telecommunications chapter 6 updated january 2009

© 2009 Pearson Education, Inc. Publishing as Prentice Hall6-1

Raymond Panko’sBusiness Data Networks and Telecommunications, 7th edition

May only be used by adopters of the book

Telecommunications

Chapter 6Updated January 2009


The Public Switched Telephone Network (PSTN)

Carriers

Telephony

Television

© 2009 Pearson Education, Inc. Publishing as Prentice Hall 6-3

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

1. Customer PremisesEquipment

1. Customer Premises Equipment

Customer premisesEquipment (CPE) consists

Of telephones, wires,And other infrastructure

on the customer premises.

It is owned by the customer.


6-2: Customer Premises Equipment at a Business Site

Most businesses have a PBX (private branch exchange).It acts like an internal switchboard

Businesses use 4-pair UTP for in-building telephone wiring.Have long used 4-pair UTP for telephony.

Only recently was this 4-pair UTP used for data.


6-1: Elements of the PSTN

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).



3. Transport Core

3. TrunkLine

3.Switch

The Transport Core connects end officeswitches and core switches.

Trunk lines connect switches.



• Telephone Company Switch



4. Signaling System

Transport is the actual transmission of voice.

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

SS7 in the United States, C7 in Europe


Transport Versus Signaling

• Transport

– The carriage of voice during a conversation

• Signaling

– Supervisory communication to set up a connection, monitor connection quality, collect billing information, closing a connection, etc.

A frequent point of confusion


6-3: Points of Presence (POPs)

Local, long-distance, and internationalcarriers connect at POPs

(points of presence)

This permits their subscribersto call one another.


Circuits


6-4: Circuit Switching


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

Voice uses about 30% of capacity, on average.Data only uses about 5% of capacity, on average.

Circuit switching is not too wasteful for voice,but it is very wasteful for data transmission.


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

Dial-Up Circuits Leased Line Circuits

Is it a circuit with reserved capacity?

Yes, by definition Yes, by definition

Operation Dial-up. Separate circuit for each call

Permanent circuit, always on

Speed for Carrying Data

Up to 33.6 kbps 56 kbps to gigabit speeds

Number of Simultaneous Voice Calls per Circuit

One Several due to multiplexing


6- 7: Time Division Multiplexing (TDM) in T1 Lines



• Calculation

– Each conversation gets an 8-bit time slot in each frame

– There are 8,000 frames per second

– So each conversation gets 64 kbps


6-8: Local Loop Technologies

Technology Use Status

1-Pair Voice-Grade UTP

Residences Already installed, so no installation cost

2-Pair Data-Grade UTP

Businesses for high-speed access lines

Must be pulled to the customer premises. (This is expensive)

Optical Fiber Businesses for high-speed access lines

Must be pulled to the customer premises. (This is expensive)

Note: Within buildings, corporate telephony uses 4-pair UTP


Analog versus Digital Transmission


6-9: Analog Telephone Transmission

Speaking creates pressure waves, which hit the microphone in the handset.

The microphone generates an analogous electrical signal.

This is called an analog signal.

Handset


6-10: The PSTN: Mostly Digital with Analog Local Loops

The PSTN today is almost entirely digital.

This includes switches (3) and trunk lines (4).

It also includes digital leased access lines to businesses (5).


6-10: The PSTN: Mostly Digital with Analog Local Loops

Only the residential telephone (1) and the1-pair voice-grade UTP line going to residences (2)

are analog today.

Digital subscriber lines (which we will see later)Send digital signals over these 1-pair VG UTP lines.


6-11: Codec at the End Office Switch

A codec at the end office switch translates between theanalog customer signals and digital signals in the PSTN core

ADC is analog to digital conversion.

DAC is digital to analog conversion.


6-12: Frequency Division Multiplexing (FDM) in Microwave Transmission

Box

Microwave providesPoint-to-point

Terrestrial Transmission


6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)

Box

For telephone transmission, a filter at the end office switch

Bandpass filters the voice to fit into 4 kHz channels.Even when microwave is not used, this saves capacity



Box

More precisely, it cuts off all signal below about 300 Hzand above about 3,400 Hz

This gives “guard bands” below 300 HzAnd from 3,400 Hz to 4 Hz

Voice still sounds good because most energy is 30 Hz to 3,400 Hz

0 to 4 kHz



Box

The signal isSampled 8,000

Times per second.

So each sampleIs 1/8000 second



Box

In eachsampling period,

only the amplitudeof the signalIs sampled



Box

The filter candistinguish 256loudness levels.

Each loudnesslevel is representedas a binary numberbetween 0 and 255.

0 = 000000001 = 00000001

255=11111111

This requires oneoctet of storage

per sample.



Box

This sampling method,which is called pulse

code modulation (PCM),produces 8 bits

per sample.

Times 8,000samples per second –

this means64 kbps per conversation


6-14: Digital-to-Analog Conversion (DAC)

Box

Signals arriving from the PSTN are digital.

The DAC converts the 8 bits of each sample into a loudness level.

Not smooth, but sounds smooth at 8,000 samples/second.


Cellular Telephony


6-15: Cellular Technology

Customer has a mobile phone.

A city is divided into smallgeographic regions called cells.

Each cell has a cellsitewith an antenna and equipment

to serve mobile phones in the cell


6-15: Cellular Telephony

• Cellsites



A mobile telephone switching office (MTSO)coordinates activity among the cellsites.

The MTSO also connects mobile customerswith wired PSTN customers via a POP.



• Cellsites connect to the MTSO using a landline or a point-to-point radio system called microwave.

• Here is a microwave dish.

• It is covered with cloth, which does not interfere with radio transmission.

New



Channels can be reused in different cells.

This permits more customers to be served.

Serving more customers through channel reuseIs the whole reason for cellular service.



GSM cellular technology cannot use thesame channel in adjacent cells.

CDMA can use the same channel in adjacentcells, providing more channel reuse

and so more customers.



When a mobile phone travels between cells,it is handed off to the cellsite in the new cell.

In this figure, there is handoff betweenthe cellsite in Cell O and the cellsite in Cell P.



In handoff, a mobile phone moves from one cellto another cell in the same city.

In roaming, a mobile phone is taken to a different city.

In handoff, a mobile phone moves from one cellto another cell in the same city.

In roaming, a mobile phone is taken to a different city.


6-16: Handoff and Roaming in 802.11 Wireless Networking and Cellular Telephony

802.11 WLANs Cellular Telephony

Relationship Handoff and roaming mean the same thing

Handoff and roaming mean different things

Handoffs (means the same in both)

Wireless host travels between access points in an organization

Mobile phone travels between cellsites in the same city

Roaming (means different things)

Wireless host travels between access points in an organization

Mobile phone travels to a different city


Voice over IP (VoIP)


6-17: Voice over IP (VoIP)

In voice over IP (VoIP),calls are digitized, packetized, and

transported over an IP network:either an internal IP network or the Internet.

In voice over IP (VoIP),calls are digitized, packetized, and

transported over an IP network:either an internal IP network or the Internet.



The user either has aPC with multimedia hardware

and VoIP softwareor

an IP telephone that can beplugged into an IP network

via a wall jack.

Either must have a codec

The user either has aPC with multimedia hardware

and VoIP softwareor

an IP telephone that can beplugged into an IP network

via a wall jack.

Either must have a codec



A media gateway connectsa VoIP network to the PSTN.

This gives VoIP users accessTo PSTN users.

The media gateway must translatebetween both signaling technology

and transport technology.

A media gateway connectsa VoIP network to the PSTN.

This gives VoIP users accessTo PSTN users.

The media gateway must translatebetween both signaling technology

and transport technology.


VoIP

• VoIP means that a firm does not have to maintain two networks—an IP network for data and a circuit-switched voice network.

– This should reduce costs considerably by only requiring the maintenance of a single network.

• In addition, VoIP’s packet switching should be more efficient than the PSTN’s circuit switching.

• But companies have concerns about sound quality and the high availability expected of telephone service.


6-18: VoIP Signaling and Transport

The most popular SIGNALING protocol in VoIP is SIP.This figure shows how a sender initiates a connection using SIP.The initiator sends a SIP INVITE message to its SIP proxy server.

The initiator’s SIP proxy server passes the INVITE to the receivers’ server.The receiver’s SIP proxy server passes the INVITE to the receiver.

If the receiver accepts the INVITE, the conversation begins.

The most popular SIGNALING protocol in VoIP is SIP.This figure shows how a sender initiates a connection using SIP.The initiator sends a SIP INVITE message to its SIP proxy server.

The initiator’s SIP proxy server passes the INVITE to the receivers’ server.The receiver’s SIP proxy server passes the INVITE to the receiver.

If the receiver accepts the INVITE, the conversation begins.


6-19: VoIP Codecs

Codec Transmission Rate

G.711 64 kbps

G.721 32 kbps

G.722 48, 56, 64 kbps

G.722.1 24, 32 kbps

G.723 5.33, 6.4 kbps

G.723.1A 5.3, 6.3 kbps

G.726 16, 24, 32, 40 kbps

G.728 16 kbps

G.729AB 8 kbps

The two phones mustuse the same codec

to encode anddecode voice.

They must agree onone of several standard

codec protocolsthrough negotiation.

Generally,more compression

gives lowersound quality but

lowers transmission cost


Wired “Last Mile” Services

Telephone Modems

ADSL Modem Service

Cable Modem Service

Fiber to the Home


6-20: “Traditional” Technologies for the Last Mile

• The Last Mile

– The access line to your home

– Traditionally, a 1-pair VG UTP line from the telephone company

– In the 1960s, a few businesses started getting 2-pair data-grade UTP and optical fiber

– Given the cost of upgrading the 1-pair VG UTP plant, 1-pair VG UTP seemed eternal



• Telephone Service and Cable TV

– 1950s brought cable television service

• Used coaxial cable with a central wire and a coaxial conductive ring or mesh



• Telephone Service and Cable TV

– A static situation emerged

– Telephone companies controlled broadcast telephone service

– Cable companies controlled television delivery service


6-20: “Traditional” Technologies for Data Transmission in the Last Mile

• Telephone modems

– Convert digital computer signals to analog and send these over the telephone access line

– They also convert incoming analog signals into digital signals for the computer

TelephoneModem

Telephone Line

DigitalComputer

Signal:1011001101010

AnalogTelephone

Signal:



• Telephone modems

– Limited to 33.6 kbps sending / 56 kbps receiving

– Cannot use your telephone for calls while using the telephone modem


6-21: Asymmetric Digital Subscriber Line (ADSL)

Like telephone modems,ADSL also uses the existing 1-pair voice-grade

UTP line going to the home;but it offers higher speeds than telephone modems

ADSL


Telephone Modems and ADSL

• Both use the 1-pair VG UTP line running to the subscriber’s home

– Already installed, so no extra cost of running a new line

• Telephone modems send analog signals

– This is what the traditional telephone system expects

• ADSL

– Send digital signals for data (digital subscriber line)

– Requires special equipment at the end office switch (DSLAM)



Unlike telephone modem services,ADSL provides simultaneous voice and data.

The phone line is not tied up



Speed is asymmetric.Faster downstream (to home) speed

than upstream (from the home) speed.This is ideal for World Wide Web downloads.

Speeds are increasing rapidly in both directions.



Home user needsa splitter for eachtelephone outlet

Connects a phoneto the splitter voice port

Connects anADSL modem

To the splitter data port



End office switchneeds a DSLAM

(DSL access multiplexer)

Connects voice callsto the PSTN

Connects data callsto a data network


6-22: Cable Modem Service

Cable modem service is providedby the cable television company,

not by a telephone company

Generally is faster than ADSLbut also more expensive

CableModem



Optical fiber brings signalsto and from the neighborhood.

Thick coaxial cables carry signalsin the neighborhood.



Thin coaxial drop cables carrysignals from the trunk cable

to individual residences.

Subscriber needs a cable modemto receive data service.


ADSL Versus Cable Modem Service

• Generally, cable modem service is somewhat faster and more expensive than ADSL service

– However, price and performance ranges overlap

– And performance is increasing rapidly

• In cable modem service, all subscribers in a neighborhood must share the speed

– However, cable modem speed to the neighborhood is very high, so cable modem subscribers usually still get higher-than-ADSL speeds

– And other subscribers cannot read a subscriber’s transmissions, which are encrypted

© 2009 Pearson Education, Inc. Publishing as Prentice Hall

Fiber to the Home

• Some carriers are beginning to replace their 1-pair voice grade UTP residential wiring with optical fiber

• This is called fiber to the home or fiber to the premises

• Download speeds of 100 Mbps or more

• Substantially more expensive than DSL service

6-66


Wireless Access Service

Figure 6-23: Wireless Technologies for the Last Mile


6-23: Wireless for the Last Mile

• 3G Cellular Data Transmission

– 2G cellular service is for voice, texting, and photographs

• Can send data via a cellular modem, but only at 10 kbps

– 3G cellular was created to send data faster

• Most current services offer low DSL speeds at higher prices

• 2 Mbps to 3 Mbps speeds are arriving but will be even more expensive

• Consumer usage is dominating with downloading music, videos, and games

© 2009 Pearson Education, Inc. Publishing as Prentice Hall


• Cellular companies are using many 3G technologies

• Cellular companies will eventually introduce faster 4G service

– 100 Mbps or more

– Beginning to converge on Long-Term Evolution (LTE)

– Which is IP-based

6-69

New



• WiMAX Metropolitan Area Networks

– Designed to compete with DSL, cable modem service, and 3G and 4G cellular service

– Designed to serve a metropolitan area

– Users can get service anywhere,not just at hotspots



• Satellite Access Service

– Very expensive because of long transmission distance to satellites

• Hundreds to thousands of miles from the user site

• One-way transmission,which is used in televisiondelivery, is not tooexpensive

• Two-way datatransmission is complexand therefore expensive


The Market Situation


6-24: The Market Situation

• The Triple Play

– The goal of access carriers

• Telephony companies

• Cable television companies

• Wireless access companies

– Provide telephony, data, and video in a package

– Video is the hardest

• People want multiple incoming TV signals

• They also want HDTV


6-24: The Market Situation

• The International Situation

– United States ranks 16th internationally in broadband speed and availability

– Korea and Japan provide 50 Mbps speeds or faster at prices comparable to U.S. prices (for lower speeds)

– Leadership in speed brings leadership in applications

telecommunications chapter 6 updated january 2009

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