Chapter 7: Modems and Other Communications Devices

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Chapter 7 Modems and Other Communications
Devices

• A Guide to Operating Systems Troubleshooting
  and Problem Solving

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Chapter Objectives

• Understand analog modem architecture
• Relate the classic Hayes AT modem command set to
  computer communications applications
• Understand the basics of digital modem
  architecture
• Describe the basics of telephone line data
  communications
• Understand basic modem communications in
  different operating systems

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Modem Hardware Basics

• A modem consists of three basic hardware or
  software components
• data pump - the hardware or software portion of a
  modem that is responsible for converting digital
  data into analog signals for transmission
• controller - defines an individual modem
  personality and interprets AT commands, handles
  communications protocols
• UART (Universal Asynchronous Receiver-Transmitter)
  - a hardware device that handles data flow
  through a serial port or modem.

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More About Modems

• A modem is a MODulator-DEModulator
• Modem classification
• Internal modem (inside the computer)
• External modem (outside the computer)

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Modem Standards and Protocols
Table 7-1 Modem Standards and Protocols
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Software-Based Modems

• Winmodems - replace one or more components (data
  pump, controller, UART) with software
• Digital Signal Processor (DSP) - a hardware data
  pump used in software-driven modems such as the
  3Com Winmodem. Controller functions are handled
  by software.
• Host Signal Processor (HSP) - software approach
  to handling data pump and controller duties by
  utilizing the hosts CPU
• Apples Geoport Telecom Adapter for AV computers
  uses onboard DSP, rather than the computers CPU

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Disadvantages of Software Modems

• Software takes up memory and processor cycles,
  although the cycles have become increasingly
  cheap
• Dependence on particular operating systems
• Windmodems will not work on a Mac or even on a PC
  running Linux or OS/2
• Geoport Adapter will not work on a PC operating
  system

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Hayes at Command Set

• Hayes (AT) command language - a modem control
  command set which begins each command with AT and
  allows communications software or users to
  control many modem functions directly
• Hayes compatible simply means that the modem
  supports all or part of the Hayes AT command set

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AT Command Set Summary
Table 7-2 AT Command Set Summary
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Digital Modems

• Digital modem - a misnomer (there is no actual
  modulation or demodulation of analog signals)
• Data is digital from start to finish
• Digital modems can be network devices or serial
  devices
• When connecting to TCP/IP networks, they connect
  via PPP (point-to-point protocol)

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Integrated Services Digital Network (ISDN)

• ISDN uses standard copper telephone line with
  digital equipment (ISDN routers and terminal
  adapters) for high speed computer communications,
  video conferencing, and Internet connections
• ISDN routers and terminal adapters typically
  include analog telephone jacks in order to plug
  in a conventional telephone or modem for use over
  the digital line

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ISDN Terminal Adapters

• An ISDN terminal adapter can be thought of as an
  ISDN modem
• It is a serial device with its own UART
• It accepts AT commands, and appears in the
  Windows and Mac OS modems control panel
• PPP stacks are built into the operating system

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ISDN Routers and Bridges

• Routers and bridges are network devices
• Bridges simply connect or bridge one part of a
  network to another
• Routers can direct network packets to different
  networks, and can route multiple protocols
• Routers and bridges use their own built-in PPP
  stacks to make a connection
• They are programmed with the number, username,
  and password for the login account

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Typical ISDN Bridge and LAN Connection
15
Typical Traceroute Routine
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Cable Modems and xDSL Modems

• A Cable modem connects to your television cable
  and uses one or more channels to transfer data at
  high speeds
• Initial cable modem designs were hybrid devices
  that required an analog telephone line
• xDSL stands for Digital Subscriber Line, the x
  represents the type of digital line being used
• xDSL uses standard telephone line to transmit
  high-speed data

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Data Communications Over Phone Lines

• flow control - allows two modems to halt and
  resume the flow of data during communication
• Software flow control
• A popular software flow control method for modems
  is called Xon-Xoff
• Xon-Xoff uses the control-s character (ASCII 19)
  to stop the flow of data (Xoff), and Ctrl-Q
  (ASCII 17) to resume (Xon)

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Hardware Flow Control

• Hardware flow control halts and resumes the
  movement of data by changing the voltage on
  specific pins in the serial interface
• Controlling data flow with hardware eliminates
  the problems of the modem confusing data with
  control signals
• It is more reliable and permits faster modem
  performance

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Error Correction

• Modems transfer bits (ones and zeroes) over the
  phone line
• Three possible errors can occur
• a bit can be lost
• an extraneous bit can be introduced
• a bit can be flipped (changed from zero to one or
  from one to zero)

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Error Correction

• The most basic form of error correction involves
  the start bit and stop bit
• Each eight-bit byte is framed by a start bit at
  the beginning and a stop bit at the end

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Data Byte Framing With Start and Stop Bits

• There are always eight bits between the start and
  stop bit, even when communicating with systems
  that only require seven.
• This akes it possible to use the eighth bit for
  another form of error checking - parity checking
• Parity can be either even or odd

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Data Compression

• PKZIP or WINZIP
• As an example, compression can reduce the size of
  a TIF (Tagged Image Format) file,, by more than
  90 percent
• Modem data compression is done on the fly
• Data does not have to be manually compressed
  before it is transmitted

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Modems and the Operating System

• Intrinsic serial communication protocols are used
  to manage data flow through the serial port
• Communications programs - applications external
  to the operating system, which handle the
  specifics of communicating with a remote host

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Modems with Mac OS

• The Mac OS has two major TCP/ IP stacks for
  Internet or remote host access
• MacTCP control panel
• TCP/IP control panel
• Newer versions of the Mac operating system have
  built-in PPP and Modems control panels
• On systems that have Mac OS version 7.53 or 7.55,
  users can switch between MacTCP and TCP/IP using
  the Network Software Selector

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UNIX and Modems

• In UNIX modems have two purposes
• A dial-in device using a serial connection
• A dial-out device
• UNIX uses a daemon called getty

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Chapter Summary

• Analog modems have a three-part architecture,
  consisting of a data pump, controller, and UART
• The Hayes (AT) modem command set is used to
  control modem settings
• Digital modems are not actually modems at all,
  they are entirely digital and do not perform the
  modulation/ demodulation required for analog
  devices

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Chapter Summary

• Data communication over telephone lines involves
  software flow control, hardware flow control,
  error correction, and data compression
• All the operating systems covered in this book
  include a communications component and method for
  setting up dial-up networking