Telecommunications Chapter 6

Telecommunications

Chapter 6

6-2

Telecommunications

• From Chapter 1:

– Data communications

– Telecommunications: Voice and Video Communications

Technical Elements of the Public Switched Telephone Network

6-4

Figure 6-1: Elements of the Public Switched Telephone Network (PSTN)

1. Customer PremisesEquipment

1. Customer Premises Equipment

6-5

Figure 6-2: Customer Premises Equipment

PSTN

PBX

Handset

4-Pair UTPTelephone Wiring

Site

A typical business site.

The private branch exchange is an internal switch for the site.

4-pair UTP was created for business premises telephone wiring

Company is essentially its own telephone company that connects to the outside PSTN

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2. & 3. End OfficeSwitch (Class 5)

2.Access Line(Local Loop)

2.Access Line(Local Loop)

The Access System consists ofthe access line to the customer

(called the local loop)and termination equipment at the end office

(nearest telephone office switch).

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3. Transport Core

3. TrunkLine

3.Switch

The Transport Core connects end officeswitches and core switches.

Trunk lines connect switches.

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Figure 6-1: Elements of the PSTN

• Telephone Company Switch

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4. Signaling System

Transport is the actual transmission of voice.

Signaling is the control of calling(setup, teardown, billing, etc.).

SS7 in the United StatesC7 in Europe

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Figure 6-3: Points of Presence (POPs)

In the U.S., competingcarriers connect at

points of presence (POPs).

Local Access and Transport Area (LATA)

LocalCarrier 1Switch

LocalCarrier 2Switch

POP

LocalCarrier 1Customer

LocalCarrier 2Customer

Other Local Area

Other Country

POPLong-Distance

Carrier A

InternationalCarrier X

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Figure 6-4: Circuit Switching

A circuit is an end-to-endconnection between two subscribers.

Capacity is reserved on alltrunk lines and switches along the way.

Capacity must be paid for even if it is not used.

The PSTNhas traditionally used

circuit switching.

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Figure 6-5: Voice and Data Traffic

Full-Duplex (Two-Way) Circuit

Voice Traffic:Fairly Constant Use;Circuit Switching Is

Fairly Efficient

Data Traffic:Short Bursts,

Long Silences;Circuit Switching Is

Inefficient

Full-Duplex (Two-Way) Circuit

The reserved capacity of circuit switchingis OK for voice, but not for bursty data transmission.

6-13

Figure 6-6: Dial-Up Circuits Versus Leased Line Circuits

Dial-Up Circuits Leased Line Circuits

Operation Dial-Up. Separatecircuit for each call.

Permanent circuit,always on.

Speed for CarryingData

Up to 56 kbpsResidence can onlySend up to 33.6 kbps

56 kbps to gigabitspeeds

Number of VoiceCalls Multiplexed

One Several due to multiplexing

There are two types of circuits between customer premises:ordinary dial-up circuits and leased line circuits.

6-14

Figure 6-7: Local Loop Technologies

Technology Use Status

1-Pair Voice-GradeUTP

Residences Already installed

2-Pair Data-GradeUTP

Businesses forLowest-speedaccess lines

Must be pulled to thecustomer premises(this is expensive)

Optical Fiber Businesses forhigher-speedaccess lines

Must be pulled to thecustomer premises(this is expensive)

Residential 1-pair voice-grade UTP is already installed.This makes it inexpensive to use

Business 2-pair data-grade UTP and fiber for leased linesmust be installed; this is expensive.

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Figure 6-8: Analog Telephone Transmission

SoundWave

Analog(Analogous)

Electrical Signal

Analog signals rise and fall in intensity with the human voice.No resistance to errors as there is in digital transmission.

Initially, the entire PSTN was analog.

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Figure 6-9: The PSTN: Mostly Digital with Analog Local Loops

Trunk Line(Digital)

LocalLoop

(Analog)

LocalLoop

(Digital)

PBX(Digital)

ResidentialTelephone(Analog)

Today's Telephone Network: Predominantly Digital

Switch(Digital)

Switch(Digital)

Switch(Digital)

Today, everything is digital except for thelocal loop access line and residential telephones.

The actual local loop line can carry either analog or digital signals,but the equipment at both ends is analog.

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Figure 6-10: Codec at the End Office Switch

Analog Signal

Local LoopTelephone

Home

Codec

DAC Digital Switch

ADC

End Office

DigitalSignal

A codec at the end office translates betweenresidential analog and PSTN digital signaling.

ADC = analog to digital conversionDAC = digital to analog conversion

6-18

Figure 6-11: Frequency Division Multiplexing (FDM) in Microwave Transmission

Channel 1 / Circuit A

Channel 2 / Circuit D

Channel 3 / Circuit C

Channel 4 / Unused

Channel 5 / Circuit E

Each circuit is sent in a separate channel.If channel bandwidth is large,there will be fewer channels.

Voice uses 4 kHz-wide channelsto allow more channels.

Microwave usesradio transmission

for PSTN trunk lines

Box:Codec Operation

6-19

Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)

Subscriber

Analog ElectricalSignal

Analog VoiceSignal

Filter atEnd Office Switch

Step 1: Bandpass Filtering

At the end office, the voice signal is bandpass-filteredto limit its bandwidth to 4 MHz.

This permits more calls to be multiplexed on trunk lines

Filter atEnd Office Switch

Box:Codec

Operation

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0 Hz

Signal

300 Hz 3,400 Hz (3.4 kHz) 20 kHz

FrequencyBandwidth (3.1 kHz)

Energy Distribution ofHuman Speech Along theFrequency Spectrum

Step 1: Bandpass Filtering

Actually, to provide a safety margin, the signalis filtered to between about 300 Hz and 3.4 kHz

instead of from 0 Hz to 4 kHz.

Box:Codec Operation

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SignalAmplitude

Time

AnalogSignal

Sample

Intensity of Sample(125/255 or 01111101)

Durationof Sample

(1/8000 sec.)

0

255 (maximum)

Step 2: Pulse Code Modulation (PCM) Sampling

Nyquist found that signals must besampled at twice their highest frequency.

For a top frequency of 4 kHz,there must be 8,000 samples per second.

Each sample is 1/8000 second.

Box:Codec Operation

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SignalAmplitude

Time

AnalogSignal

Sample


Durationof Sample

(1/8000 sec.)

0

255 (maximum)


In each samplingperiod, the intensity

of the signal ismeasured.

In pulse codemodulation, the

signal is measuredas one of 256

intensity levels.

One byte storesone sample.

Box:Codec Operation

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SignalAmplitude

Time

AnalogSignal

Sample


Durationof Sample

(1/8000 sec.)

0

255 (maximum)


Pulse CodeModulation (PCM)

produces8,000 one-byte

samples per second.

This is 64 kbpsof data.

Box:Codec Operation

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ADC Recap

• First, Bandpass-Filter the Incoming Signal to 4 kHz

– Really about 300 Hz to 3.4 kHz

– To reduce transmission requirements

• The Codec then Uses PCM for the Conversion

– Samples at twice the highest frequency (4 kHz so 8,000 samples/second)

– Loudness is recorded with 8 bits per sample (to give 256 loudness levels)

– Generates 64 kbps of traffic (8 bits/sample times 8,000 samples per second)

Box:Codec Operation

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Figure 6-13: Digital-to-Analog Conversion (DAC)

To Customer:Generated “analog” signal(Sounds smooth because

the sampling rateis very high)

From digital PSTN network:Arriving digital signalfrom the PSTN Core

(8,000 Samples/Second)

00000100 00000011 00000111

DACat EndOfficeSwitch

One 8-BitSample

One 8-BitSample

Box:Codec Operation

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Figure 6-14: Cellular Telephony

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Channel47

Mobile Telephone Switching Office

Cellsite

PSTN

HandoffIn cellular technology, the regionis divided into smaller cells.

In each cell, a cellsite servescellphones in the cell.

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• Cellsites

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A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Channel47


Cellsite

PSTN

Handoff

Channels can be reused in different cells.Channel reuse supports more customers.

This is the reason for using cells.(Channel 47 is reused in cells A, D, and F)

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A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Channel47


Cellsite

PSTN

Handoff

When a subscriber moves from onecell to another in a cellular system,

this is called a handoff.

When a subscriber moves fromone city to another, this is roaming.

(In WLANs, handoffs and roamingmean the same thing.)

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A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Channel47


Cellsite

PSTN

Handoff

The Mobile TelephoneSwitching Office (MTSO)

coordinates the cellsites andimplements signaling and handoffs.

The MTSO also connectscellphones to the PSTN

(called the wireline network).

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Cellular Technologies

• GSM is the worldwide standard for cellular voice– Uses time division multiplexing (TDM)

– Uses 200 kHz channels

– Divides each second into many frame periods

– Divides each frame into 8 slots

– Gives same slot in each frame to a conversation

Slot 1Conversation

A

Slot 2Conversation

B

Slot 8Conversation

H

……Slot 1

ConversationA

Time Frame 1 Frame 2

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• Cannot use the same channel in adjacent cells

– So can only reuse a channel about every 7 cells

– For example, suppose there are 50 cells

• Channel can be reused 50 / 7 times

• This is 7 (not precise, so round things off)

• So each channel can support 7 simultaneous customers in these 7 cells

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• Code Division Multiple Access (CDMA)

– Also used in the United States

– A form of spread spectrum transmission

– Unlike traditional spread spectrum technology, multiple users can transmit simultaneously

– 1.25 MHz channels

– Can support many users per channel

• Can use the same channel in adjacent cells– So can only reuse a channel in every cell

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Figure 6-15: Voice over IP (VoIP)

PC withMultimedia Hardwareand VoIP Software

IP Telephonewith

Codec andTCP/IP Functionality

MediaGateway

PSTN

Internet

VoIP carries telephone calls overLANs and the Internet

With IP, there is no wasted capacityas there is with circuit switching.

This reduces cost.

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IP Telephonewith


MediaGateway

PSTN

Internet

Stations can be special IP telephoneswith IP functionality

Or a PC with multimedia hardwareand VoIP software

IP phones need a codec to convertvoice analog signals from the microphone

into digital IP signals

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IP Telephonewith


MediaGateway

PSTN

Internet

A media gateway connects aVoIP network to the PSTN

Handles transport and signaling differences

6-37

Figure 6-16: Speech Codes

Codec Transmission Rate

G.711 64 kbps (pulse code modulation)

G.721 32 kbps (adaptive PCM)

G.722 46, 56, or 64 kbps

G.722.1 24, 32 kbps

G.723.1A 5.3, 6.3 kbps

There are several codec standards.They differ in transmission rate, sound quality, and latency.

Both sides must use the same codec standard.

6-38

Figure 6-17: VoIP Protocols

Transport is the transmission of voice(carries codec data).

Signaling is call supervision.

Signaling: SIP or H.323(Call setup, breakdown, accounting, and other supervisory tasks)

IPHdr

UDPHdr

RTPHdr

Codec DataStream

Transport(Voice transmission)PC with Multimedia and

VoIP SoftwareIP Telephone

VoIP Transport Packet

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IPHdr

UDPHdr

RTPHdr

Codec DataStream




2. The UDP header is followed by aReal Time Protocol (RTP) header, which contains

a sequence number and timing information.Receiver uses timing information to smooth out sound playback.

1. VoIP transport packets use UDP at the transport layer.(There is no time for retransmissions to repair errors.)

The receiver puts in fill sounds for lost packets.

3.The applicationmessage is acodec data

stream

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IPHdr

UDPHdr

RTPHdr

Codec DataStream




Signaling is call supervision.The H.323 signaling standard came first for VoIP signaling.

SIP is simpler and now dominates VoIP signaling

6-41

Video over IP

• The Other VoIP

– It’s not just voice over IP

– Video Telephones

– Video Conferencing

• PC to PC

• Multiparty

• Sometimes room-to-room

– Video Downloads on Demand

6-42

Figure 6-18: Residential Internet Access Services

• Telephone Modems

• Broadband Internet Access

• Asymmetric Digital Subscriber Line (ADSL)

• Cable Modem Service

• 3G Cellular Data Service

• WiMAX (802.16d and 802.16e)

• Broadband over Power Lines

• Fiber to the Home (FTTH)

Note:Speeds and Prices

Change Rapidly

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Figure 6-19: Telephone Modem Connection to an ISP

Client A

TelephoneModem Telephone

PSTN (Digital)

33.6 kbpsDigital

AnalogAccess

Line

Analog

56 kbps

Telephone modemsconvert digital computer

signals to analogtelephone signals.

6-44


Digital Leased Line(No Modem)

ISP

PSTN (Digital)

56 kbpsDigital

33.6 kbps

ISP does not have a modem.

It has a digital leased line socan send at 56 kbps.

(There is no bandpassfiltering on digital leased lines.)

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Client A

TelephoneModem

Digital Access Line(No Modem)

TelephoneISP

PSTN (Digital)

33.6 kbps56 kbps

DigitalAnalogAccess

Line

Digital

Analog

56 kbps

Circuit

Dial-up circuits connect the client with the ISP.56 kbps downstream, 33.6 kbps upstream

33.6 kbps

6-46

Telephone Modem Limitations

• Very low transmission speeds

– Long delays in downloading webpages

• Subscriber cannot simultaneously use the telephone line for voice calls

• Still used by 30% to 40% of Internet users.

6-47

Figure 6-20: Amplitude Modulation

Client A

Binary Data

Modem

Amplitude Modulation

Telephone

Modulated AnalogSignal

1 0 1 1

PSTNSerialCable

TelephoneCable

Modulation is the conversion of binary computer signalsinto analog signals that can travel over an ordinary access line.

Demodulation, at the other ends, converts the modulatedsignals back to digital computer signals.

6-48

Figure 6-20: Amplitude Modulation

Client A

Binary Data

Modem

Amplitude Modulation

Telephone

Modulated AnalogSignal

1 0 1 1

PSTNSerialCable

TelephoneCable

In amplitude modulation, there aretwo amplitude (loudness levels)—

one for 1 and one for 0

1011 is loud-soft-loud-loud

6-49

Figure 6-21: Asymmetric Digital Subscriber Line (ADSL)

PCADSL

Modem Single Pair ofVoice-GradeUTP Wires

Splitter DSLAM

Telephone

Telephone CompanyEnd Office Switch

SubscriberPremises

DataWAN

PSTN

ADSL ALSO uses the existing residential local loop technology.Inexpensive because no need to pull new wires, but

1-pair voice-grade UTP is not designed for high-speed transmission.

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PCADSL

Modem Single Pair ofVoice-GradeUTP Wires

Splitter DSLAM

Telephone


SubscriberPremises

DataWAN

PSTN

1.Subscriber needs an ADSL modem.

Also needs a splitter for each telephone wall outlet.

2.Telephone carrier needs a digital subscriber line

access multiplexer (DSLAM) to separate the two signals.

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Unlike telephone modems, ADSL serviceprovides simultaneous voice and data transmission.

PCADSL

Modem

Ordinary TelephoneService

Single Pair ofVoice-GradeUTP Wires

Downstream DataUp to 1.5 Mbps

Upstream DataUp to 512 kbps

Splitter DSLAM

Telephone


SubscriberPremises

DataWAN

PSTN

Downstream DataUp to 3 Mbps

6-52


PCADSL

Modem

Ordinary TelephoneService

Single Pair ofVoice-GradeUTP Wires

Downstream DataUp to 1.5 Mbps

Upstream DataUp to 512 kbps

Splitter DSLAM

Telephone


SubscriberPremises

DataWAN

PSTN

Downstream DataUp to 3 Mbps

Speed is asymmetricFaster downstream than upstream

(Up to 3 Mbps versus up to 512 kbps)Ideal for Web accessAcceptable for e-mail

Good for residential use

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PCCable

Modem

Subscriber Premises

NeighborhoodSplitter Cable Television

Head End

OpticalFiber to

Neighborhoods

ISP

Coaxial Cablein Neighborhood

(Shared Throughput)

Coaxial CableDrop Cable

UTPor

USB

Maximum downloadthroughput is about 5 Mbps

Figure 6-22: Cable Modem Service

Cable modem service brings high-speedoptical fiber lines to the neighborhood.

6-54

PCCable

Modem

Subscriber Premises


Head End

OpticalFiber to

Neighborhoods

ISP

Thick Coaxial Cablein Neighborhood

(Shared Throughput)

ThinCoaxial CableDrop Cable

UTPor

USB


In the neighborhood,thick coaxial cablebrings service to

households.

This bandwidth isshared by

everyone in theneighborhood.

A thin coax linegoes to each

home’scable modem.

6-55


PCCable

Modem

Subscriber Premises


Head End

OpticalFiber to

Neighborhoods

ISP

Thick Coaxial Cablein Neighborhood

(Shared Throughput)

ThinCoaxial CableDrop Cable

UTPor

USB

Maximum downloadthroughput is about 5 Mbps

Downstream speeds up to 5 Mbps.Upstream speeds up to about 1 Mbps.

6-56

ADSL versus Cable Modem Service

• Do Not Over-Stress the Importance of Sharing– Cable modem service usually is still faster than ADSL

service

– DSLAM sharing can slow ADSL service too

• The Bottom Line Today:– Cable modem service usually is faster

– ADSL service usually is cheaper• ADSL offers more speed-price options

• Both are improving rapidly in terms of speed and (sometimes) price

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Figure 6-23: Third-Generation (3G) Cellular Data Services

• Cellphone connects to computer via a cellphone modem or USB

• Traditional GSM and CDMA

– Limited to only about 10 kbps

– Far too slow for usability

6-58

Figure 6-23: Third-Generation (3G) Cellular Data Services

• Both GSM and CDMA are evolving

• Second Generation (now dominant)– Only 10 kbps data transmission

• Third Generation– Low end: comparable to telephone modem service

– High end: comparable to low-speed DSL service

• Future– Speeds comparable to high-end DSL or cable modem

service

– 100 Mbps or more (fast enough for good video)

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• WiMax (802.16)

– Wireless Internet access for metropolitan areas

– Basic 802.16d standard: ADSL speeds to fixed locations

• Will use dish antennas

• Just reaching the market

– 802.16e will extend the service to mobile users

• Will use omnidirectional antennas

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• Satellite InternetAccess

– Very expensive

– Often needed to serve rural areas

New

6-61


• Broadband over Power Lines

– Broadband data from your electrical company

– It already has transmission wires and access to residences and businesses

– It can modulates data signals over electrical power lines

– It works, but has very limited availability and is slow

– Especially promising for rural areas

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– Carrier runs fiber to the home

– Provides speeds of tens of megabits per second for high-speed video, etc.

• Less if fiber only goes to the curb (FTTC)

• Or to the neighborhood (FTTN)

– Much faster than other residential internet access services

– Could dominate residential (and business) Internet access in the future

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Internet Access and VoIP

• Most ISPs are Planning to or Already Provide VoIP Telephone Service

– An alternative to the local telephone company service

– Media gateways will interconnect with the PSTN

– Should be less expensive that traditional phone service

– Questions remain• Voice quality and reliability• 911 and 911 location discovery• Regulation and taxation• Laws that require wiretapping with warrants

Topics Covered

6-65

Telecommunications

• Data Communications versus Telecommunications

• The PSTN’s Technical Elements

– Customer premises equipment (PBX and 4-pair UTP)

– Access system (local loop)

– Transport core

– Signaling (call setup and management)

• POP to interconnect carriers

6-66

Telecommunications

• Access Lines

– For residences, 1-pair voice-grade UTP

• DSL uses existing residential access lines to carry data by changing the electronics at each end (DSL modem in the home and DSLAM at the end office switch)

• DSL is cheap because 1-p VG UTP is already in place

– For businesses,

• 2-pair data-grade UTP for speeds up to a few Mbps

• Optical fiber for faster speeds

• Usually must be pulled into place, so expensive

– Eventually, fiber to the home (FTTH), FTTC, FTTN

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PSTN Transmission

• Circuit Switching

– Reserved capacity end-to-end

– Acceptable for voice, but not for bursty data transmission

– Dial-up and leased line circuits

• Analog and Digital Transmission

– Analog signals on the local loop

– ADC and DAC at the end office switch

– ADC: bandpass filtering and sampling for 64 kbps

– DAC: sample values are converted to sound levels

6-68

Cellular Telephony

• Cells Allow Channel Reuse

– Channel reuse allows more customers to be served with a limited number of channels

• GSM: most widely used technology for cellular telephony

• CDMA for greater channel reuse

• Handoffs and Roaming

6-69

VoIP

• To allow voice to be carried over data networks

• Converge voice and data networks

• Phone needs a codec

• Transport: UDP header followed by RTP header

• Signaling: H.323 and SIP

• Video over IP

6-70

Residential Internet Access Services

• Telephone Modems

• Asymmetric Digital Subscriber Line (ADSL)

• Cable Modem Service

• 3G Cellular Data Service

• WiMAX (802.16 and 802.16e)

• Broadband Over Power Lines


Documents

Telecommunications Chapter 6