Modems and Other
Communications Devices
Guide to Operating Systems
Third Edition
Objectives
After reading this chapter and completing
the exercises you will be able to:
• Explain analog modem architecture
• Use the classic Hayes AT modem command set
with computer communications applications
• Describe digital modem architecture for highspeed communications through IDSN, cable, DSL,
and satellites
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Objectives (continued)
After reading this chapter and completing
the exercises you will be able to:
• Explain the basics of telephone-line data
communications
• Configure modem and internet communications
in different operating systems
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Analog Modem Architecture
• Modem
– hardware and software that connects two
incompatible systems
• Plain old telephone service (POTS) lines – old
term
• Public switched telephone network (PSTN)
lines - modern term
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Analog Modem Hardware
Basics
• Components:
– data pump, controller, and UART
• Data pump
– component that performs modulation/demodulation
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Analog Modem Hardware
Basics (continued)
• Controller
– provides the modem’s identity
– interprets Attention (AT) commands
• Protocol
– specifies how networked data is formatted
– how it is transmitted
– how it is interpreted at the receiving end
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Analog Modem Hardware
Basics (continued)
• International Telecommunications Union
(ITU)
– establishes modem protocol standards
– compose the ITU-T standards
• Latest standards define a 56 Kbps to 64
Kbps protocol
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Analog Modem Hardware
Basics (continued)
• Data terminal equipment (DTE)
– data transfer rate
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Analog Modem Hardware
Basics (continued)
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Analog Modem Hardware
Basics (continued)
• Modem is called the data communications
equipment (DCE)
• Speed is the DCE communications rate
• UART
– electronic chip
– the Universal Asynchronous Receiver-Transmitter
– converts data from the computer
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Analog Modem Hardware
Basics (continued)
• Asynchronous communication
– most common method for today’s desktop
computers
– uses fairly accurate clocks (timers) at both ends of
the connection to synchronize data
• Synchronous communication
– sends information in blocks (frames) of data that
include embedded clock signals
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Software-Based Modems
• Winmodems
– software-based modems
– controllerless modem that retains a hardware data
pump (Digital Signal Processor (DSP)
– implements the controller functions in software
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Software-Based Modems
(continued)
• Host Signal Processor (HSP) modems
– dispense with the controller and data pump
hardware entirely
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Software-Based Modems
(continued)
• Disadvantage
– software takes up memory and processor cycles
• Advantage
– their dependence on particular operating systems
– cost savings and upgradeability
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Hayes AT Command Set for
Analog Modems
• Hayes command or Attention (AT) commands
– created by Dennis Hayes
– begin with the letters AT
– tells the modem to interpret the next character string
as a command
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Hayes AT Command Set for
Analog Modems (continued)
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Hayes AT Command Set for
Analog Modems (continued)
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Digital Modems
• Digital modems
– digital devices that use digital transmission media
– network or serial devices
– connect via Point-to-Point (PPP)
• Telecommunications networks:
– ISDN
– Cable networks
– DSL
– Satellite
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ISDN
• Integrated Services Digital Network (ISDN)
– uses standard copper telephone line pair
– digital equipment on either end of the connection
– ISDN router to route the transmission to the right
place
– terminal adapter (TA), a type of digital modem
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ISDN (continued)
• Basic rate interface (BRI)
– aggregate data rate of 144 Kbps
– three channels
• Primary rate interface (PRI)
– supports faster data rates
– aggregate bandwidth equal to 1.544 Mbps
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ISDN (continued)
• PRI
– used for LAN-to-LAN connectivity
– ISP sites
– videoconferencing
– corporate sites that support telecommuters
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Cable Modems
• Cable modem
– used to attach to cable data services
– external device
– plugs into a USB port or network interface card
– connected to the coaxial cable used for the cable
TV system
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Cable Modems (continued)
• Data Over Cable Service interface
Specifications (DOCSIS)
– DOCSIS 1.0
– DOCSIS 1.1
– DOCSIS 2.0
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DSL Modems
• Digital Subscriber Line (DSL)
– high-speed digital data communications service
– challenging ISDN and cable modems
– digital technology that works over copper wire
– must install an intelligent adapter in your computer
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DSL Modems (continued)
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DSL Modems (continued)
• Dedicated to a single user
• Employs the full bandwidth
• Connected by means of a combined DSL
adapter and router
• Router
– device that can be used to direct network traffic
and create a firewall
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DSL Modems (continued)
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DSL Modems (continued)
• DSL services:
– Asymmetric Digital Subscriber Line (ADSL)
– G.lite Asymmetric Digital Subscriber Line (G.lite
ADSL)
– Integrated Services Digital Network Digital
Subscriber Line (IDSL)
– Rate Adaptive Digital Subscriber Line (RADSL)
– High Bit-Rate Digital Subscriber Line (HDSL)
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DSL Modems (continued)
• DSL services (continued):
– Symmetric high Bit-Rate Digital Subscriber Line
(SHDSL)
– Very High Bit-Rate Digital Subscriber Line (VDSL)
– Symmetric Digital Subscriber Line (SDSL)
• ADSL is the most commonly used version of
DSL
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DSL Modems (continued)
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Satellite
• Available
– more and more rural areas
– metropolitan areas
• 500 Kbps for downloading speeds
• 50 Kbps for uploading
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Satellite
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Data Communications
Techniques
• Flow control
– way to halt and resume the flow of data
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Software Flow Control
• Xon-Xoff
– uses Ctrl+S character (ASCII 19) to stop the flow
of data (Xoff)
– Ctrl+Q (ASCII 17) to resume (Xon)
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Hardware Flow Control
• Halts and resumes the movement of data
– changing the voltage on specific pins in the
serial interface
– eliminates the problem of the modem confusing
data with control signals
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Hardware Flow Control
(continued)
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Error Correction
• Possible errors
– a bit can be lost
– an extraneous bit can be introduced
– a bit can be flipped
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Error Correction
• Basic error correction
– start and stop bits
– each eight-bit byte is framed by a start bit and a stop bit
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Error Correction (continued)
• Parity checking
– data verification process
– ensures data integrity
– system of data bit comparisons between the sending
and receiving computer
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Error Correction (continued)
• Link Access Protocol for Modems (LAPM)
– used to construct data into discrete frame-like
units for transmission over communications lines
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Data Compression
• Modems compress data sent
• Data compression routine for a picture with a
lot of blue
– the “compressed” representation of the screen
shows a blue dot and a number that represents
the number of times the blue dot is repeated
– does it “on the fly,” compressing the data while
you send it
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Data Compression
(continued)
• Lempel-Ziv-Welch (LZW) compression
method
– compresses data as it is sent
– can detect when the data is already compressed
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Modems and the Operating
System
• Available in all operating systems
• Installation CD-ROMs shipped with most all
modems
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Configuring Dial-up Connections
in Windows 2000/XP/Server 2003
• See page 352 for setup information on Windows 2000,
Windows XP, and Windows Server 2003
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Configuring Dial-up
Connections in UNIX/Linux
• Two purposes
– dial-in device
– dial-out device
• For dial-in connectivity, UNIX uses a daemon
– an internal, automatically running program called
getty
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Configuring Dial-up Connections
in UNIX/Linux (continued)
• Minicom
– create a modem configuration – see Figure 7-8
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Configuring Dial-up Connections
in UNIX/Linux (continued)
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Configuring Dial-up Connections
in UNIX/Linux (continued)
• UUCP
– UNIX to UNIX Copy Protocol
– found on almost all UNIX versions
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Configuring Modems and
Scripts in NetWare 6.x
• Components for Novell Internet Access
Server
– WMDMMGR.EXE
– MOEDMMGR.DLL
– Modem configuration files
– A sample PPP login script
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Configuring Modems and Scripts
in NetWare 6.x (continued)
• See page 357-358
– recommended steps for modifying a modem configuration
file or PPP login script
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Configuring Modems in Mac
OS X
• Provide information to configure the device
– telephone number for the ISP
– user name for the ISP account
– password for the ISP account
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Configuring Modems in Mac
OS X (continued)
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Configuring Modems in Mac
OS X (continued)
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Summary
• Modems open WAN communications
• Most PCs have a built-in analog modem
• Analog modem architecture
• Hayes AT modem command set
• Digital versus analog modems
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Summary (continued)
• See Table 7-4 for a review of analog and digital
remote access speeds
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Summary (continued)
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