1

Introduction to Telecommunications

� Lecture 1

� Paul Flynn

2

The Public Switch Telephone Network(PSTN)

Composed of the following major network elements:

lines

trunks

repeater

transmission equipment (e.g. repeaters, multiplexers, DCS)

switch switch

switch

switches

PBX

SS7 & IN

network signaling and control (e.g. SS7 and IN)

PCMODEM

station equipment (e.g. telephone sets, MODEMs, FAX machines, …)

3

Telephony Equipment

� Telephone set

� PBX (Private Branch Exchange)Advanced features and call routing

10s to 100s of telephone sets

� Telephone Exchange

4

Basic Call Progress: On-Hook

TelephoneExchange

Local Loop2 wires

LocalLoop

-48 DC Voltage-48 DC VoltageDC Open CircuitDC Open CircuitNo Current FlowNo Current Flow

5

TelephoneExchange

LocalLoop

LocalLoop

Basic Call Progress: Off-Hook

DC CurrentDC CurrentDial ToneDial Tone

Off-HookOff-HookClosed Closed CircuitCircuit

6

Basic Call Progress: Dialing

Dialed DigitsDialed DigitsPulses or Pulses or

TonesTones

DC CurrentDC Current

TelephoneExchange

LocalLoop

Off-HookOff-HookClosed Closed CircuitCircuit

7

DC CurrentDC Current

TelephoneExchange

LocalLoop

Basic Call Progress: Switching

AddressAddresstoto

PortPortTranslationTranslationLocal

Loop

Off-HookOff-HookClosed Closed CircuitCircuit

8

Basic Call Progress: Ringing

Ring BackRing BackToneTone

DC CurrentDC CurrentDC Open Cct.DC Open Cct.Ringing ToneRinging Tone

TelephoneExchange

LocalLoop

LocalLoop

Off-HookOff-HookClosed Closed CircuitCircuit

9

Basic Call Progress: Talking

Voice EnergyVoice EnergyDC CurrentDC Current

Voice EnergyVoice EnergyDC CurrentDC Current

TelephoneExchange

LocalLoop

LocalLoop

Off-HookOff-HookClosed Closed CircuitCircuit

10

Analog Telephony—Signaling

� Supervisory

� Addressing

� Call progress

11

Off-Hook Signaling

� Loop Start (almost all telephones)

Seizure is detected when current flows through local loop, due to off-hook

12

Analog Telephony— Supervisory Signaling

� Loop startCurrent flow sensed

SwitchSwitch SwitchSwitch

13

Loop StartLoop

(Local or Station)

+–

+–

AC

+–

Station PBX or Central Office

DC Current

Ringing

SwitchSwitch

SwitchSwitch

SwitchSwitch

14

Signaling and Addressing

Analog Transmission“In-Band” Signaling0–9, *, # (12 Digits)

Digital Transmission“Out-of-Band”

Message-BasedSignaling

ISDNDial Pulse DTMF

15

Pulse Dialing

Make(Circuit Closed)

Break(Circuit Open)

Off-Hook Dialing Inter-Digit Next Digit

Pulse Period(100 ms)

US:60/40 Break/Make

700 ms

16

Tone Dialing

1 2 3 A

4 5 6 B

7 8 9 C

* 0 # D

Dual Tone Multifrequency (DTMF)

1209 1336 1477 1633

697

770

852

941

Timing:Timing:60 ms Break60 ms Break40 ms Make40 ms Make

17

Network Call Progress Tones

ToneTone Frequency (Hz)Frequency (Hz) On TimeOn Time Off TimeOff Time

DialDial

BusyBusy

Ringback, NormalRingback, Normal

Ringback, PBXRingback, PBX

Congestion (Toll)Congestion (Toll)

Reorder (local)Reorder (local)

Receiver Off-hookReceiver Off-hook

No Such NumberNo Such Number

350 + 440350 + 440

480 + 620480 + 620

440 + 480440 + 480

440 + 480440 + 480

480 + 620480 + 620

480 + 620480 + 620

1400 + 2060 + 2450 +26001400 + 2060 + 2450 +2600

200 to 400200 to 400

ContinuousContinuous

0.50.5 O.5O.5

22 44

11 33

0.20.2 0.30.3

0.30.3 0.20.2

0.10.1 0.10.1

Continuous, Freq. Mod 1HzContinuous, Freq. Mod 1Hz

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Voice Channel Bandwidth

OutputVoltage

orEnergy

Frequency(K-Hertz)

1 2 3 4.2

Voice SignalVoice Signal

Voice ChannelVoice Channel

Tone DialingSignals

Systems ControlSignals

19

Local Access Network

Telephone Exchange

Feeder Route Boundary

40,000 to 50,000 Lines

Serving AreaBoundary

20

Summary

� Analog voice technology dates back to the 1900’s

� Information exchange based on voltage, current flow, grounding, etc.

21

Digital Telephony

Digital Trunking

ExchangeExchangeExchangeExchange

Analog Loop Digital Network

POTSExchangeExchange

L.E.L.E.

Digital Loop Digital Network

ISDNSwitchSwitch

22

Digital Telephony

= Sample

8 kHz (8,000 Samples/Sec)Codec Technique

Sampling StageAnalog Audio Source

Pulse Code Modulation—Nyquist Theorem

Voice Bandwidth = Voice Bandwidth = 300 Hz to 3400 Hz300 Hz to 3400 Hz

23

Pulse Code Modulation—Analog to Digital Conversion

Stage 1

Quantizing Stage

Quantizing Noise

100100111011001

A—Law (Europe)A—Law (Europe)A—Law (Europe)A—Law (Europe)

µ—Law (USA–Japan)µ—Law (USA–Japan)µ—Law (USA–Japan)µ—Law (USA–Japan)

24

Digital Telephony Summary

� Analog telephony emulation

Voice encoding

Limited signaling

Loop consolidation

25

Voice Coding and Compression

� Speech coding schemes

� Digitizing voice

� Voice compression

26

Voice Compression Technologies

Bandwidth(Kbps)

Quality

UnacceptableUnacceptable BusinessQuality

BusinessQuality

TollQuality

TollQuality

8

16

32

24

64

0

*PCM (G.711)

*PCM (G.711)

*ADPCM 32 (G.726)

*ADPCM 32 (G.726)

*ADPCM 24 (G.726)

*ADPCM 24 (G.726)

*ADPCM 16 (G.726)

*ADPCM 16 (G.726) *

LDCELP 16 (G.728)*

LDCELP 16 (G.728)

*CS-ACELP 8 (G.729)

*CS-ACELP 8 (G.729)*

LPC 4.8*

LPC 4.8

(Cellular)(Cellular)

27

Digitizing Voice: PCM Waveform Encoding� Nyquist Theorem: sample at twice the

highest frequency Voice frequency range: 300-3400 Hz

Sampling frequency = 8000/sec (every 125us)

Bit rate: (2 x 4 Khz) x 8 bits per sample

= 64,000 bits per second (DS-0)

� By far the most commonly used methodCODEC

PCM64 Kbps

= DS-0

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Waveform Coders

Sam

pli

ng

Sam

pli

ng

Fil

teri

ng

Fil

teri

ng

En

cod

ing

En

cod

ing

Qu

anti

zin

gQ

uan

tizi

ng

1110010010010110

WaveformENCODER

WaveformDECODER

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Voice Compression

� Objective: reduce bandwidth consumption

Compression algorithms are optimized for voice

Unlike data compression: these are “loose”

� Drawbacks/tradeoffsQuantization distortion

Tandem switching degradation

Delay (echo)

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Digital Speech Interpolation (DSI)

� Voice Activity Detection (VAD)

� Removal of voice silence

� Examines voice for power, change of power, frequency and change of frequency

� All factors must indicate voice “fits into the window” before cells are constructed

� Automatically disabled for fax/modem

31

Voice Activity Detection

Voice “Spurt” Silence

Pink Noise

Time

Voice Activity(PowerLevel) SID Buffer SID

Hang Timer No Voice Traffic Sent

B/W Saved

- 54 dbm

- 31 dbm

Voice “Spurt”

32

Bandwidth Requirements

Voice Band Traffic

Encoding/Encoding/CompressionCompression

ResultResultBit RateBit Rate

G.711 PCMG.711 PCMA-Law/A-Law/uu-Law-Law

64 kbps (DS0)64 kbps (DS0)

G.726 ADPCMG.726 ADPCM 16, 24, 32, 40 kbps16, 24, 32, 40 kbps

G.729 CS-ACELPG.729 CS-ACELP 8 kbps8 kbps

G.728 LD-CELPG.728 LD-CELP 16 kbps16 kbps

G.723.1 CELPG.723.1 CELP 6.3/5.3 kbps6.3/5.3 kbpsVariableVariable

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Voice Quality

Compression MethodCompression Method MOS ScoreMOS Score DelayDelay(msec)(msec)

64K PCM (G.711)64K PCM (G.711) 4.44.4 0.750.75

32K ADPCM (G.726)32K ADPCM (G.726) 4.24.2 11

16K LD-CELP (G.728)16K LD-CELP (G.728)

8K CS-ACELP (G.729)8K CS-ACELP (G.729) 4.24.2 1515

8K CS-ACELP (G.729a)8K CS-ACELP (G.729a) 1515

3–53–54.24.2

3.63.6

Anything Above an MOS of 4.0 Is “Toll” Quality

34

Voice Network Transport

� Voice Network Transport is typically TDM circuit-based:

T1/E1

DS3/E3

SONET (OC3, OC12, etc.)

� But can also be packet-based:ATM

Frame Relay

IP

35

Voice Transport and Delay

Time (msec)

0 100 200 300 400

CB ZoneCB ZoneCB ZoneCB Zone

Cumulative Transmission Path Delay

Satellite QualitySatellite QualitySatellite QualitySatellite Quality

Fax Relay, BroadcastFax Relay, BroadcastFax Relay, BroadcastFax Relay, BroadcastHigh QualityHigh QualityHigh QualityHigh Quality

Delay Target

500 600 700 800

36

The Public Switch Telephone Network(PSTN)

Composed of the following major network elements:

lines

trunks

repeater

transmission equipment (e.g. repeaters, multiplexers, DCS)

switch switch

switch

switches

PBX

SS7 & IN

network signaling and control (e.g. SS7 and IN)

PCMODEM

station equipment (e.g. telephone sets, MODEMs, FAX machines, …)

Structure of the tel. network

Telephone networks

Telephone networks

Fiber to the curb (FTTC)

40

European Numbering Plan defines format for dialed numbers in Europe As of 1/95

00 + 353 + 71 - 91 - 55363

(N digits are 2-9, X digits are 0-9)

international calls are initiated by dialing 00, followed by country and city code (if any)

phone number shortage crisis – Why?

International preliminary code

Station number

Exchange code

Area code (geographic region)

International Country Code

41

Network Architecture

� ISDN is an access specification to a network

ISDNPBX

PRANT1BRA

Common ChannelCommon ChannelSignalling Network andSignalling Network and

DatabaseDatabase

Circuit Switched Services

Dedicated Circuit Services

Public PacketPublic PacketNetworkNetwork

TelcoTelcoSwitchSwitch

TelcoTelcoSwitchSwitch

42

Public ISDN and SS7

TransmissionNetwork

PBXPBX22PBXPBX11

Signaling System 7DSS1 DSS1

SwitchSwitchSwitchSwitch

SignalingNetwork

BRI BRI

PRI PRI

DSS1 is a Public ISDN Protocol

43

ISDN and SS7 “The Bridge Between the Islands”

ExchangeExchange

Exchange

ISDN—PRI

SS7Signaling Network

Voice Transmission

Voice Transmission

PBX1

Voiceand

Signaling

SCP

SSPSSP

SSP

SCP

STP

STP

STP

STPSTP

STP

44

Convergence