Modems and Other Communications Devices

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Modems and Other Communications Devices

• Guide to Operating Systems
• Third Edition

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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 modems 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 todays 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 CtrlS character (ASCII 19) to stop the flow
  of data (Xoff)
• CtrlQ (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