Modems and Beyond Chapter 5 Copyright 2001 Prentice Hall Revision 2: July 2001

Modems and Beyond

Chapter 5

Copyright 2001 Prentice HallRevision 2: July 2001

2Modems

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

Modulation: converting outgoing digital device signals to analog transmission line signals

Demodulation: converting incoming analog transmission line signals into digital device signals

DigitalSignal

Modem

AnalogSignal

3Modulation

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

1

0

1

1

4Modem Forms

Internal Modem– On printed circuit board inside PC– Does not take up desktop space– Complex access--must open computer

Phone Line toTelephone

Phone Line toWall Jack

5Modem Forms

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

PhoneLine to

Telephone

Wall Power:usually uses

“brick” transformer

Serial Cableto Serial

Port

Phone Line toWall Jack

6Modem 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

7Modem Standards

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

Training Period– When two modems first start talking, they negotiate

standards to use; settle on highest common standards

8Modem 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)

9Modem 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

10Modem 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

11New Modem Standards

V.92– Can transmit faster than 33.6 kbps but only if the line

has unusually high top frequency cutoff– If transmits faster, reception speed falls below 56 kbps– Modem on hold: can talk a short time without breaking

the connection– Cuts call setup time in half

V.44– Can compress webpages better, cutting download time

in half.

12V.90 Modems

Telephone Bandwidth is Limited – Limits speed to about 35 kbps

Upload (Sending) Speed is Limited– Analog-to-digital converter (ADC) at the

telephone company filters & converts your incoming signal

– Limits you to about 35 kbps

35 kbps

TelephoneNetwork

TelephoneNetworkADCADCPCPC

V.90modem

33.6 kbps

13V.90 Modems

However, Receive at 56 kbps– ISP connects with digital line to the Telco– Sends at 56 kbps– No analog-to-digital converter to filter and so

limit speed

TelephoneNetwork

TelephoneNetworkDACDACPCPC

V.90modem ISPISP

DigitalLink

NoADC!

56 kbps

14V.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


V.90modem ISPISP

Digital Switches and Trunk Lines

56 kbps

15V.90 Modems

Receive at 56 kbps– Telephone company transmits in analog to

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

limit speed to less than 56 kbps. No filtering.

TelephoneNetwork


V.90modem ISPISP

DigitalLink

56 kbps

16V.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

PCPCV.90

modem ISPISP

DigitalLink

DACDAC

17V.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


V.90modem ISPISP

DigitalLink

NoV.90

Modem!

18V.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

19Alternatives 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

20Transmission 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

– Transmission line costs must always be considered in addition to hardware

External Modem High-Speed ports

21Ports

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– Faster version coming (USB-2, ~480 Mbps)

Firewire (IEEE 1394)– 400 Mbps and faster– Not available on most new PCs

Ethernet NIC (10 Mbps)– Network interface card used in PC networks– Printed circuit board– Must be installed inside PC systems unit

22ISDN

ISDN line from telephone company (dial-up digital service)– 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

23ISDN

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

BondingBonding

24ISDN Costs

Must install an ISDN line– Will cost more than telephone line (3X)

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

telephony

25ISDN 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– Good if usage is small per day, say to upload

sales data from retail once per night– Good for backup to always-on services

Need “ISDN modem” (expensive)

26ISDN Modem

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

– 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.)

StationAnalog

Analog

Digital

DigitalCodec

Modem DSU

27ISDN Modem

Codec– Translates analog device signal into digital signal for

propagation– Divides each second into 8,000 sampling periods– Constantly samples the intensity of the voice analog

signal in each sample– Measures voice intensity as 8-bit value (0-255)– Sends 8 bits 8,000 times per second (64 kbps)

Sample

1/8,000 sec Sampling Period

IntensityValue

0110010

28DSU (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

29Cable Modem Service

Service of Cable Television Companies– 10 Mbps downstream

Capacity is shared by multiple subscribers (real throughput 300 – 500 kbps)

Shared within Blocks of 500 Houses

– 64 kbps to 256 kbps upstream– Does not tie up telephone line– Always available

Cable TVNetwork

Cable TVNetwork

Also ISPFunctions

Also ISPFunctions

CableModem

CableModem

30Cable 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

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

31Digital Subscriber Lines (DSLs)

Offered by Telephone Companies– subscriber lines: connect customer premises to

telephone system– Entirely digital– Different subscriber lines for home & business

TelephoneNetwork

TelephoneNetwork ISPISPDSL

Modem

DSLModem

DSL

32Digital Subscriber Lines (DSLs)

Residential customers: multiplexes regular phone Can use existing phone line coming into house Can use Internet without tying up phone

– Most common Asymmetric DSL (ADSL)

TelephoneNetwork


Modem

DSLModem Existing

Phone Line

33Digital Subscriber Lines

Lower-speed upstream and higher-speed downstream About $50 per month includes ISP

– Businesses pay more for higher speeds (up to 1.5 Mbps) DSL service requires DSLAM (Digital Subscriber Line

Access Multiplexer)

G.Lite (G.992.2) Standard– Up to 1.5 Mbps downstream speed

TelephoneNetwork


Modem

DSLModem

ADSL

64 or more kbps

384 kbps-640 kbps


Digital Subscriber Line Access Multiplexer– Telephone company must install at end

switching office to serve DSL users

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

DSLAMDSL

DataNetwork

VoiceNetwork

Computer

Phone

36Wireless Internet Access

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

ISP

37GEO Satellites

In geosynchronous orbit– Appear to be stationary in the sky– 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

38VSATs

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

dishes, so inefficient– Less precise focus then large dishes, satellites

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

39LEO Satellites

Low Earth Orbit (LEO) Satellites– Below First Van Allen Radiation band,

peaks at 3,000 km (1,800 miles)– A typical orbit is 1,000 km or 600 miles

Medium Earth Orbit (MEO) Satellites– Between 1st and 2nd Van Allen Radiation Band,

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

OmnidirectionalAntenna

40Small Office Service

Serial Router Connects Internal Stations to Shared Broadband Access Service

SmallOffice

Serial Router

SharedBroadband

Line

ISP


Serial Routers (Also Called Access 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)

Documents

Modems and Beyond Chapter 5 Copyright 2001 Prentice Hall Revision 2: July 2001