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

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