EE 489 Telecommunication Systems Engineering Introduction to Analog Telephony Concepts

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EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering

EE 489EE 489Telecommunication Systems EngineeringTelecommunication Systems Engineering

Introduction to Analog Telephony ConceptsIntroduction to Analog Telephony Concepts

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Network TypesNetwork Types

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Trade-off Between Switching and Trade-off Between Switching and Transmission Costs in Network ArchitectureTransmission Costs in Network Architecture

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Trade-off Between Switching and Trade-off Between Switching and Transmission Costs in Network ArchitectureTransmission Costs in Network Architecture

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Network TypesNetwork Types

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Concept of a “Homing Plan”Concept of a “Homing Plan”

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PSTN HierarchyPSTN Hierarchy

Local Exchanges, orCentral Offices (C.O.’s)

Primary SwitchingCentres

Regional Exchanges

National Exchanges

International Exchanges

Also connected to submarine cables and

satellite links.

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World Numbering PlanWorld Numbering Plan

• World is divided into zones, with each zone assigned a single digit zone codezone code.

• Each country within a zone is assigned a country codecountry code (usually 2-3 digits).– 1st digit is the country’s zone code.

• Regions or “numbering plan areasnumbering plan areas” (NPAs)within countries are assigned a 1-4 digit “area codearea code” or “routing coderouting code”.– NPA size and shape driven by numerous factors:

• Size and shape• Present and future numbering capacity• Political boundaries• Population demographics

• Local exchanges are assigned codes (3 digits in N.A.), followed by several digits assigned to each phone (4 digits in N.A.)

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World Numbering Plan (2)World Numbering Plan (2)

• World Numbering Zones

Code Zone

1 N.A. & Carib.

2 Africa

3 & 4 Europe

5 S.A. & Cuba

6 South Pacific

7 Former USSR

8 N. Pac., E. Asia

9 Far & Mid East

0 Spare

• Sample Country CodesCode Country

20 Egypt

231 Liberia

33 France

351 Portugal

44 UK

55 Brazil

593 Ecuador

60 Malaysia

886 Taiwan

966 Saudi Arabia

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Network ArchitectureNetwork Architecture

Customer terminals 20%

Outside plant, cables 29%

Switching equipment 25%

Multiplexing and Transmission equipment 15%

Buildings, land, other 11%

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Telephone SystemTelephone System

• Early telephone system– Powered by self-contained local battery– Ringing created by cranking generator

• Today’s telephone system– Powered through the line by battery at the central office (-

48V)– Circuit is closed when handset is lifted from the cradle (“off off

hookhook”)

• Transmitter – carbon granule microphone– Air pressure of sound waves impact on diaphragm, varying

pressure on carbon granules– Resistance of electrical current passing through carbon

granules varies the current (analog)

• Receiver– Varying electrical current passing through windings on

magnet, moves a diaphragm. Same as in a music loudspeaker.

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Telephone System (2)Telephone System (2)

• “PSTNPSTN”, or “POTSPOTS” simplified circuit model of any connection:

coil (ZB)

central battery

speech current

The coil is a “transmission bridge transmission bridge coilcoil” with a high impedance (ZB) preventing the speech current from shorting out at the central battery.

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Establishing A Call Establishing A Call (Conventionally)(Conventionally)1. Calling customer takes phone off hook which closes the circuit to

the C.O. (“looping the circuit”).

2. C.O. detects the “loop” and indicates readiness with dial tone.

3. Calling customer hears dial tone and dials number.• The network converts (“translates”) the phone number to a physical

equipment address

4. The network checks on the called party status and decides on a routing for the connection.

5. If connection possible, the called party is alerted.– Large 20 Hz alternating current is applied to line (“ringing current”).

6. “Ring tone” is returned to the caller.

7. The called party picks up the handset and closes his/her loop.

8. Exchange detects second loop and “trips” or stops ringing, then establishes call.

9. One party opens loop by hanging up, and exchange clears connection.

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Subscriber Line SignallingSubscriber Line Signalling

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Loop and Disconnect SignallingLoop and Disconnect Signalling

“pulse diallingpulse dialling” :

• Line is rapidly disconnected and reconnected in sequence with one pulse for digit value “1”, two pulses for digit value “2”, etc.

• Each pulse lasts 0.1 second.

• Inter-digit pause (IDP) must be >0.5 second.– If not, current digit may combine with previous digit.

• Ten digit phone number typically takes 6-15 seconds total.

• This is the kind of signalling old “rotary dial” phones produced.

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Dual-Tone Multi-Frequency SignallingDual-Tone Multi-Frequency Signalling

DTMF signallingDTMF signalling” or “tone signallingtone signalling”.

• Faster than pulse dialling (1-2 seconds for ten digit number).– Reduces call set-up time.

• Each digit produced by combination of 2 pure frequency tones.– Reduces chances of error or interference.

1208 Hz 1336 Hz 1477 Hz 1633 Hz

697 Hz 1 2 3 spare

770 Hz 4 5 6 spare

852 Hz 7 8 9 spare

941 Hz * 0 # spare

High Frequency Tone

Low

Fre

quen

cy T

one

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Address SignallingAddress Signalling

Dial Pulsing

Dual-Tone Multi-frequency (DTMF)

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DTMF ReceiverDTMF Receiver

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The Concept and Implications of The Concept and Implications of “two-wire” (2W) to “four-wire” (4W) “two-wire” (2W) to “four-wire” (4W) conversionconversion

Or…why your phone only needs one twisted pair and why you sometimes hear an echo

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2-W to 4-W Conversion2-W to 4-W Conversion

• For short distances, two-way communication is possible on a single pair of wires (bi-directional transmission).

• Problems occur however when amplification (in past) or digital regeneration (nowadays) is needed.– Amplifiers or regenerators in the network are uni-directional.

A Hybrid TransformerHybrid Transformer is used to convert a 2-wire circuit at the phone/terminal end to a 4-wire system in the switching network:

Balance Network (ZB)

2-W Line (ZLine)4-wire portion

Send Amplifier

Receive Amplifier

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2-W to 4-W Conversion (2)2-W to 4-W Conversion (2)

2-wires 2-wiresHybrid Hybrid

Amplifier or Regenerator

2-wires 2-wires

receivetransmit

Amplifier

2-wires 2-wires

receive transmit

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2-Wire to 4-Wire Conversion2-Wire to 4-Wire Conversion

•Any telephone call undergoes 2W-4W conversions:

- from the phone (4W) to the subscriber line (2W)

- from the subscriber line (2W) to the network interface (4W)

Overall structure of any phone connection

“2W”

“2W”

“4W”

“4W”

“4W”

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2-W to 4-W Conversion (3)2-W to 4-W Conversion (3)

• Balance network has a balance impedance of ZZBB.

• If ZZBB=Z=ZLineLine then half the signal goes to the line and half goes to the balance network with little or no coupling (reflection) to the local receiver.

• But by design, we use ZZBBZZLineLine to create “sidetonesidetone”.– Reflections from the C.O. return to the station set.– Talker hears his/her own voice.– Useful because acts (almost subconsciously) as a signal to

the talker that the line is live.– No sidetone makes the line feel “dead” and unnatural (IP

telephony often sounds like this since there’s no sidetone).– Today’s electronic phones have a small sidetone network

within them to create sidetone.

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2W to 4W conversion: The “Hybrid Coupler” 2W to 4W conversion: The “Hybrid Coupler”

CircuitCircuit

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2W to 4W conversion: The “Hybrid Coupler” 2W to 4W conversion: The “Hybrid Coupler”

CircuitCircuit

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Electronic Hybrid CouplerElectronic Hybrid Coupler

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Concept of Hybrid Return Loss Concept of Hybrid Return Loss

Echo return loss (ERL) = average attention of power reflected at the 2W-4W interface

Singing Return Loss (SRL) = minimum attenuation to reflected power at any frequency

coming back from the 2W-4W interface

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Introduction to the “Subscriber Loop” PlantIntroduction to the “Subscriber Loop” Plant

• Wire network from the central office to the station sets.

• Largest portion of capital expenditure (50%?) and workforce requirements (30%-40%?).

• Prime candidate for replacement by optical fibre but costs often prohibitive.

• Main goal is to design and work with length limits.– Limited by resistance and attenuation along the line.

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Subscriber “Loop” Plant (N. Subscriber “Loop” Plant (N. America)America)

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Subscriber “Loop” Plant (UK)Subscriber “Loop” Plant (UK)

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Three Main Design Three Main Design Goals/MethodsGoals/Methods• (A) (D.C.) Resistance Limit Requirement (A) (D.C.) Resistance Limit Requirement

– Keep total line resistance below a target level by choosing the appropriate wire gauges.

– Historically 1300 limit but now ~1700 .

• (B) (A.C.) Attenuation Limit Requirement(B) (A.C.) Attenuation Limit Requirement– Keep total signal loss below a target maximum level.– North America usually uses 8 dB maximum loss at 1000 Hz.– Elsewhere usually uses 7 dB maximum loss at 800 Hz.

• ““Uni-Gauge” Design MethodUni-Gauge” Design Method– In principle could mix and match wire gauges in loop makeup

to satisfy (A) and (B) at minimum cost of the copper used.– Actual practice has been to keep to a single size wire (often

26 gauge) as much as possible (better economically) and add battery boost, range extenders, amplifiers, or “load coils” as needed.

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More on Resistance DesignMore on Resistance Design

• How do we determine the target resistance?– We need a high enough current at the customer premises

to operate the station set (20mA minimum20mA minimum in North America).

– Use V=IR, with a known battery voltage of –48V.– 48V 20mA x R R 2400 total– Budget 400 for the battery feed bridge at the C.O.– Budget 300 for other miscellaneous wire resistances

(e.g. subset wiring, etc.).

The subscriber loop’s wire resistance must not exceed 1700 1700 .

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American Wire Gauge (AWG) American Wire Gauge (AWG) DataData

Loop Resistance Data

1 WireAWG mm inches /1000 ft /mile /km /km

19 0.910 0.036 16.1 85 53 26.522 0.644 0.025 32.4 171 106 5324 0.511 0.020 51.9 274 168.5 8426 0.405 0.016 83.5 440 268 13428 0.302 0.012 132 697 433 216.5

Diameter 2-W Loop Resistance

Loop Attenuation Loss Data (@ 1kHz)

1 WireAWG mm inches dB/1000 ft dB/mile dB/km dB/km

19 0.910 0.036 0.21 1.11 0.71 0.35522 0.644 0.025 0.32 1.69 1.01 0.50524 0.511 0.020 0.41 2.16 1.27 0.63526 0.405 0.016 0.51 2.69 1.61 0.80528 0.302 0.012 0.615 3.25 2.03 1.015

Diameter 2-W Loop Loss

•N.B.: Each change of 3 gauge numbers is a factor of 2 in wire area (cross section), this a factor of 2 in resistance / unit length

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Subscriber “Loop” PlantSubscriber “Loop” Plant

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Extending Loop Length: Load Extending Loop Length: Load CoilsCoils• Amplifiers can be used to add gain (7dB each is common).

• “Line loadingLine loading” by adding inductive coils at fixed intervals.– Decreases velocity of signal propagation.– Increases impedance.– Acts as high frequency filter but generally outside speech

band.

• Transmission line theory says that attenuation is lowest when:l g r c where Inductancel

Conductannceg Resistancer Capacitancec

We can change l by adding coils periodically along the line.

“Line LoadingLine Loading” reduces attenuation.

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Inductive Loading Inductive Loading - - Lumped “Load Lumped “Load Coils”Coils”

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Effect on Transfer Function of Twisted PairEffect on Transfer Function of Twisted Pair

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Example Calculation of Cutoff Example Calculation of Cutoff FrequencyFrequency

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Inductive Loading - Inductive Loading - “Load Coils”“Load Coils”

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Inductively Loaded Line Inductively Loaded Line Notation Notation Example #1: “19H8819H88” means 19 gauge wire loaded every 1830 m (H) with 88mH inductors.

Letter Code

Spacing (ft)

Spacing (m)

A 700 213.5B 3000 915C 929 283.3D 4500 1372.5E 5575 1700.4F 2787 850H 6000 1830X 680 207.4Y 2130 649.6

Example #2: “26B6626B66” means 26 gauge wire loaded every 915 m (B) with 66mH inductors.

Some Cable Conductor Properties:

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Psophometric and “C Weighting” Noise Psophometric and “C Weighting” Noise FiltersFilters

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ExampleExample

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Network Loss PlanningNetwork Loss Planning

• Received Volume Control– Subscribers must have a received signal level within an

appropriate range.– i.e. Not too loud and not to quiet.

• Stability or Oscillation Control: “Singing”– Manage reflections that can result if there’s a poor

mismatch of the 2-wire line impedance and the hybrid balance impedance.

– Singing can result.

• Talker Echo– Talker should not hear his/her own voice reflected back

(with a significant enough delay).

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Volume ObjectivesVolume Objectives

• Reference Equivalent (RE) or Overall R. E. (ORE)– A measure of perceived loudness of the signal.– ITU in Geneva used group of telephone users to judge

loudness.– Measured by adjusting an attenuator in a simulated

network.– Rated “highest tolerable volume”, “preferred volume” and

“lowest tolerable volume”.– Results showed that attenuator settings of <6dB were too

loud and >21dB were too faint.

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Overall Loudness Rating (OLR)Overall Loudness Rating (OLR)

• New standard circa 1990.

• Loss accumulated from speaker’s mouth and listener’s ear.

• OLR = SLR + CLR + RLRSLR – Send Loudness RatingCLR – Circuit Loudness RatingRLR – Receive Loudness Rating

Mouth toInterface

Loss

Interface toInterface

Loss

Interfaceto EarLoss

OLROLR Good/ExcellentGood/Excellent Poor/BadPoor/Bad

5-15dB 90% 1%

20dB 80% 4%

25dB 65% 10%

30dB 45% 20%

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StabilityStability

• Long distance connections all have 2-W to 4-W to 2-W conversion (as do most local connections).

• If there’s a poor mismatch of the 2-W line impedance with the hybrid balance impedance, signal energy passes across the hybrid reflecting from one 4-W direction into the other.

2-wires 2-wiresHybrid Hybrid

Amplifier

2-wires 2-wires

receivetransmit

Amplifier

2-wires 2-wires

receive transmit

Reflection(ZB ZL)

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Stability (2)Stability (2)

• Reflection at the hybrid re-inserts the signal with “balance balance return lossreturn loss” (BRLBRL or BBSS) into the return side of the 4-W loop.

1020log B LS

B L

Z ZB

Z Z

Minimum return loss seen atthe hybrid in any frequency

in the voice-band

• Additional 3+dB loss at hybrid when converting 4-W signal to 2-W signal, and another 3+dB going from 2-W to 4-W (6db total).

• Total trans-hybrid loss of returned signal:3 3STHL dB B dB

6STHL B dB Ideal loss

7STHL B dB Loss in practice (~3.5 db splitting loss)

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3dB

3dB

BS+6dB

G

Net Gain of one side of 4-W loop(total amplifier gain minus line losses)

T2-W to 2-W total attenuation

6T dB G

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• Total round-trip closed loop loss (“singing marginsinging margin”):

2( 6 )Sm B dB G 2( )Sm T B

• Generally found to be adequate if:

2( ) 6ST B dB

• Otherwise, singingsinging may result.– out of control runaway oscillation in the loop.– can continue even after the original impulse ceases.

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• Loss in a 4-W circuit may depart from its nominal value for a number of reasons:– Variation in line losses and amplifier gain with time,

temperature, etc.– Gain or loss will differ at different frequencies (usually

tested at 800 Hz and/or 1600 Hz).– Measurement errors.– Circuit errors.

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• Define new term for variance of round trip loss:

2 2 22 (2 )STot B Tn 2

SB Variance of trans-hybrid loss

2T Variance of gain or

loss in each 4-W sectionn Number of 4-W sections

• What if we want only 0.1% chance of singing?

2(T+BS)

Recall:We have singing ifm = 2(T+BS) < 6dB

Recall:3 standard deviationsfrom the mean isequivalent to 0.1%

6dB

0.1% 3Tot

2( ) 3 6S TotT B dB

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Typical values: 2.5SB

dB 1T dB 6SB dB

2 2 22 (2 )STot B Tn 2 22 (2 )

STot B Tn

2 22 2.5 (2 1)n 12.5 4n

2( ) 3 6S TotT B dB 2( 6) 3 12.5 4 6T n

1.5 12.5 4 3T n

This is basis for a generally acceptedapproximation or rule of thumb for 99.9%chance of stability (i.e. no singing): 4 0.5T n

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• Example: We have a long distance circuit with 6 4-W sections. What is the minimum attenuation (T) required for a 99.9% chance of not singing?

4 0.5T n

4 0.5 6T

7T dB

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Echo-Delay PhenomenaEcho-Delay Phenomena

• If the reflection at the hybrid is strong enough, telephone users will hear it.

• Talker echoTalker echo is when talker hears his/her own voice.

• Listener echoListener echo is when listener hears talker’s voice twice.

Talker Listener

Be+6dB

TTalker Echo:

Loss = Be + 2T

Listener Echo:

Loss = 2Be + 2T

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Echo-Delay (2)Echo-Delay (2)

• Recall Bs:– Balance Return Loss– Minimum return loss seen at any voice-band frequency

• What is Be?– Hybrid Echo Return LossHybrid Echo Return Loss– Average return loss in voice-band.

Frequency

B(f)

Return Lossat Frequency f

10

( ) ( )( ) 20log

( ) ( )B L

B L

Z f Z fB f

Z f Z f

Be (echo)

BS (stability)Why BWhy Bee and not B and not BSS??

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• SubjectiveSubjective annoyance of echo depends on relative echo level and on the delay.– The stronger the echo and the longer the delay, the more

troublesome the echo.

One-Way DelayOne-Way Delay Loss to Satisfy 50%Loss to Satisfy 50%

10 ms 11.1 dB

20 ms 17.7 dB

30 ms 22.7 dB

40 ms 27.2 dB

50 ms 30.9 dB

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• The echo objective is for 99% of all connections to have acceptable or better echo effects.

• Factors to consider:• Be = Expected hybrid echo return loss.

Be = Standard deviation of Be

• T = Nominal 2W-2W loss in connection.

T = Standard deviation of T

• Ē(t) = E = Expected echo attenuation for delay t at which 50% of users find echo tolerable.

E = Standard deviation of E

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• For the connection to be acceptable, we want:

2eE B T 2 0eM B T E

Mean margin against

objectionable echoM has a standard

deviation:M 2 2 2 2(2 )

eM B T En

If we want 99% chance of acceptable echo in all connections then:

Recall:2.33 std. dev.from the mean isequivalent to 1% 1%

0

2.33M

2eM B T E E 2eB T

2 2.33e MB T E

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EE 489 Telecommunication Systems Engineering Introduction to Analog Telephony Concepts

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