Telephony

TABLE OF CONTENTS

I. TelephonyII. Switching SystemIII. Subscribers’ LoopIV. Decibels and Noise LimitsV. Telephone TrafficVI. Numbering ConceptsVII.SignalingVIII.Multiplexing

Telephony is the art of projecting a sound wave into a given apparatus, transforming the sound wave into an electrical wave, passing the electrical wave over a wire or channel and then transforming the electrical wave back into sound wave.

Three Functional Parts:1. Subscriber2. Exchange 3. Interexchange

Telephone Set is an instrument used to transmit and receive information or calls sequentially or simultaneously, thus permitting one party to communicate with another

Main Functions of a Telephone Set:1. to call the local exchange2. to transmit the desired subscriber's number3. to receive different tone signals4. to transmit and receive speech5. to order the clearing of the connection

Telephone Set Wires1. Tip: Green – used to transmit the signal2. Ring: Red – used to receive the signals from the far end3. Slave: Yellow – used as a spare or for special purpose

applications4. Ground: Black or White

Parts of a Telephone Set1. Transmitter (mouthpiece) – used to converts sound wave to

electrical wave

2. Receiver (earpiece) – used to converts the varying electric current that represents the transmitted speech signal into an audible sound that will recreate the voice.

3. Ringer (bell) – a signaling system from the telephone exchange to a called phone to impart that a call should be answered.

4. Switch-hook (plunger) – a part of telephone where the handset is place when not in used and lifted up the phone when making a call.

Two Switch Hook Conditions of a Telephone

1. On-hook Condition – when the telephone is idle, the switch is open and no dc current flowing through the line.

2. Off-hook Condition – when the telephone handset is lifted up from the cradle, the current is flowing through the line, the telephone resistance appears between the tip and the ring causes the loop to close and alerts the central office to supply current and the dial tone.

5. Dial Pad (keypad) – a part of telephone where a desire subscribers number can be dialed.

Dialing – referred to as addressing a telephone system.

Two Type of Dialing1. Pulse Dialing is defined as a momentary on-hook condition that causes loop making and breaking from the telephone set dialer toward the central office.

Rotary Dial Pad is marked from 0 to 9 with each number representing a corresponding number of pulses that would be generated once the dial number is rotated from a starting position towards the fixed point.

State of Pulses:1. Make – circuit closed/off-hook, 40 %2. Break – circuit opened/on-hook condition, 60 % (make and break time = 0.1 sec)

Interdigit Delay – continuous make of 500 ms

2. Tone Dialing is a type of dialing employing two tones or two frequencies to represent a single digit.

Also known as Dual Tone Multi-Frequency (DTMF) dialing or touch tone dialing.

1209 Hz 1336 Hz 1477 Hz 1633 Hz697 Hz 1 2 3 A770 Hz 4 5 6 B852 Hz 7 8 9 C941 Hz * 0 # D

6. Protection Circuits

Over-voltage Protection – it protects the circuit for over voltage supply coming from the central office.Polarity Reversal – it protects the circuit from any reversed voltage polarity.

Problem Solving:1. How long does it take to dial 765-8340 in second using

pulse dialing?

2. How about if using a touch-tone dialing? How long does it take to dial the same number in 1?

Switching a method of connecting the calling party to the called party.

Types of Switching:1. Manual Control System – a call is being carried out by an

operator using switchboards and wires with jack.

2. Progressive Control or Step by Step Switching – also known as Strowger Switching System, named after the inventor Almon B. Strowger, 1889.

It is the oldest automatic switching system, electromechanical in nature, in which intelligence is located in relays mounted on each switch.

It also known as direct control switch with three stages:1. Line Equipment- includes the line finder2. Switch Train – consists of selectors arranged to operate in a sequence connecting the calling line to the connector group of the called line.3. Connectors – are switches that complete the connection to the called party.

3. Common Control Switching Also known as Crossbar Switching System.

It is a switching system, still mechanical in nature but the intelligence of the system is separated from the actual switch.

It utilizes a switching matrix, which externally managed by common control, to route telephone calls.

4. Stored Program Control (SPC) or Electronic Switching System (ESS)

Is the first generation ESS was employed in the year 1960.

It is a switching system control is in electronic circuit instead of mechanical devices, and the network or matrix is replaced with tiny glasses-encapsulated reed switches.

The final generation is known as digital switch.

Features of digital switches:1. Call waiting2. Caller ID3. Call forwarding4. Teleconferencing5. Abbreviated Dialing6. Malicious Call Trace7. Video conferencing8. Automatic Call back9. Call hold

Exchange Hierarchy1. International Exchange/Center – a center to which the

tertiary centers are connected to establish trunk connections and these center access the gateway facilities for international trunk connections.

2. Quaternary Exchange/Center – a center to which tertiary centers are connected to establish trunks connections

3. Tertiary Exchange/Center – a center to which secondary centers are connected to establish trunk connections.

4. Secondary Exchange/Center – a center to which the primary centers are connected to established a trunk connections

5. Primary Exchange/Center – a center to which local exchanges are connected and via which trunk connections are established

6. Local Exchange – an exchange of a local network to which the subscribers are directly or indirectly connected

7. Tandem Exchange/Center – an exchange used for connecting local exchange within a multi-exchange network.

Two Major Groups of Exchange

1. Public Network – anyone can be connected to it.

2. Private Network – telephone network privately owned by a certain organization.

Subscriber Loop - the connection between the telephone set and the central office switching equipment. Also known as local loop.

Basic Subscriber Loop Design Requirements:1. Enough power or current2. An adjustable gain or loss in the loop3. Minimum power loss4. Minimum amount of noise, echo, crosstalk and any

form of interference.

Rtotal = R tel + R cable + R misc + R switch

I= 24 mA to 60 mA ( optimal value is 35 mA )R tel = 150 (old)

= 600 to 900 ohms ( new)Ohm’s Law: V=IR

Where:R misc – resistance of the main distribution frame plus drop

wire resistance (neglected)R tel – resistance of the telephone setR switch – switching office equipment resistanceR cable – cable resistance used and varies according to size

Three Types of Design1. Normal Loop – a simple design because there is no need

to add any devices to amplify or attenuate the transmitted signal or to boost up the current to meet the design requirements.

Limitations: R switch is 1300 ohms; R L ≤ 1300 Ω2. Long Loop Design – a device has to be added to increase

the amount of current in the loop.Limitations: R L ≥ 1300 Ω, the current in the loop reduced

to less than 24 mA.Devices: Dial long lines, loop range extender and voice

repeater3. Special Services Design – when R L exceeds the 2800

ohms limit, special equipment to permit proper voice transmission and proper performance of the circuit used.

Devices: foreign exchange, off-premises extensions, private line circuits and wide area telephone service

Common Methods to attain longer loops without exceeding loss limits:

1. by increasing conductor diameter2. by using special devices3. use inductive loading

Loaded Cable vs. Non loaded CableLoading – is a scheme to improve transmission of cable pairs

by the addition of inductive devices in series with the cable pair.

Loading Coil – coil of wire around a magnetically permeable core constituting inductances which can be inserted in a circuit at regular intervals to improve transmission.

1. Loaded Cable – a loop with loading coil to extend the cable length

2. Non-loaded Cable – a loop or cable pair without loading coil

Subscriber loop length limits:1. Attenuation Limit (Transmission Design)Attenuation – refers to loop AC loss at reference frequency

measured in decibels. It is a function of the diameter and length of the wire pair.

Reference: 1000 Hz – North America = 8 dB 800 Hz – Europe = 7 dB

2. Signaling Limit (Resistance Design)It is based on dc resistance. The function of IR drop and

conductivity of the loop conductor diameter and gauge.

Reference: Imin = 20 mA – North America

Problem Solving:1. If the central office supply is -48 volts, the total resistance is

2400 ohms, the switching office and the telephone set resistances has 400 ohms and 300 ohms respectively. Considering a North American standard will the subscriber have dial tone? What is the maximum loop resistance in order to maintain the dial tone or other signaling element on the cable pair?

2. From the table, using 19 H 44 for the design of a subscriber loop for an 8 dB loss limit, determine the loop limit for this specification. How many inductive coil should be added and what is the resistance equivalent of each loading coil.

dB - decibel is a ratio of two power or voltage values and is a standard unit of measurement in the communications industry.- is a one-tenth of a bel and was named in honor of Alexander Graham Bell-dB as a unit is not a definite or absolute unit of measurement. -used to express transmission gains and losses

Mathematically: dB=10 log (P1/P2)

if P1 > P2, power gainif P1 < P2, power loss

dB=20 log (V1/V2) if V1 > V2, voltage gainif V1 < V2, voltage loss

dB=20 log (I1/I2) if I1 > I2, current gainif I1 < I2, current loss

Properties of logarithm1. log (AxB)= log A + log B 2. log (A/B)= log A - log B 3. log (Ax)=x log A4. log (1)= 05. log (A-1)= log (1/A) =0-log A= - log A

Power Levels (an actual or definite amount of power)1. dBm -means that the 0-dB point reference in 1 mW or dB

above or below the reference power of one milliwatt.dBm=10 log (P1/P2) where: P2 is one milliwatt

2. dBw -reference with one watt.dBw=10 log (P1/P2) where: P2 is one watt

3. dBk -dB in reference (above or below) one kilowatt.dBk=10 log (P1/P2) where: P2 is one kilowatt

4. dBv -represents the voltage level in dB with reference to 1 voltdBv=20 log (V1/V2) where: v2 is one volt

Power Levels (an actual or definite amount of power)

1. dBc -dB with reference to the nominal carrier value.dBc=10 log (P1/Pc); dBc=20 log (V1/Vc)=20 log

(I1/Ic) where: Pc ,Vc & Ic -Carrier power, voltage & current

respectively

2. dBa -dB above an adjusted reference noise; Noise level that has same interfering effect in a F1A handset as a -85 dBm, 1000 Hz tone

dBa= pure test tone (in dBm) +85 dBa= F1A weighted noise (in dBm) + 82dBa= 601A/F1A weighted (in dBm) + 77

3. dBaO -dB adjusted at 0 dBm level point. The dBa0 value state the noise would be test point level had been 0 dBm.

dBaO=dBa- TPL where: TPL- test point level

4. dBrn -dB above or below reference noise. Noise level that has same interfering effect in a 144 Handset as a -90 dBm, 1000 Hz tone, 144 weighted.

dBrn= 10 log (Po/1pWp) where: 1pWp= 1 picowatt

5. dBrnC-dB above reference noise, C-message weighted. Noise level that has same interfering effect with C-weighting as a -90 dBm, 1000 Hz tone dBrnC= pure test tone (in dBm)+90 dBrnC= F1A weighted noise (in dBa)+6 dBrnC= 601A/F1A weighted (in dBa)+6 dBrnC=10 log pWp where: pwp= picowatt

6. dBrnCO -dBrnC at 0 dBm level point. The dBrnC value states what the noise would be if test level had been 0 dBm

dBrnCO=dBrnC- TPLwhere: TPL- test point level

Definition of TermspWp - picowatt, psophometrically weighted. A definite amount of

noise power as measured with psophometric weighted network.VU - Actual amount of power (similar to dBm) as measured on a

VU meter specifically damped to give meaningful readings of voice and music.

Test Tone - pure signal at a single frequency. Standard Test tone is 0dBm, 1000 Hz.

Relative Level - difference between two powers. Power at one point compared to power at some reference point, not a definite amount of power.

Important Points to Remember“dBm” -is a symbol used to indicate the level of a signal in the system, relative

to 1 mW.“dB” -is the symbol used to indicate the gain or the loss in the system.“dB” and “dBm” can be added and subtracted as one unit from the other.

dBm= dBm ± dBdBm + dBm≠dB INVALID!!!

Problem Solving:1. What is the level of the signal at the output of the amplifier if the

input signal value is 1mW and if the gain of the amplifier is 3dB?

2. What is the noise level in dBm at the output of the amplifier if the noise power is given by 10-3 W?

3. Convert 13mW to (a) dBm (b) dBrn.If the attenuation in a telephone cable is 1dB/mile and the transmitted voltage is 1 volt, calculate the voltage received 1000 miles away.

4. The loss of a cable is measured by applying a signal at one end and measuring the result at the other end. Calculate the loss in dB. Impedance of the two cable ends are equal.

Signal applied Signal measured(a) 10 W 0.3 x 10 -5 W(b) 160 mV 1mV(c) 3mA 0.1 x 10 -4 A

TELEPHONE TRAFFIC

1. Traffic The total occupied time of circuit, switch or other paths.

2. Traffic Intensity The product of all calls and the average holding time of all calls.

Mathematically:A= C x T

Where: A=Traffic flow, ErlangC=Calling rate per hourT=The average holding time per call.

Traffic Units:1. Erlang-Preferred unit named after the Danish

Mathematician, Agner Krakup Erlang2. Call Hour (Ch)-One Ch is the quantity represented by one

or more calling having an aggregate duration of 1 hour.3. Call Second (Cs)-One Cs is the quantity represented by

one or more calls having an aggregate duration of 1 second.

4. Cent Call Second (CCs)-One CCs is the quantity represented by one 100’s call or by aggregate of 100 Cs of traffic.

5. Call Minute (Cm)- One Cm is the quantity represented by one or more call aggregate of 1 minute.

6. Equated Busy Hour Call (EBHC)-One EBHC is the average intensity in one or more traffic paths occupied in the busy hour by one 2 minute call or for an aggregate duration of 2 minutes.

Conversion: 1 Erlang=1 Ch=3600 Cs=36 CCs=60 Cm=30 EBHC

3. Traffic Density Traffic per unit time.

Mathematically: Td= A/t

Unit: CCs/BH or Erlang/BH

4. Grade of service A measure of the probability, that, during a specified period of peak traffic, a call offered to a group of trunks or circuits will fail to find an idle circuit at the first attempt.Typical GOS: P=0.01

An average of one call out of 100 will be blocked or lost during busy hour.

5. Call Congestion:P=number of lost calls/total number of offered calls.

Definition of Terms

1. AttemptAny effort on the part of a traffic source to seize a circuit,

switch or other traffic channel, whether or not the attempt is successful.

2. CallAny actual engagement or seizure of a circuit, switch or other

traffic channel.3. Calling RateThe number of calls per unit time. Calls per traffic source.4. TrafficRefers to the average of all user requests being serviced by

the network.5. Traffic Rate The busy hour traffic density per traffic source. Density per

traffic source.

Definition of Terms

6. Lost Calls or Blocked CallsRefers to calls that fail at the first trial.7. Busy Hour The continuous 60 minute period in a day during which the

highest usage occurs.8. Call ConcentrationThe ratio of the busy hour to the total day calls. It is the

reciprocal of length of day.9. Holding timeLength of time during which call engages a traffic path or

channel.10. Full availabilityEach inlet has access to any outlet. Every free inlet is at all

times able to test every outlet.

Definition of Terms

11. Limited AvailabilityWhen not all the free outlets in a switching system can be

reached by inlets.12. Carried Traffic The volume of traffic actually carried by the switch.13. Offered TrafficThe volume of traffic offered by the switch.14. Occupation TimeThe total amount of time that a circuit is occupied.

NUMBERING CONCEPTDefinition Terms:1. Subscriber Number – number to be dialed or called to

reach a subscriber in the same local network or numbering area.

2. Numbering Area – area which any two subscribers use the same dialing procedure to reach another subscriber in the telephone network.

3. Trunk Prefix or Toll Access Code – digit or combination of digits to be dialed by a calling subscriber in his own country but outside his own numbering area.

4. Trunk Code or Area Code – digit or combination of digits (not including the trunk prefix) characterizing the called numbering area with a country.

5. Country Code – combination of one to three digits characterizing the called country.

NUMBERING CONCEPTNumbering Functions:1. Call routing2. Addressing of called and calling party3. Activates necessary apparatus for charging

If the numbering is 6 digits, it would start from 000000 to 999999.

If the numbering is 7 digits, it would start from 0000000 to 9999999.

NUMBERING CONCEPTNumbering Concepts:1. Uniform Numbering – a scheme in which the length of the

subscriber numbers are uniform inside a given numbering area.

2. Non-uniform Numbering – a scheme in which the subscriber numbers vary within a given numbering area.

International Telephone NumberCCITT Recommendation E161 recommends that not more

than 12 digits make up an international number. This excludes the international prefix that switches the call to the transmit exchange for international calls.

4 Elements:1. International access code2. Country Code3. Area Code4. Telephone Number

NUMBERING CONCEPTCharging of Call:1. Flat Rate – fixed fee for unlimited number of calls.2. Message – number calls is counted on a call meter.

Charging Long Distance Calls – dependent upon duration and distance.

Two Ways of Time Zone Metering1. Bulk Billing (Multi-metering) – a call meter counts how

many rate pulses are received during the call.2. Toll Ticketing – accounts for individual calls; a computer or

an operator notes which subscriber has called to whom, where and for how long.

Channel and Circuits Types:

1. Switched Circuit – a call is automatically switched through to its destination after dialing has been completed.

2. Leased (Dedicated) Circuit – a permanent circuit for private use within a communication network, with the line directly between the two locations or routed through a serving central office.

SIGNALING

Refers to specific signals on the transmission line that are used for controlling the connection from the calling telephone to the called telephone and signals that are used to indicate the status of a call as it is being interconnected

Provides a means for operating and supervising a telephone communication system

Established connections, announces incoming calls and reports the fact that a line is busy

Signaling Functions

1. Ringing Signals*used to operate a visible or audible alarm to alert someone of an incoming call

2. Supervisory Signals*used to convey information regarding switchboards conditions

3. Address Signals*used dialing or digital information which is necessary to establish the desire connections

Subscriber Loop Signaling- 3 Methods

1. Wet-Dry*signaling information is indicated by the presence (wet) and the absence (dry) of a battery and ground condition on the line at the called end of the trunk.

2. Reverse Battery*loop signaling is accomplished by reversing the polarity of the battery on the line to indicate supervisory conditions

3. High-Low Method*by representing the on-hook and off-hook condition in terms of resistance values

Other Signaling Method1. E & M Signaling

*employs 2 leads to connect the signaling equipment to trunk circuit, M lead transmit battery or ground to the distant end of the circuits and while incoming signals are received on the E lead as either a ground or open condition

2. Out Band Signaling*make use of one or more AC tones which lie within the passband of the transmission facility but just outside the voice band

3. In-band Signaling*signaling tones are transmitted within the speech band, usually 1600, 2400 or 2600 Hz

Other Signaling Method4. Common Channel Signaling (CCS)

*all signaling for a number of voice path is carried over one channel, instead of within each individual channel

5. Alternate Channel Signaling*2 channels are used, one carries the signaling signal while other carriers the voice signal. An error detector monitors error rate in the signaling channel. If error is tolerable, the system is normal; if more than 3 errors occur for a period of 10 second, a line switch unit automatically switches the signaling channel information to the alternate voice channel switches, the signaling and the alternate-channel information to the signal element.

Telephone Audible Tone Signaling

Frequency (Hz)350 440 480 620 Cadence

Dial Tone X X continuousBusy Tone X X 0.5 Sec-ON/

0.5 Sec-OFFRinging Tone X X 2 Sec- ON/

4 Sec- OFF

MULTIPLEXINGThe process of transmitting two or more signals over the same

communication channel.

MultiplexerThe device that accepts many inputs but will only give one

output.

DemultiplexerThe device that accepts one input but separate the signals

into many outputs.

Different Types of MultiplexingFDM - Frequency Division MultiplexingTDM - Time Division MultiplexingWDM - Wavelength Division Multiplexing

FREQUENCY DIVISION MULTIPLEXING

Analog scheme for multiplexing (input is analog and the output is still analog)

A method of multiplexing in which the total frequency spectrum available is divided into channels, each which occupies a particular frequency range all of the time.

Introduce in the telephone networks in 1930s.

Applications: Frequency Modulation and Telemetry.


Message ChannelBasic building block of the FDM hierarchy.For voice transmission utilizing voice band frequencies.Voice band frequency: 300-3400 Hz; 0-4000 Hz

Basic GroupNext higher level in the FDM hierarchy.Composed of 12 voice band channels60-108 kHz

Basic Super GroupThe third level in the FDM hierarchyComposed of 60 Voice channels.From 312-552 kHz


Basic Master GroupThe fourth level in the FDM hierarchyComposed of 6000 Voice band channels.Jumbo Group

The Fifth Level in the FDM HierarchyConsists of 3600 Voice channelsSuper Jumbo GroupThe sixth in the FDM hierarchyConsists of 10800 voice channels


For CCITT Level No. of Voiceband channelsBasic Group 12Basic Super Group60Basic Master Group 600Jumbo Group 3600Super Jumbo Group 10800

For AT&T:Level No. of Voiceband ChannelsGroup 12Super Group 60Master Group 300Super Master Group 900

TIME DIVISION MULTIPLEXINGA type of multiplexing wherein each signal can occupy the

entire bandwidth of the channel but transmitted for only a brief period of time.

It is used for both digital and analog signal inputs but the output is digital.

Operation:To transmit multiple digital signals, the data to be transmitted

is formatted into serial data words. One byte may be transmitted during the time interval assigned to a particular channel. Each timeslot might contain 1 byte from each channel. One channel transmit 8 bits and then halts while the next channel transmits 8 bits, and then the third channel as well transmits 8 bits and so on. This process is known as interleaving.

A frame is one complete cycle of operation.

“Plan as if you’ll live forever; Act as if you’ll die tomorrow”

*** END of PRESENTATION***

Thank you!

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Telephony