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Modems and Other Communications Devices

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Title: Modems and Other Communications Devices


1
Modems and Other Communications Devices
2
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
    high-speed communications through IDSN, cable,
    DSL, and satellites

3
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

4
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

5
Analog Modem Hardware Basics
  • Components
  • data pump, controller, and UART
  • Data pump
  • component that performs modulation/demodulation

6
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

7
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

8
Analog Modem Hardware Basics (continued)
  • Data terminal equipment (DTE)
  • data transfer rate

9
Analog Modem Hardware Basics (continued)
10
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

11
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

12
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

13
Software-Based Modems (continued)
  • Host Signal Processor (HSP) modems
  • dispense with the controller and data pump
    hardware entirely

14
Software-Based Modems (continued)
  • Disadvantage
  • software takes up memory and processor cycles
  • Advantage
  • their dependence on particular operating systems
  • cost savings and upgradeability

15
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

16
Hayes AT Command Set for Analog Modems (continued)
17
Hayes AT Command Set for Analog Modems (continued)
18
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

19
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

20
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

21
ISDN (continued)
  • PRI
  • used for LAN-to-LAN connectivity
  • ISP sites
  • videoconferencing
  • corporate sites that support telecommuters

22
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

23
Cable Modems (continued)
  • Data Over Cable Service interface Specifications
    (DOCSIS)
  • DOCSIS 1.0
  • DOCSIS 1.1
  • DOCSIS 2.0

24
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 or to your
    computer

25
DSL Modems (continued)
26
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

27
DSL Modems (continued)
28
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)

29
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

30
DSL Modems (continued)
31
Satellite
  • Available
  • more and more rural areas
  • metropolitan areas
  • 500 Kbps for downloading speeds
  • 50 Kbps for uploading

32
Satellite
33
Data Communications Techniques
  • Flow control
  • way to halt and resume the flow of data

34
Software Flow Control
  • Xon-Xoff
  • uses CtrlS character (ASCII 19) to stop the flow
    of data (Xoff)
  • CtrlQ (ASCII 17) to resume (Xon)

35
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

36
Hardware Flow Control (continued)
37
Error Correction
  • Possible errors
  • a bit can be lost
  • an extraneous bit can be introduced
  • a bit can be flipped

38
Error Correction
  • Basic error correction
  • start and stop bits
  • each eight-bit byte is framed by a start bit and
    a stop bit

39
Error Correction (continued)
  • Parity checking
  • data verification process
  • ensures data integrity
  • system of data bit comparisons between the
    sending and receiving computer

40
Error Correction (continued)
  • Link Access Protocol for Modems (LAPM)
  • used to construct data into discrete frame-like
    units for transmission over communications lines

41
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

42
Data Compression (continued)
  • Lempel-Ziv-Welch (LZW) compression method
  • compresses data as it is sent
  • can detect when the data is already compressed

43
Modems and the Operating System
  • Available in all operating systems
  • Installation CD-ROMs shipped with most all modems

44
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

45
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

46
Configuring Dial-up Connections in UNIX/Linux
(continued)
  • Minicom
  • create a modem configuration see Figure 7-8

47
Configuring Dial-up Connections in UNIX/Linux
(continued)
48
Configuring Dial-up Connections in UNIX/Linux
(continued)
  • UUCP
  • UNIX to UNIX Copy Protocol
  • found on almost all UNIX versions

49
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

50
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

51
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

52
Configuring Modems in Mac OS X (continued)
53
Configuring Modems in Mac OS X (continued)
54
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

55
Summary (continued)
  • See Table 7-4 for a review of analog and digital
    remote access speeds

56
Summary (continued)
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