Telephone Instruments and Signals

TELEPHONE INSTRUMENTS & SIGNALS

Copy of the original phone of Graham Bell at the Musée des Arts et Métiers in Paris

1896 Telephone (Sweden)

INTRODUCTION

• Communication– The process of conveying information from one

place to another

• Components– Source of information– Transmitter– Receiver– Destination– Transmission medium

• Telecommunication– A long distance communications– ‘tele’ Greek word for distant or afar

• Telephone– One of the most remarkable devices ever

invented

• Originally developed for conveying human speech information (voice)

• Now extensively used to transport data using data modems

• PTN– Public telephone network– Global communications network

Rapid Development of Telephone System

• 1876 - Alexander Graham Bell & Thomas A Watson invented the telephone

• 1877 – there are only 6 telephones in the world

• 1881 – 3,000 telephones

• 1883 – 133,000 (in US)

Telecommunication Giant Evolution

• AT&T –American Telephone & Telegraph Company– Referred to as the Bell Telephone System– In 1982 - $155 billion assets, 1M employees,

100,000 vehicles

• Comparison with Microsoft assets in 1988– $10 billion

• 1.5 billion telephone sets are operating in the world today

The Subscriber Loop

• POTS – Plain Old Telephone Service– Simplest and most straightforward form of

telephone service

– Involves subscribers accessing the PTN through a pair of wires called the local subscriber loop (or simply local loop)

The Subscriber Loop

• Local Loop– Most fundamental component of a telephone

circuit

– Unshielded twisted pair(UTP) transmission line

– Cable pair – 2 insulated conductors twisted together

The Subscriber Loop• Comprised of several lengths of copper wire

interconnected at the junction & cross connect boxes located in manholes.

• Provides the means to connect telephone set of the subscriber’s location to the closest telephone office– A telephone office – end office/local exchange/central

office

• Connected to an electronic switching system (ESS) – Enables the subscriber to access the PTN (will be

discussed later)

Standard Telephone Set

• Telephone – ‘tele’ – from afar, phone – sound/ voice– An apparatus for reproducing sound,

especially that of the human voice (speech) at a great distance by means of electricity; consisting of transmitting & receiving instruments connected by a line or wire which conveys the electric current

Standard Telephone Set

• Basic telephone set

– Simple analog transceiver designed for converting speech of acoustical signals to electrical signals

– Added features: multiple line selection, hold, caller ID

Standard Telephone Set

• Butterstamp telephone(1878)– First telephone set that combined a transmitter

and receiver into single handheld unit

• Rotary Dial Telephone– Introduced by western electric company also

called the bell system

• Touch-Tone Telephone

http://en.wikipedia.org/wiki/Telephone

Quality of Transmission

• Depends on – the received volume– Frequency response of the telephone

circuit– Degree of the interference

Functions of the Telephone Sets• Notify the subscriber when there is an incoming call

with an audible signal – bell or visible signal – flashing light

• Provide a signal to a telephone network verifying when the incoming call has been acknowledged & answered– Receiver is lifted off hook

• Convert speech energy to electrical energy (Tx) and vice versa (Rx)– Microphone, Speaker

Functions of the Telephone Sets

• Incorporate some method of inputting and sending destination telephone numbers– From tel. Sets to central office– ‘pulses’ rotary dialer– ‘frequency tones’ touch tone

• Regulate the amplitude of the speech signal – To avoid crosstalk

• Notify the tel. Office when a subscriber wishes to place an outgoing call (handset lifted off hook)– Dial tone

Functions of the Telephone Sets

• Ensure that a small amount of the transmit signal is fed back to the speaker– Feedback signal- sidetone/talkback

• Provide an open circuit (idle condition) to the local loop when the telephone is not in use (on hook)– Closed ckt – busy (off hook)

• Provide a means of transmitting & receiving call progress signals between the central office switch & the subscriber– On & off hook signal, busy, ringing, dial tone

Components of a Telephone Sets

• Transmitter

• Receiver

• Electrical network for equalization

• Associated circuitry – to control sidetone level– to regulate signal power

• Signaling circuitry

Essential Components

• Ringer Circuit– Electronic oscilllator– To alert the destination party of the incoming calls

• On/Off Hook Circuit– Simple single throw-double-pole STDP switch– Placed across the ‘tip’ and ‘ring’– On hook (idle, open switch), off-hook (in use, closed

switch)

Essential Components

• Equalizer circuit– Combination of passive components (resistors &

capacitors)– To regulate the amplitude & frequency response of

the voice signals

• Speaker/receiver– Converts electrical signals received from the local

loop to acoustical signals (sound waves)– Enclosed in the handset of the telephone along with

a microphone

Essential Components

• Microphone– Transmitter– Converts acoustical signals in the form of sound pressure

waves from the caller to electrical signals

• Hybrid network– Hybrid coil/duplex coil– Special balanced transformer used to convert 2-wire

(local loop) circuit into 4-wire (telephone set) circuit– Outgoing signal –1 to 2V, incoming – half of outgoing

signal

Essential Components

• Dialing circuit

– Enables the subscriber to output signals representing digits

– Rotary dialer/Electronic dial pulsing circuit/touch tone keypad

Basic telephone call procedures

• Step 1: Calling station goes off hook• Step 2: Switching machine returns an audible

dial tone to the calling station• Step 3: Caller dials the destination #• Step 4: Switching machine removes the dial

tone from the loop• Step 5: Switch interprets the # and locates

the local loop for the destination #

Basic telephone call procedures

• Step 6: Switch check the destination for the DC current– If off hook, busy signal sends to the calling station– If on hook, ringing signal and ring back signal will

be sent

• Step 7: When destination answers the call, it completes the loop causing DC current to flow.

Basic telephone call procedures

• Step 8: Removes the ringing and ring back signal

• Step 9: When either end goes on hook– The switch machine detects an open

circuit, then drops the connection through the switch

Call Progress Tones & Signals

• Acknowledgement & status signals that ensure the processes necessary to set-up and terminate a telephone call are completed in an orderly & timely manner

• Signaling– Station signaling –exchange of signaling message

over local loops between stations & tel. company switching machines

– Intraoffice signaling – exchange of signaling message bet. switching machines

Categories of Signaling• Alerting signals

– Indicate request for service• Ex. Going off hook, ringing the destination

• Supervising signals– Provide call status information

• Ex. Busy, ring back signal

• Controlling signals– Provide information in the form of announcement

• Ex. Change of number, number no longer in service

• Addressing signals– Provide routing information (calling & called numbers)

Essential Call Progress Signals

• Dial tone• Dual tone

multifrequency tones• Multifrequency tones• Dial pulses• Station busy – slow

busy• Equipment busy - fast

busy– Congestion tone

• Ringing • Ring back• Receiver on hook• Receiver off hook

Essential Call Progress Signals

• Dial tone– 2 frequencies – 350 & 440Hz

• Dual tone multifrequency tones (touch tones)– Introduced in 1963 with 10 buttons– Each digit is represented by the linear addition of two

frequencies– Used to transfer digits & control signals between

telephone sets & local switching machines

DTMF Keypad Frequencies

1209 Hz 1336 Hz 1477 Hz 1633 Hz

697 Hz 12

ABC

3

DEFA

770 Hz4

GHI

5

JKL

6

MNOB

852 Hz7

PRS

8

TUV

9

WXYC

941 Hz * 0 # D

Essential Call Progress Signals

• Multifrequency tones

– Combination of two frequencies that fall within the same speech BW so that they can be propagated over the same circuits as voice (in-band signaling)

– Used to transfer digits and control signals between switching machines

– Transmit the calling & called numbers from originating tel. office to the dest. tel. office

Multifrequency CodesFrequencies (Hz) Digit or Command

700 + 900 1

700 + 1100 2

700 + 1300 3

700 + 1500 4

900 + 1100 5

900 + 1300 6

900 + 1500 7

1100 + 1300 8

1100 + 1500 9

1100 + 1700 Key pulse (KP)

1300 + 1500 0

1500 + 1700 Start (ST)

2600 IDLE

Multifrequency

• Rate of transmission – 7 digits per second

• KP - Used to indicate the beginning of a sequence of MF digits

• ST – used to indicate the end sequence of dialed digits– Indicates the beginning of the processing of the

signal (telephone circuit perspective)

Dial Pulses

• Rotary dial pulsing• The process begins when the telephone set

is lifted off hook• When a digit is dialed

– The loop circuit alternately opens (breaks) & closes (makes) a prescribe number of times

– Rate – 10 make/break cycle per second• 100 ms per pulse cycle• Break time 61 ms, make time 39 ms• Interdigit time – 300 ms

DTMF

Subscriber

Min. digit duration

50 ms

Min interdigit duration

45 ms

Max interdigit duration

3 s

Essential Call Progress Signals

• Station busy – Sent from the switching machine back to the

calling station whenever the called tel # is off hook– 2 tone signal, 480 Hz and 620 Hz, – 0.5 s on/off– 60 pulse-per-min

• Equipment busy– Congestion tone or no circuit available tone– Blocking condition – 0.2 s on, 0.3 s off– 120 pulse-per-min

Essential Call Progress Signals

• Ringing signal– 20 Hz, 90 Vrms– ‘on’ for 2 sec– ‘off’ for 4 sec

• Ring back– 440 Hz & 480 Hz– ‘on’ for 2 sec– ‘off’ for 4 sec

• Receiver on/off hook–On hook

• used to terminate a call

–Off-hook •Causes DC current (20-80mA)to flow on the loop •Request for service

Non-Essential Signaling & Call Progress Tones

• Call waiting tones

• Hold

• Stutter dial tone (for voice mail)

• Calling card service tone

• Intrusion tones