ModemsModems

How They WorkHow They Work

ModemModem

A device used by a PC to A device used by a PC to communicate over a phone line or communicate over a phone line or cablecable

Can be connected to a USB port, a Can be connected to a USB port, a serial port, a network interface cardserial port, a network interface card

Can be installed as an internal Can be installed as an internal modem (modem card)modem (modem card)

ModulationModulation

Conversion of digital signal (from Conversion of digital signal (from computer) to analog signal (for computer) to analog signal (for phone line use)phone line use)

Digital dataDigital data One state – on or offOne state – on or off 1’s and 0’s1’s and 0’s

Analog dataAnalog data Varying statesVarying states

DemodulationDemodulation

Conversion of analog signal (from Conversion of analog signal (from telephone line) to digital signal (for telephone line) to digital signal (for computer use)computer use)

MOMOdulate/dulate/DEDEModulateModulate That’s where the word That’s where the word modemmodem

comes fromcomes from

ModemsModems

Sound traveling over phone lines has an Sound traveling over phone lines has an infinite number of different states just as infinite number of different states just as voices do.voices do.

Computers communicate with all 1’s and Computers communicate with all 1’s and 0’s0’s

In order to communicate over phone lines In order to communicate over phone lines the simple on/off states of 1’s and 0’s the simple on/off states of 1’s and 0’s have to be convertedhave to be converted

Modems use different characteristics of Modems use different characteristics of waves to represent the 1’s and 0’swaves to represent the 1’s and 0’s

ModemsModems

Have an RJ-11 connector to connect Have an RJ-11 connector to connect to the wall outlet (for the phone)to the wall outlet (for the phone)

Have an additional one to attach to a Have an additional one to attach to a telephonetelephone

Modems have to transmit and Modems have to transmit and receive datareceive data

ModemsModems

Half-duplexHalf-duplex Modem can communicate in only one Modem can communicate in only one

direction at a timedirection at a time Like a CB radio or walkie talkieLike a CB radio or walkie talkie

Full duplexFull duplex Modem can communicate in both Modem can communicate in both

directions at oncedirections at once Like regular phone voice Like regular phone voice

communicationscommunications

ModemsModems When dialup connection first established, When dialup connection first established,

noises you hear are the modems noises you hear are the modems handshakinghandshaking or or trainingtraining

The modems are establishing the rules of The modems are establishing the rules of communication between themcommunication between them

They each determine what speeds and They each determine what speeds and protocols of data transmission they can protocols of data transmission they can supportsupport

They agree on one that is mutually They agree on one that is mutually functional, usually the fastest one that is functional, usually the fastest one that is agreeable to bothagreeable to both

Modem SpeedsModem Speeds bps – Bits Per Secondbps – Bits Per Second How speed is measuredHow speed is measured Older modems expressed speed as Older modems expressed speed as baud baud

raterate Number of times a signal changes in one Number of times a signal changes in one

secondsecond Bps = baud rateBps = baud rate One signal represents one bitOne signal represents one bit

Newer modemsNewer modems One signal can represent more than one bitOne signal can represent more than one bit Baud rates differ from bps ratesBaud rates differ from bps rates

Modem SpeedsModem Speeds

When speed measured in baud rates, bps When speed measured in baud rates, bps rate always a multiple of the baud raterate always a multiple of the baud rate

Most common bps rate = 56.6 KbpsMost common bps rate = 56.6 Kbps Still a few 28.8 or 33.6 Kbps modems out Still a few 28.8 or 33.6 Kbps modems out

therethere Maximum speed often included in Maximum speed often included in

manufacturer’s name for the modemmanufacturer’s name for the modem U.S. Robotics 56K Fax Internal PnP U.S. Robotics 56K Fax Internal PnP

modemmodem

Ceiling On Modem Ceiling On Modem SpeedsSpeeds

Many factors contributeMany factors contribute Analog phone lines designed with Analog phone lines designed with

sufficient quality to handle voice sufficient quality to handle voice communicationscommunications

Not able to handle high speed Not able to handle high speed transmissions of datatransmissions of data

Older phone lines (pre-1940’s) were Older phone lines (pre-1940’s) were analog from start to finishanalog from start to finish

New ones now convert to digital for New ones now convert to digital for much of the pathmuch of the path

Ceiling On Modem Ceiling On Modem SpeedsSpeeds

Signal is converted to digital soon Signal is converted to digital soon after leaving house or officeafter leaving house or office

Converted back just before going Converted back just before going into the house or office destinationinto the house or office destination

One limitation results from method One limitation results from method used when analog signal converted used when analog signal converted to digitalto digital

Ceiling On Modem Ceiling On Modem SpeedsSpeeds

Phone company takes sampling of Phone company takes sampling of analog signal (8000/second) and analog signal (8000/second) and converts to digital 8-bit byteconverts to digital 8-bit byte

Limits signal to 8000 bytes (64,000 Limits signal to 8000 bytes (64,000 bits) per secondbits) per second

Additional bits and bytes sent with Additional bits and bytes sent with data to regulate the transmission data to regulate the transmission means that maximum rate over phone means that maximum rate over phone line is about 56,000 bps or 56Kline is about 56,000 bps or 56K

Ceiling On Modem Ceiling On Modem SpeedsSpeeds

Other factors that limit speedOther factors that limit speed NoiseNoise Lines bumping together in the windLines bumping together in the wind Bad wiringBad wiring InterferenceInterference

Clean lineClean line A line that consistently delivers a high-A line that consistently delivers a high-

quality resultquality result

DSL – Digital Subscriber DSL – Digital Subscriber LineLine

Phone line is digital from start to Phone line is digital from start to finishfinish

No analog conversionNo analog conversion No limit of 8,000 bytes/secondNo limit of 8,000 bytes/second

Serial Port SettingsSerial Port Settings

Control how serial ports Control how serial ports communicatecommunicate

Five protocols that control these port Five protocols that control these port settingssettings Bits per second (port speed)Bits per second (port speed) Data bitsData bits ParityParity Stop bitsStop bits Flow controlFlow control

Bits Per Second (Port Bits Per Second (Port Speed)Speed)

Port speedPort speed should usually be about should usually be about four times the modem speedfour times the modem speed

Common valuesCommon values 2400; 4800; 9600; 19,200;2400; 4800; 9600; 19,200; 38,400; 57,600; 115,200; 230,400; 38,400; 57,600; 115,200; 230,400;

460,800;460,800; 921,600 bps921,600 bps

Data BitsData Bits

How many bits needed to send How many bits needed to send one single character of data?one single character of data?

Only seven required for ASCII Only seven required for ASCII characterscharacters

In most cases, you should use 8In most cases, you should use 8 Common valuesCommon values

77 88

ParityParity

Error checking?Error checking? What format?What format? In most cases, let the modem to In most cases, let the modem to

error checking and use no parity error checking and use no parity checking on the serial portchecking on the serial port

Common valuesCommon values OddOdd EvenEven NoneNone

Stop BitsStop Bits

What code to indicate that a What code to indicate that a string of bits (for a character) is string of bits (for a character) is starting or ending?starting or ending?

Most cases: 1 bitMost cases: 1 bit Common valuesCommon values

1 bit1 bit 1.5 bits1.5 bits 2 bits2 bits

Flow ControlFlow Control

How will flow of data be controlled?How will flow of data be controlled? Stops flow of data if receive port is Stops flow of data if receive port is

being overloadedbeing overloaded Controlled by software (Xon/Xoff) Controlled by software (Xon/Xoff)

or hardware (RTS/CTS)or hardware (RTS/CTS) Always use RTSAlways use RTS Common valuesCommon values

Xon/Xoff (software control)Xon/Xoff (software control) RTS/CTS (hardware control)RTS/CTS (hardware control)

Modem FeaturesModem Features

Caller ID supportedCaller ID supported Can interrupt data connection Can interrupt data connection

temporarilytemporarily Must have callwaitingMust have callwaiting

Display ReadoutDisplay Readout Lights or LCD to list what modem is Lights or LCD to list what modem is

doingdoing Flash ROMFlash ROM

Allows upgrade to modem for future Allows upgrade to modem for future standardsstandards

Modem FeaturesModem Features

Plug and Play for WindowsPlug and Play for Windows Makes installation more automaticMakes installation more automatic

Voice/data capabilityVoice/data capability Allows use of built-in speaker and Allows use of built-in speaker and

microphone so modem can be used microphone so modem can be used as telephoneas telephone

Auto-answerAuto-answer Allows modem to answer an incoming Allows modem to answer an incoming

call even if you are away from the PCcall even if you are away from the PC

Dial-up NetworkingDial-up Networking

Must have TCP/IP installed and bound Must have TCP/IP installed and bound to the modem (will look at later on)to the modem (will look at later on)

Under Windows XP, this is found under Under Windows XP, this is found under Network Connections on the Control Network Connections on the Control PanelPanel

Uses Point-to-Point Protocol (PPP) to Uses Point-to-Point Protocol (PPP) to send data packets over a telephone linesend data packets over a telephone line

See fig 13-7 p. 547See fig 13-7 p. 547 PPP adds its own header and trailer to PPP adds its own header and trailer to

the packetsthe packets

Dial-up NetworkingDial-up Networking Three layersThree layers

DataData Network protocolNetwork protocol Line protocolLine protocol

PPP is the data link layer in the OSI PPP is the data link layer in the OSI modelmodel

To send, data is “enclosed’ in TCP/IP To send, data is “enclosed’ in TCP/IP protocols (or others, perhaps)protocols (or others, perhaps)

The TCP/IP packet is then enclosed in The TCP/IP packet is then enclosed in the PPP and sent to the modem for the PPP and sent to the modem for transmissiontransmission

Dial-up NetworkingDial-up Networking

Receiving modem receives the Receiving modem receives the packetpacket

Receiving computer strips off PPP Receiving computer strips off PPP and sends packet to the network and sends packet to the network still packaged in TCP/IPstill packaged in TCP/IP

Network sends packet to Network sends packet to destination computerdestination computer

Destination computer removes Destination computer removes TCP/IP from data and uses dataTCP/IP from data and uses data

Creating a Dial-up Connection Creating a Dial-up Connection in Windows XPin Windows XP

Double click the Double click the Network and Network and Internet ConnectionsInternet Connections icon in the icon in the Windows XP Control PanelWindows XP Control Panel

Click Click Set up or change your Set up or change your Internet connectionInternet connection

Click the Click the SetupSetup button in the top button in the top section of the windowsection of the window

Follow the wizard through the Follow the wizard through the installation processinstallation process