the first technological development in the field of modern telecommunications. Telegraphy was introduced in 1837 in Great Britain and in 1845 in France. In March 1876, Alexander Graham Bell demonstrated his telephone set and the possibility of telephony, I.e. long- distance voice transmission. His demonstration laid the foundation for telephony.

Cont…. Graham Bell demonstrated a point-to-point telephone connection. A network using point-to-point connections is shown below. In such a network, a calling subscriber choose the appropriate link to establish connection with the called subscriber. In order to draw the attention of called subscriber before information exchange can begin, some form of signalling is required with each link.

Cont.. In figure below, there are 6 entities and 15 point-to-point links. In a general case with n entities, there are n(n-1)/2 links. Let us consider the n entities in some order. In order to connect the first entity to all other entities, we require (n-1) links. With this, the second entity is already connected to the first. We now need (n-2) links to connect the second entity to the others. For the third entity we need (n-3) links, for the fourth (n-4) links and so on. follows:

Cont.. Networks with point-to-point links among all the entities are known as fully connected networks. The number of links required in a fully connected network becomes very large even with moderate values of n. For example we requires 1225 links for fully connection 50 subscribers

Cont.. This image shows the overhead Telephone and Telegraph Wires above Broadway, circa From the Book of Old New York

Component of Telecommunication System Subscriber end instrument (ex. telephone) Subscriber loop system Switching system Transmission system Signaling system

Modern Telephone Modern telephone instruments are largely electronic. Wire coils that performed multiple functions in older sets have been replaced by integrated circuits that are powered by the line voltage. Mechanical bell ringers have given way to electronic ringers. The carbon transmitter dating from Edison’s time has been replaced by electret microphones, in which sound waves cause a thin, metal- coated plastic diaphragm to vibrate, producing variations in an electric field across a tiny air gap between the diaphragm and an electrode. The telephone dial has given way to the keypad, which can usually be switched to generate either pulses similar to those of the dial mechanism or dual-tone signals as in AT&T’s Touch-Tone system. Finally, a number of other features have become available on the telephone instrument, including last-number recall and speed- dialing of multiple telephone numbers.

1209 Hz on 697 Hz to make the 1 tone

DTMF The DTMF telephone keypad is laid out in a 4×4 matrix of push buttons in which each row represents the low frequency component and each column represents the high frequency component of the DTMF signal. Pressing a key sends a combination of the row and column frequencies. For example, the key 1 produces a superimposition of tones of 697 and 1209 hertz (Hz). Initial pushbutton designs employed levers, so that each button activated two contacts. The tones are decoded by the switching center to determine the keys pressed by the user.

▶ How Wired Telephone Transmission Works - [360p].mp4 ▶ How Wired Telephone Transmission Works - [360p].mp4 How a cellular telephone works - [360p].mp4