University of the East
College of Engineering
ECE Department
TELEPHONE
Assignment No. 1
ECN 512 1ECM
10:30 12:00PM / EN421
Name: FAJARDO, Shiela Monique A.
Student Number: 20101115224
Date Submitted: June 18, 2014 Grade
Instructor: Engr. Edelito A. Handig
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FAJARDO
Telephony and Mobile Communications
Questions
Brief history of telephone
1874 Alexander Graham Bell and Thomas A. Watson met.
1876 Alexander Graham Bell and Thomas A. Watson invented the telephone.
1878 First telephone exchange was installed
March 10, 1876 Bell succeeded in transmitting speech in his lab at 5 Exeter Place in
Boston. Bell was 29 years old and Watson was only 22. Bells patent number 174,465, has
been called the most valuable ever issued.
1877 There were only 6 telephones in the world.
1881 3,000 telephones were producing revenues.
1883 There were over 133,000 telephones in the United States.
1881 Bell and Watson left the telephone business. This proved to be a big financial
mistake, as the telephone company they left evolved into the telecommunications giant
American Telephone and Telegraph Company (AT&T). AT&T owned most of the local
operating companies, thus it was often referred to as the Bell Telephone system or
sometimes Ma Bell. AT&T once called the Bell system as the worlds most
complicated machine.
1982 Bell system grew to a $155 billion in assets ($256 billion in todays dollars) with
over 1 million employees and 100,000 vehicles.
January 1, 1983 AT&T officially divested the Bell system, the telecommunications
industry continued to grow.
Parts and functions of a telephone network
Parts of a telephone network
Instruments any device used to originate and terminate calls to transmit and receive
signals into and out of the telephone network.
Local loops the dedicated cable facility used to connect an instrument at a subscribers
station to the closest telephone office
Exchanges central station where subscribers are interconnected, either temporarily or
on a permanent basis.
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Telephony and Mobile Communications
Trunk circuits similar to a local loop except trunk circuits are used to interconnect two
telephone offices. The primary difference between a local loop and a trunk is that a local
loop is permanently associated with a particular station, whereas a trunk is a common-
usage connection.
Functions of a telephone network
1. Supplies the means and facilities for connecting the subscribers at the beginning
of a call and disconnecting them at the completion of the call.
2. Switching function identifies and connects the subscriber to a suitable
transmission path.
3. Signaling function supplies and interprets control and supervisory signals
needed to perform the operation.
4. Transmission function involves the actual transmission of a subscribers
messages and a necessary control signals.
Parts and functions of a telephone set
Parts of a telephone set
Ringer circuit the purpose is to alert the destination party of incoming calls.
On-off hook circuit also called switch hook; is a single throw double pole switch placed
across the tip and the ring. The switch is mechanically connected to the telephone
handset so that when the telephone is idle (on hook), the switch is open. When the
telephone is in use (off hook), the switch is closed completing an electrical path through
the microphone between the tip and the ring of the local loop.
Equalizer circuit combinations of passive components that are used to regulate the
amplitude and frequency response of voice signals.
Hybrid circuit special balanced transformer used to convert a two-wire circuit (the local
loop) into a four-wire circuit (the telephone set) and vice versa, thus enabling full duplex
operation over a two wire circuit. In essence, the hybrid network separates the
transmitted signals from the received signals.
Speaker the receiver of the telephone. This converts electrical signals received from the
local loop to acoustical signals (sound waves) that can be heard by a human being.
Microphone the transmitter of the telephone. This converts acoustical signals in the
form of sound pressure waves from the caller to electrical signals that are transmitted
into the telephone network through the local subscriber loop.
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Telephony and Mobile Communications
Dialing circuit enables the subscriber to output signals representing digits, and this
enables the caller to enter the destination telephone number.
Functions of a telephone set
1. Notify the subscriber when there is an incoming call with audible signal, such as
bell, or with a visible signal, such as flashing light.
2. Provide a signal to the telephone network verifying when the incoming call has
been acknowledge and answered.
3. Convert speech (acoustical) energy to electrical energy in the transmitter and vice
versa in the receiver.
4. Incorporate some method of inputting and sending destination telephone
numbers (either mechanically or electrically) from the telephone set to the central
office over the local loop.
5. Regulate the amplitude of the speech signal the calling person outputs onto the
telephone line. This prevents speakers from producing signals high enough in
amplitude to interfere with other peoples conversations taking place on nearby
cable pairs (crosstalk).
6. Incorporate some means of notifying the telephone office when a subscriber
wishes to place an outgoing call.
7. Ensure that a small amount of the transmit signal is fed back to the speaker,
enabling talkers to hear themselves speaking.
8. Provide an open circuit (idle connection) to the local loop when the telephone is
not in use, and a closed circuit (busy condition) to the local loop when the
telephone is in use.
9. Provide a means of transmitting and receiving call progress signals between the
central office switch and the subscriber, such as an on and off hook, busy ringing,
dial pulses, touch-tone signals, and dial tone.
Parts and functions of a central office
Parts of a central office
Main distribution frame is a signal distribution frame for connecting equipment (inside
plant) to cables and subscriber carrier equipment (outside plant).
Telephone switching exchange switch for short, is the main piece of equipment
located within the central office building
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Telephony and Mobile Communications
Functions of a central office
1. To provide a path for a call to be completed between two parties. Subscribers
connected to the same central office can communicate with each other by means
of the central office switch, which can connect any line to any other line. Modern
switches are digital, so the analog local-loop signals are digitized as they enter
the central office.
To process a call, the switch must provide three primary functions:
a. Identify the subscribers.
b. Set up or establish a communications path.
c. Supervise the calling processes.
2. It provides circuit switching. It connects to the local subscriber lines each
subscriber is normally connected via a separate twisted-pair line (local loop) to a
central office (end office).
References
Blake, R. (2001). Electronic Communication Systems: Digital Modulation (2nd ed., pp.
146-147, 582). New York, USA: Cengage Learning
Tomasi, W. (2004). Electronic Communication Systems Fundamentals Through Advanced:
Telephone Instruments and Signals (5
th
ed., pp. 688-693, 744-751)
Telephone switching exchange. Retrieved June 17, 2014 from
http://www.frankoverstreet.com/pages/co/what-is-a-central-office.aspx.
