Point-to-Point CommunicationPoint-to-Point Communication

2Terminal-Host CommunicationTerminal-Host Communication ComponentsComponents

TerminalTerminal Host (locus of processing)Host (locus of processing) Transmission line (here, phone line and modem)Transmission line (here, phone line and modem) Telephone line acts as a point-to-point linkTelephone line acts as a point-to-point link

PhoneLine

Modem Modem

TerminalHost

3Terminal EmulationTerminal Emulation

People Already Have PCsPeople Already Have PCs

Host operating systems only work with terminalsHost operating systems only work with terminals

Do not want to buy a terminal as wellDo not want to buy a terminal as well

PCs can emulate (act like) terminalsPCs can emulate (act like) terminals

Only requires software (a communication Only requires software (a communication program)program)

Turns an expensive PC into a cheap terminalTurns an expensive PC into a cheap terminal

4VT100 TerminalsVT100 Terminals VT100 Emulation Only Needs SoftwareVT100 Emulation Only Needs Software

Communications programCommunications program Terminal emulation softwareTerminal emulation software

Most Hosts Support VT100 TerminalsMost Hosts Support VT100 Terminals

Lowest Common DenominatorLowest Common Denominator Slow: maximum speed of 19 kbps, usually slowerSlow: maximum speed of 19 kbps, usually slower Uses inefficient asynchronous ASCII transmission, Uses inefficient asynchronous ASCII transmission,

discussed laterdiscussed later No graphics or even multiple fonts: plain text onlyNo graphics or even multiple fonts: plain text only No colorNo color

5Terminal Emulation SoftwareTerminal Emulation Software File TransferFile Transfer

Transfer whole files with error correctionTransfer whole files with error correction UploadUpload: from PC to host: from PC to host DownloadDownload: from host to PC: from host to PC

Terminal emulation program and host file transfer Terminal emulation program and host file transfer program must support the same file transfer program must support the same file transfer protocol standardprotocol standard KermitKermit XMODEM, YMODEMXMODEM, YMODEM

IBM 3270 TerminalsIBM 3270 Terminals

6

DigitalSignal(1101)

Modem

AnalogSignal

Analog and Digital TransmissionAnalog and Digital Transmission

DigitalSignal(1101)

Modem

AnalogSignal

Modulation

Demodulation

7Digital and BinaryDigital and Binary

Digital TransmissionDigital Transmission

Can have multiple voltage levels, say 4Can have multiple voltage levels, say 4

Change to one at start of each bit cycleChange to one at start of each bit cycle

If 4, changes can represent 2 bits each:If 4, changes can represent 2 bits each:

00, 01, 10, 1100, 01, 10, 11

00

11

0110

VoltageLevel

Time

8Baud Rate and Bit RateBaud Rate and Bit Rate

Baud RateBaud Rate Number of times line changes per secondNumber of times line changes per second Let baud rate be 4 (4 changes per second)Let baud rate be 4 (4 changes per second) Let bits per line change be 2Let bits per line change be 2 Bit rate = 8 bits per secondBit rate = 8 bits per second Bit rate = x2 Baud rate in this exampleBit rate = x2 Baud rate in this example

00

11

0110

One Second

9Wave CharacteristicsWave Characteristics

AmplitudeAmplitude (power) (power) FrequencyFrequency (cycles per second, Hertz) (cycles per second, Hertz) WavelengthWavelength (meters) (meters)

Amplitude(power)

Wavelength(meters)

Frequency (Hz)

One Second

10Wave CharacteristicsWave Characteristics

PhasePhase Fully cycle is 360 degreesFully cycle is 360 degrees Phase is degrees different from reference wavePhase is degrees different from reference wave Human ears cannot hear. Equipment canHuman ears cannot hear. Equipment can

ReferenceWave

180degreesout ofphase

0o

180o

0o

90o

180o

270o

0 and rising

Highest

0 and falling

Lowest

11Wave CharacteristicsWave Characteristics

AmplitudeAmplitude

Frequency and WavelengthFrequency and Wavelength

Not independentNot independent

As frequency rises, wavelength fallsAs frequency rises, wavelength falls(Shorter guitar strings produce higher notes)(Shorter guitar strings produce higher notes)

Their product is constant--the speed of light, Their product is constant--the speed of light, sound, etc.sound, etc.

PhasePhase

12Frequency ModulationFrequency Modulation

LowFrequency

(0)

HighFrequency

(1)

FrequencyModulation

(1011)

Wavelength

Wavelength

1

0

1

1

Vary the frequency (wavelength) to represent 1 and 0Vary the frequency (wavelength) to represent 1 and 0

13Amplitude ModulationAmplitude Modulation

LowAmplitude

(0)

HighAmplitude

(1)

AmplitudeModulation

(1011)

Amplitude (low)

Amplitude (high)

14Phase ModulationPhase Modulation

In Phase(0)

180 degreesout of phase

(1)

FrequencyModulation

(1011)

15Complex ModulationComplex Modulation

HighAmplitude

LowAmplitude

90 DegreesOut of Phase,High Amplitude

In Phase

180 Degrees Out of Phase

Vary both amplitude and phaseVary both amplitude and phaseSeveral values (not just two) on each dimensionSeveral values (not just two) on each dimension

16Modem StandardsModem Standards Modems at Two Ends Must CommunicateModems at Two Ends Must Communicate

Must follow same standardsMust follow same standards

Most modem standards set by ITU-TMost modem standards set by ITU-T

Multiple category of standardsMultiple category of standards:: Modem speed (modulation)Modem speed (modulation) Error correction and compressionError correction and compression FacsimileFacsimile Etc.Etc.

When buying a modem, must check for standard(s) followed in each category

17Modem Speed StandardsModem Speed Standards Set by the ITU-TSet by the ITU-T

Govern how modulation is doneGovern how modulation is done

Standards for Standards for speedspeed governs modulation for data governs modulation for data transmissiontransmission

V.92V.92 56.6 kbps plus quick connect, 56.6 kbps plus quick connect, modem on hold, PCM upstreammodem on hold, PCM upstream

V.V.9090 56.6 kbps 56.6 kbps V.34V.34 28.8 kbps/33.6 kbps28.8 kbps/33.6 kbps V.32 bisV.32 bis 14.4 kbps14.4 kbps

18Modem Speed StandardsModem Speed Standards

Most data modems are also Most data modems are also fax modemsfax modems V.14V.14 14.4 kbps14.4 kbps V.29V.29 9,600 bps9,600 bps

19Error Correction and CompressionError Correction and Compression ITU-T StandardsITU-T Standards

V.42 Error detection and correction V.42 bis Data compression (up to 4:1) V.44 Data compression (20 to 120%

more than V.42 bis) Independent of speed standards (but V.44 only Independent of speed standards (but V.44 only

with V.92)with V.92)

Microcom StandardsMicrocom Standards Microcom Network Protocol (MNP)Microcom Network Protocol (MNP) Both error correction and compressionBoth error correction and compression Several levelsSeveral levels Independent of speed standardsIndependent of speed standards

20Modem IntelligenceModem Intelligence

ComputerComputer Can Send Commands to ModemCan Send Commands to Modem Dial a number, including how long to wait, etc.Dial a number, including how long to wait, etc. Called intelligent modemsCalled intelligent modems

Hayes Developed the first Command SetHayes Developed the first Command Set Most modems follow the same command setMost modems follow the same command set We call them “Hayes compatible”We call them “Hayes compatible” Commands start with “AT”Commands start with “AT”

Other Standards for Fax ModemsOther Standards for Fax Modems Class 1 and Class 2: extensions to HayesClass 1 and Class 2: extensions to Hayes

21Telephone Bandwidth is LimitedTelephone Bandwidth is Limited

Telephone TransmissionTelephone Transmission Cuts off sounds below 300 HertzCuts off sounds below 300 Hertz Cuts off sounds above about 3,400 HzCuts off sounds above about 3,400 Hz BandwidthBandwidth is the difference between the highest is the difference between the highest

and lowest frequencies (3400-300): about 3,100 and lowest frequencies (3400-300): about 3,100 HzHz

SoundLoudness

Frequency (Hz)

0 300 3400 20,000

Bandwidth3,100 Hz

22Telephone Bandwidth is LimitedTelephone Bandwidth is Limited

Speed is LimitedSpeed is Limited

Maximum speed is related to bandwidth Maximum speed is related to bandwidth (Shannon’s Law)(Shannon’s Law)

Maximum speed for phone lines for transmission Maximum speed for phone lines for transmission is a little over 30 kbpsis a little over 30 kbps

So modems can’t get much fasterSo modems can’t get much faster

23Another Look at CompressionAnother Look at Compression

With 4:1 Compression, a V.34 Modem Can With 4:1 Compression, a V.34 Modem Can Receive Data at 115.2 kbps from the PCReceive Data at 115.2 kbps from the PC

However the ~30 kbps limit of the phone However the ~30 kbps limit of the phone system is not exceeded. Still transmit at 33.6 system is not exceeded. Still transmit at 33.6 kbps.kbps.

115.2 kbps 33.6 kbps

~35 kbpsMaximum

Compressionin Modem

2456 kbps Analog Modems56 kbps Analog Modems

From home, you transmit From home, you transmit

Analog-to-Digital Converter (ADC)Analog-to-Digital Converter (ADC) Filters your signal to a bandwidth of ~3.1 kHzFilters your signal to a bandwidth of ~3.1 kHz This limits you to 33.6 kbpsThis limits you to 33.6 kbps

TelephoneNetwork

TelephoneNetworkADCADCPCPC

V.34modem

33.6 kbps

2556 kbps Analog Modems56 kbps Analog Modems

But ISP Can Connect DigitallyBut ISP Can Connect Digitally Signal travels through phone system at 56 kbpsSignal travels through phone system at 56 kbps At user end, digital-to-analog converter (DAC)At user end, digital-to-analog converter (DAC) Sends signal to analog modem at wide bandwidthSends signal to analog modem at wide bandwidth Modem can receive at 56 kbpsModem can receive at 56 kbps

TelephoneNetwork

TelephoneNetworkDACDACPCPC

56 kbpsmodem ISPISP

DigitalLink

56 kbps

2656 kbps Modems56 kbps Modems What they can doWhat they can do

Send at 33.6 kbps Send at 33.6 kbps (V.92 with PCM upstream can (V.92 with PCM upstream can go up to 48 kbps)go up to 48 kbps)

Receive at 56 kbps Receive at 56 kbps (V92 with V.44 compression (V92 with V.44 compression can go up to 120 kbps)can go up to 120 kbps)

ProblemsProblems past:past: competing standards from Rockwell, U.S. competing standards from Rockwell, U.S.

Robotics (V.90 ended them)Robotics (V.90 ended them) present: ISPs must present: ISPs must support V.92support V.92 (all support V.90) (all support V.90)

Users and ISPsUsers and ISPs Users V.90 analog modem or V.92Users V.90 analog modem or V.92 ISPs V.90 digital modem or V.92ISPs V.90 digital modem or V.92

2756 Kbps Modems56 Kbps Modems

Telephone companyTelephone company No changesNo changes needed, although ... needed, although ...

Many not have an internal ADC conversion Many not have an internal ADC conversion between ISP and customer (some do)between ISP and customer (some do)

May not have long transmission line from last May not have long transmission line from last switch to the customer premises (local loop)switch to the customer premises (local loop)

Not all phone lines to customer premises will support Not all phone lines to customer premises will support 56 kbps modems56 kbps modems

Even when they do, speeds may only be 40-50 kbpsEven when they do, speeds may only be 40-50 kbps

28Half-Duplex TransmissionHalf-Duplex Transmission

Sender and receiver Sender and receiver must take turnsmust take turns sending sending Like an old one-lane roadLike an old one-lane road No interruption for error handling or flow controlNo interruption for error handling or flow control

A B A B

Time 1Only one side

May communicateA does

Time 2Only one side

May communicateB does

29Full-Duplex TransmissionFull-Duplex Transmission

Both Sides Both Sides May Transmit SimultaneouslyMay Transmit Simultaneously Needed for error correction, flow controlNeeded for error correction, flow control Now almost universal in modem communicationNow almost universal in modem communication

A B A B

Time 1Both sides may communicate

Both do

Time 2Both sides may communicate

A does

30Asynchronous TransmissionAsynchronous Transmission

ASCII Character SetASCII Character Set 7-bit is the standard7-bit is the standard 8-bit extended ASCII is popular8-bit extended ASCII is popular Bits transmitted backwardBits transmitted backward

Parity for Error DetectionParity for Error Detection Only for 7-bit ASCIIOnly for 7-bit ASCII

Start/Stop Bits for FramingStart/Stop Bits for Framing

Each frame is exactly 10 bits longEach frame is exactly 10 bits long

31Asynchronous TransmissionAsynchronous Transmission

ASCII Character SetASCII Character Set

Created for sending printed American textCreated for sending printed American text

Each character is a Each character is a 7-bit code7-bit code (e.g., 1010101) (e.g., 1010101)

This allows 2^7 or 128 possible charactersThis allows 2^7 or 128 possible characters

Printing characters: A, a, !, <, %, etc.Printing characters: A, a, !, <, %, etc.

Control codes: XOFF tells other side to pauseControl codes: XOFF tells other side to pause

32Asynchronous TransmissionAsynchronous Transmission

8-bit ASCII8-bit ASCII

Used in PCs: 8 bits per character (10101010)Used in PCs: 8 bits per character (10101010)

Used for word processing format codesUsed for word processing format codes

Used in graphics that stores data in bytesUsed in graphics that stores data in bytes

33PC Serial PortPC Serial Port

Bit Transmission of ASCII CharactersBit Transmission of ASCII Characters

Transmits last bit firstTransmits last bit first

If you wish to send 1111000,If you wish to send 1111000,

The serial port transmits 0001111The serial port transmits 0001111

34ParityParity For 7-bit ASCII Only (No Parity = 8-bit ASCII)For 7-bit ASCII Only (No Parity = 8-bit ASCII)

Transmit an 8th bit per characterTransmit an 8th bit per character

Even parity: sum of data and parity bits is evenEven parity: sum of data and parity bits is even To send 1110000 (odd), send 0000111To send 1110000 (odd), send 000011111 To send 1111000 (even), send 0001111To send 1111000 (even), send 000111100

Odd parity: sum is oddOdd parity: sum is odd

If error is detected, the character is simply If error is detected, the character is simply discarded. No way to ask for retransmissiondiscarded. No way to ask for retransmission

35Start and Stop BitsStart and Stop Bits When the Data Line is at RestWhen the Data Line is at Rest

It is kept in the “1” stateIt is kept in the “1” state So “11110000” would look like 111111100001111So “11110000” would look like 111111100001111 ““00001111” would also look like 1111110000111100001111” would also look like 11111100001111

How can you tell where a character begins?How can you tell where a character begins?

SolutionSolution Add a start bit (always 0)Add a start bit (always 0) to change the line state to change the line state

End with a stop bit (always 1)End with a stop bit (always 1) to guarantee at least a to guarantee at least a one-bit rest (1) against which to detect the next start bit one-bit rest (1) against which to detect the next start bit (0)(0)

36The Final Asynchronous FrameThe Final Asynchronous Frame

Always 10 bitsAlways 10 bits Start, 7 data bits, parity, stop, orStart, 7 data bits, parity, stop, or Start, 8 data bits, stopStart, 8 data bits, stop

0 1 1 1 0 0 0 1 1 1

Start Parity

Stop7-bit ASCII Character

0 1 1 1 0 0 0 1 1 1

Start

Stop

8-bit ASCII Character

37Flow ControlFlow Control Ask the Other Device to Pause (or Slow Down)Ask the Other Device to Pause (or Slow Down)

ASCIIASCII In asynch, usually done by sending ASCII control In asynch, usually done by sending ASCII control

codescodes

XOFF tells other side to pauseXOFF tells other side to pause

XON tells the other device to resumeXON tells the other device to resume

Serial PortSerial Port Signals on the pins control when PC, modem can Signals on the pins control when PC, modem can

transmittransmit

38ISDNISDN Integrated Serviced Digital NetworkIntegrated Serviced Digital Network

Planned as eventual replacement for the Planned as eventual replacement for the worldwide analog telephone networkworldwide analog telephone network

All-digital, but expensive and difficult to installAll-digital, but expensive and difficult to install

Integrated servicesIntegrated services Automatic number identificationAutomatic number identification Call waitingCall waiting Now available only locally (business and limited)Now available only locally (business and limited) will ISDN standardize worldwide? - NO!!!will ISDN standardize worldwide? - NO!!!

39Basic Rate Interface (BRI)Basic Rate Interface (BRI)

Service Brought to User’s DeskService Brought to User’s Desk 3 Channels multiplexed onto one set of wires3 Channels multiplexed onto one set of wires Really, bidirectional, so 6 channels!Really, bidirectional, so 6 channels!

1101

BChannel

BChannel

2B+D

MultiplexedOnto One

Set of Wires

TerminalAdapter

D Channel:control signals

40ISDN BRIISDN BRI

B ChannelsB Channels 2 B-channels2 B-channels multiplexed to desktop multiplexed to desktop

64 kbps each64 kbps each

Simultaneous voice and data connectionsSimultaneous voice and data connections

Sometimes, a single 128-kbps connectionSometimes, a single 128-kbps connection

Moderately faster than modems and comparable Moderately faster than modems and comparable increase in costincrease in cost

D ChannelD Channel one 16 kbps channel, usually for control signalingone 16 kbps channel, usually for control signaling

41Connecting a Computer to the ISDNConnecting a Computer to the ISDN

Requires a Requires a Terminal AdapterTerminal Adapter Contains a DSU (data service unit)Contains a DSU (data service unit) Converts PC digital format to ISDN digital formatConverts PC digital format to ISDN digital format

1101

TerminalAdapter

DSU1101

42ISDN Costs for Internet AccessISDN Costs for Internet Access Carrier ChargesCarrier Charges

Installation of the ISDN Line by CarrierInstallation of the ISDN Line by Carrier Monthly Service Charge to CarrierMonthly Service Charge to Carrier Traffic volume charges (per kilobit)Traffic volume charges (per kilobit)

Purchase or Lease of Terminal AdapterPurchase or Lease of Terminal Adapter

Internet Service ProvidersInternet Service Providers Charge extra for ISDN accessCharge extra for ISDN access

Even more for 128 kbps service (2 B channels)Even more for 128 kbps service (2 B channels)

ISDN fees do not go to ISPISDN fees do not go to ISP

43Beyond ISDNBeyond ISDN Faster than analog phone lines but still slowFaster than analog phone lines but still slow

Better dial-up connections …. Better dial-up connections …. V.92-V.44V.92-V.44

Newer alternatives are growingNewer alternatives are growing Cable modems from cable television companiesCable modems from cable television companies DSL lines from telephone companiesDSL lines from telephone companies

Much fasterMuch faster 1-10 Mbps to customer premises1-10 Mbps to customer premises Sometimes slower uplink (33.6 kbps - 1 Mbps)Sometimes slower uplink (33.6 kbps - 1 Mbps)

More expensive than dial-upMore expensive than dial-up equivalent to the cost of two phone linesequivalent to the cost of two phone lines installation costs and/or monthly lease of modeminstallation costs and/or monthly lease of modem

44Beyond ISDNBeyond ISDN Cable Modems from Cable TV CompaniesCable Modems from Cable TV Companies

High-speed digitalHigh-speed digital access offered by access offered by cable television cable television companiescompanies

Really DSUs but called cable modemsReally DSUs but called cable modems

500 kbps to 10 Mbps downstream (to you)500 kbps to 10 Mbps downstream (to you)

Downstream speed shared by block of usersDownstream speed shared by block of users

ISDN-like speeds upstream (from you)ISDN-like speeds upstream (from you)

Typical Cable + ISP charges around $40.00Typical Cable + ISP charges around $40.00

Comcast and TCI major players Comcast and TCI major players

10 million users today and growing10 million users today and growing

45Beyond ISDNBeyond ISDN Digital Subscriber Link (DSL)Digital Subscriber Link (DSL)

High-speed digitalHigh-speed digital access offered by access offered by telephone telephone companiescompanies over the over the ordinary local loopordinary local loop

Requires equipment at telephone company switchRequires equipment at telephone company switch User needs DSL modem (really, a DSU)User needs DSL modem (really, a DSU) 128 kbps to perhaps 8 Mbps downstream 128 kbps to perhaps 8 Mbps downstream ISDN-like rates upstream are commonISDN-like rates upstream are common Several versions: Asymmetric (ADSL), etc.Several versions: Asymmetric (ADSL), etc. Microsoft, Intel, Compaq, and others join TelcosMicrosoft, Intel, Compaq, and others join Telcos Keep yourself informed: Keep yourself informed: ADSL ForumADSL Forum

46Beyond ISDNBeyond ISDN

Satellite/WirelessSatellite/Wireless Satellite link instead of wire link to ISPSatellite link instead of wire link to ISP Good for mobile usersGood for mobile users Good for intranets in a company with multiple Good for intranets in a company with multiple

dispersed sitesdispersed sites ISDN speeds or higherISDN speeds or higher High costHigh cost

47Broadband Market ForecastBroadband Market Forecast

Residential Use (2003)Residential Use (2003) 80% of Households have computers online80% of Households have computers online 77%77%dial-up connectionsdial-up connections 23% broadband (17 million)23% broadband (17 million)

Forecast 2007: 56% ( 41 million)Forecast 2007: 56% ( 41 million)

BroadbandBroadband 54%54% Cable modemsCable modems 34%34% DSL DSL 8%8% ISDN ISDN 4%4% Satellite/wireless Satellite/wireless

Cnet: Cable vs DSLCnet: Cable vs DSL