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
852 Hz
4
GHI
7
PRS
2
ABC
5
JKL
8
TUV
3
DEF
6
MNO
9
WXY
941 Hz
*
0
#
697 Hz
770 Hz
1
1633 Hz
A
B
C
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 Codes
Frequencies (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
3s
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 (2080mA)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