modems how they work. modem a device used by a pc to communicate over a phone line or cable a device...
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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