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 Components The process of conveying information from one place to another Components Source of information Transmitter Receiver Destination Transmission medium

Telecommunication Telephone 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 On/Off Hook 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 Speaker/receiver 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 Hybrid network 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 1 2 ABC 3 DEF A 770 Hz 4 GHI 5 JKL 6 MNO B 852 Hz 7 PRS 8 TUV 9 WXY C 941 Hz * # 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 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 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 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