modems and beyond chapter 5 copyright 2001 prentice hall

Modems and Beyond

Chapter 5

Copyright 2001 Prentice Hall

2Modems

Problem– Computer is digital– Telephone line is analog– Need translation device called a modem

DigitalSignal

Modem

AnalogSignal

3Modems

Note:– Device is digital;Transmission line is analog

– Cannot just say “modem translates between analog and digital” (For analog device and digital transmission line, need something else)

DigitalDevice

AnalogTransmission Line

4Modulation

A Modem is a Modulator and Demodulator

– Modulation is converting outgoing digital device signals into analog transmission line signals

– Demodulation is converting incoming analog transmission line signals into digital device signals

DigitalSignal

Modem

AnalogSignal

5Modulation

Example: Frequency Modulation– 1 is a high-frequency vibration– 0 is a low-frequency vibration

1

0

1

1

6Modem Forms

Internal Modem– On printed circuit board inside PC– Does not take up desktop space– Can be complex to install--must open up

computer

7Modem Forms

External Modem– Easy to install (just plug into serial port)– Takes up desk space– Needs wire to serial port (adds to wiring

clutter)– Needs electrical power

8Modem Forms

PC Card Modems– Fit in PC Card slots on notebooks– Easy to install– Do not take up desktop space– Expensive– Few desktop PCs have PC Card slots

9Modem Connection for Internal Modem

Phone Line toTelephone

Phone Line toWall Jack

10Modem Connection for External Modem

PhoneLine to

Telephone

Wall Power:usually uses

“brick” transformer

Serial Cableto Serial

Port

Phone Line toWall Jack

11Modem Standards

Two Modems Must Follow Same Standards– Speed standards (modulation method)– Error correction and compression standards– Facsimile standards– Modern modem standards are created by the

ITU-T

12Modem Standards

Training Period

– When two modems first start talking, they negotiate standards to use; settle on highest common standards

13Modem Speed Standards (ITU-T)

V.90– Receive at 56 kbps but send at only 33.6 kbps– Not all phone lines, ISPs support 56 kbps– Fall back to 33.6 kbps if cannot support

V.34– 33.6 kbps send and receive

V.32 bis– 14.4 kbps send and receive– bis means second (version of the standard)

14Modem Standards: Error Correction and Compression

ITU-T

– V.42: Error detection and correction– V.42 bis: Compression (up to 4:1)– Most newer modems have both

MNP Level 5

– Proprietary standard of Microcom– Once widely used– Provides both error correction and compression– Now largely superceded by V.42 and V.42 bis

New

15Modem Standards

Facsimile Modem Standards

– Most modems also act as fax modems

– Facsimile requires different modulation than data transmission

– V.14: 14.4 kbps

– V.29: 9.6 kbps

16V.90 Modems

Telephone Bandwidth is Limited (Chapter 4)– Bandwidth limits speed– Limits speed to about 35 kbps– When you transmit, limited to 35 kbps

35 kbps

17V.90 Modems

Telephone Bandwidth is Limited

– Limits occur when you transmit in analog to the phone company

Analog-to-digital converter (ADC) at the telephone company limits you to about 35 kbps

TelephoneNetwork

TelephoneNetworkADCADCPCPC

V.34modem

33.6 kbps

18V.90 Modems

Receive at 56 kbps– ISP connects with digital line to the Telco– Sends at 56 kbps– No analog-to-digital converter to limit speed

TelephoneNetwork

TelephoneNetworkDACDACPCPC

56 kbpsmodem ISPISP

DigitalLink

NoADC!

56 kbps

19V.90 Modems

Receive at 56 kbps– 56 kbps digital channels inside phone network– Only local loop is analog today– Trunk lines and switches support 56 kbps

transmission

TelephoneNetwork


56 kbpsmodem ISPISP

Digital Switches and Trunk Lines

56 kbps

20V.90 Modems

Receive at 56 kbps– Telephone company transmits in analog to

subscriber at 56 kbps– Digital to audio converter (DAC) does not limit

speed to less than 56 kbps

TelephoneNetwork


56 kbpsmodem ISPISP

DigitalLink

56 kbps

21V.90 Modem

Telephone Company Does Not Have to Do Anything Differently– ADCs and DACs are already in place for

ordinary voice service– Lack of change in phone system allowed fast

implementation

TelephoneNetwork

TelephoneNetwork

ADCADC

PCPC56 kbpsmodem ISPISP

DigitalLink

DACDAC

22V.90 Modem

ISP Does Have to Do Things Differently– Digital line to telephone network– Special equipment at ISP– Does not use a V.90 modem– If used V.90 modem, could only send at 33.6

kbps

TelephoneNetwork


56 kbpsmodem ISPISP

DigitalLink

NoV.90

Modem!

23V.90 Modem

User Needs a V.90 Modem

– Does not need a new phone line

– Not all phone lines will work, but it is never worth the cost to install a new line for V.90 modems

24Alternatives to Ordinary Phone Lines & Modems

Faster Speeds; Called Broadband Services

Also Higher Cost

– Translation devices more expensive than modems

– Faster (more expensive) transmission line to ISP

– ISP may charge more because of heavier transmission load

– Faster interface than 232 serial port may be needed

25Transmission Line Versus ISP

Transmission line may be provided by a transmission carrier, not the ISP

Users have to pay separate fees to carrier and ISP

Some ISPs also provide transmission line– But not all do– Transmission line costs must always be

considered

PCPC ISPISPTransmission Line

26Translator Devices

Device versus Line

Analog Line Digital Line

Analog Device Codec

Digital Device Modem DSU

TranslatorTransmission Line

Device

ISDN

Integrated Services Digital Network

Codecs

DSUs

28ISDN

Get ISDN line from telephone company– Multiplexes (mixes) three channels on one UTP wire

pair to the desktop

1101

BChannel

BChannel

2B+D

MultiplexedOnto One

Set of Wires

ISDNModem

D Channel:control signals

29ISDN

Two B Channels are 64 kbps– Original idea: one for voice, one for data

1101

BChannel

BChannel

30ISDN

Can now “bond” the two B channels for 128 kbps data if ISP supports it

D Channel is 16 kbps (for supervisory signaling)

TelephoneNetwork

TelephoneNetwork ISPISPISDN

Modem

ISDNModem 128

kbps

B

B

Bonding

31ISDN Costs

Must install an ISDN line– Will cost more than telephone line

Actually, could use your existing phone line– But then would would need one B channel for

telephony

32ISDN Costs

Line is expensive– $60-$80/mo plus installation fee– ISP charge is separate; may charge more for

ISDN access

Dial Up: Not always connected– Do not have to pay for full-time use

Need “ISDN modem” (expensive)

33ISDN Modem

ISDN “Modem” is a Misnomer– Modem is for for digital device, analog line– ISDN line is digital

ISDN modem really contains

– Codec to link analog telephone to digital ISDN line

– Data service unit (DSU) to translate between PC digital format and ISDN digital format (voltage levels, timing, etc.)

34ISDN Modem

Codec– Translates analog device signal into digital

signal for propagation– Constantly samples the intensity of the voice

analog signal

Sample

1/8,000 sec

0110010

35ISDN Modem

Codec– Divides each second into 8,000 sampling

periods– Only measures intensity of voice signal in each

Sample

1/8,000 sec Sampling Period

IntensityValue

0110010

36ISDN Modem

Codec– Measures voice intensity as an 8-bit intensity

value (0-255)– Overall, sends 8 bits 8,000 times per second (64

kbps)

Sample


IntensityValue

0110010

37ISDN Modem

Codec– This is why telephone channels are 64 kbps

Designed for digitized voice Often steal 8 kbps for supervisory signaling, so 56

kbps

Sample


IntensityValue

0110010

38DSU (Data Service Unit)

DSU translates between different digital formats– Device and line are both digital, but still must

have translator Different bit rates Different number of possible states Different voltage levels for the states Different ways to represent ones and zeros

DSUDSUDigital Line

Cable Modem Service

Broadband Internet Access

40Cable Modem Service

Service of Cable Television Companies– Deliver 10 Mbps downstream to the home– Capacity is shared by multiple subscribers, so

real speed is more limited– Limited to about 64 kbps to 256 kbps upstream– Does not tie up telephone line– Always available

Cable TVNetwork

Cable TVNetwork

Also ISPFunctions

Also ISPFunctions

CableModem

CableModem

41Cable Modem Sharing in Perspective

Sharing is Not as Extreme as it May First Appear– Usually, shared within Blocks of 500 Houses– Only Some Households in Block will Subscribe– Only Some Subscribers will be Online at Any

Moment– Only Some of Online Subscribers will Send and

Receive at Any Moment– Only these will Share the Capacity

So Sharing Does Not Reduce Speed Unless the Adoption Rate is Large

42Cable Modem Service

Cost is about $50 per Month

– Includes ISP service!

– Installation usually costs $100 to $150 and includes a cable modem and a network interface card

– Best alternative today beyond V.90 modem

– Cost-competitive with adding a second phone line to handle your modem communication

DSLs

Digital Subscriber Lines

44Digital Subscriber Lines (DSLs)

Offered by Telephone Companies– Lines to customer premises are subscriber

lines, which connect subscribers to the telephone system

– These are digital– Hence “digital subscriber line”

TelephoneNetwork

TelephoneNetwork ISPISPDSL

Modem

DSLModem

DSL

45Digital Subscriber Lines (DSLs)

Offered by Telephone Companies– Several types of digital lines for subscribers

Some are for homes, others for businesses

– For residential customers, usually multiplexes regular phone, high-speed data

Can use existing phone line coming into house Can use Internet without tying up phone

TelephoneNetwork


Modem

DSLModem Existing

Phone Line

46Digital Subscriber Lines

Most common for home is Asymmetric DSL (ADSL)– Upstream and downstream speeds are different– Upstream at 64 kbps or more– Downstream at 256 kbps to a few Mbps– Asymmetric speed is good for WWW– About $50 per month incl. ISP; more for faster service

TelephoneNetwork


Modem

DSLModem

ADSL

64 or more kbps

256 kbps-a few Mbps


G.Lite (G.992.2) Standard– New ADSL standard from ITU-T– Up to 1.5 Mbps downstream speed– No carrier installation is necessary– Not being widely adopted by ADSL vendors

yet

TelephoneNetwork


Modem

DSLModem

ADSL

64 or more kbps

Up to 1.5 Mbps

New


Digital Subscriber Line Access Multiplexer– Telco must install a DSLAM at end switching

office

TelephoneNetwork

TelephoneNetwork ISPISPDSLAMDSLAM

DSL

DSL

DSL


Splitting Voice and Data– Voice and data are split at home by the DSL

modem– Voice and data are also split at the telephone

company’s first switching office

DSLModem

DSLSplitter

DSL

DSLAM

VoiceNetwork

Computer

Phone

New

50DSLs Versus Cable Modem Service

Both Expected to Sell Well

Forecast for Worldwide Installations in 2004– Cable modem lines: 9 million– DSLs: 10 million– Source: Insight Research

Cable is Available Mostly in Residential Areas

Cable is only widespread in some countries, especially the United States

51Broadband Access Lines in Perspective

Limited to Transmission Between Customer Premises and the ISP– ISP Speed can Limit Service Speed

– Internet Backbone Speed can Limit Service Speed

– Server Host Access Line Speed can Limit Service Speed

– Server Host Processing Speed can Limit Service Speed

Broadband Access Lines are Not Panaceas

52Wireless Internet Access

Reach the Internet by Radio– Terrestrial (uses earth stations)– Satellite-based

ISP

53GEO Satellites

In geosynchronous orbit– Appear to be stationary in the sky

Far from the ground– 36,000 km (22,300 miles)– Need much power to send/receive– Need dish antennas to concentrate signals– Must point dish at the satellite– Impractical for portable computers

54VSATs

Very Small Aperture Terminals (VSATs)– Small dishes (1 meter in diameter or less)– Do not concentrate signal as well as large

dishes, so inefficient– Do not focus as precisely as large dishes, so

satellites cannot be spaced closely together– However, inexpensive– Used when there must be many ground stations

55LEO Satellites

Low Earth Orbit (LEO) Satellites

– Below First Van Allen Radiation band, which peaks at 3,000 km (1,800 miles)

– A typical orbit is 1,000 km or 600 miles

OmnidirectionalAntenna

New: Not in Book

56LEO Satellites

Low Earth Orbit (LEO) Satellites

– Need far less power to reach than 36,000 km (22,300 mile) geosynchronous satellites

– So can get by with omnidirectional antenna

– Can use phone of reasonable size, cost

– Access anywhere is the world, although local laws may limit legality

OmnidirectionalAntenna

57MEO Satellites

Medium Earth Orbit (MEO) Satellites

– Between First and Second Van Allen Radiation Band, which peaks at 15,000 km (about 9,000 miles)

– Typical distance is 6,000 km or 4,000 miles

– Farther away than LEOs, so need more power

– But travel more slowly across sky, so fewer hand-offs, fewer satellites needed

New

58Ports

Need port fast enough for line

232 Serial port to 115.2 kbps: only V.34, V.90 or ISDN

USB (universal serial bus)– 12 Mbps– Available on all new PCs– Fast enough for DSLs, cable modems– Faster version coming (USB-2, ~480 Mbps)

59Ports

Firewire (IEEE 1394)– 400 Mbps and faster– Not available on most new PCs– Fast enough for DSLs, cable modems

Ethernet NIC (10 Mbps)– Network interface card used in PC networks– Printed circuit board– Must be installed inside PC systems unit– Fast enough for DSLs, cable modems

60Small Office Service

Serial Router Connects Internal Stations to Shared Broadband Access Service

SmallOffice

Serial Router

SharedBroadband

Line

ISP


Serial Routers are Very Simple– One or more ports to connect the serial port to

the LAN– One port to connect to the high-speed access

line, which is serial– Much less expensive than general-purpose

routers because no complex router forwarding decision caused by multiple possible output ports

Serial Router


Serial Router Connects Internal Stations to Shared Broadband Access Service– Companies with several stations pay much

more for line than when connecting a single PC

– Each machine gets its own IP address via DHCP

– Serial router may be a user PC with added software

– Often, router is called a gateway (the old name for router)

63Connecting Two PCs at Home

Physically, One PC (the Gateway PC) Connects to the Hub or Switch and to the Broadband Line– Gateway (router) PC needs two Network Interface

Cards, one for hub, one for broadband modem

GatewayPC

BroadbandLine to ISPADSL/Cable

Modem

Hub or Switch

NIC


Gateway PC Needs Gateway Software– On NIC to broadband line, acts like a single station– Gets temporary IP address from ISP DHCP– Sends and receives to ISP using this IP address

GatewayPC

Send, Receive withTemporaryIP Address


Gateway PC Needs Gateway Software– Each PC is set up with a permanent IP address– So gateway PC has two addresses, one bound to each

NIC

GatewayProgram

TemporaryIP Address

PermanentIP Address

PermanentIP Address


Gateway Software Does Network Address Translation (NAT)– When a PC sends using permanent IP address– Gateway software replaces source IP permanent

address with temporary IP address ISP expects

GatewayPC

TemporaryIP Address

PermanentIP Address

PermanentIP Address


Gateway Software Does Network Address Translation (NAT)– When a PC receives– Gateway software replaces ISP temporary IP

destination address with receiver’s permanent IP address, passes on to PC

GatewayPC

TemporaryIP Address

PermanentIP Address

PermanentIP Address

modems and beyond chapter 5 copyright 2001 prentice hall

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