1. Modems and Other Communications Devices
Guide to Operating Systems
Third Edition

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
Guide to Operating Systems, Third Edition

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
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4. Analog Modem Architecture
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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5. Analog Modem Hardware Basics
Components:
data pump, controller, and UART
Data pump
component that performs modulation/demodulation
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6. 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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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
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8. Analog Modem Hardware Basics (continued)
Data terminal equipment (DTE)
data transfer rate
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9. Analog Modem Hardware Basics (continued)
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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
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11. 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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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
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13. Software-Based Modems (continued)
Host Signal Processor (HSP) modems
dispense with the controller and data pump hardware entirely
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14. 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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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
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16. Hayes AT Command Set for Analog Modems (continued)
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17. Hayes AT Command Set for Analog Modems (continued)
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18. Digital Modems Digital modems Telecommunications networks:
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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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
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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 Mbps
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21. ISDN (continued) PRI used for LAN-to-LAN connectivity ISP sites
videoconferencing
corporate sites that support telecommuters
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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
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23. Cable Modems (continued)
Data Over Cable Service interface Specifications (DOCSIS)
DOCSIS 1.0
DOCSIS 1.1
DOCSIS 2.0
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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 your computer
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25. DSL Modems (continued)
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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
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27. DSL Modems (continued)
Guide to Operating Systems, Third Edition

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)
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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
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30. DSL Modems (continued)
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31. Satellite Available 500 Kbps for downloading speeds
more and more rural areas
metropolitan areas
500 Kbps for downloading speeds
50 Kbps for uploading
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32. Satellite
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33. Data Communications Techniques
Flow control
way to halt and resume the flow of data
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34. 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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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
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36. Hardware Flow Control (continued)
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37. Error Correction Possible errors a bit can be lost
an extraneous bit can be introduced
a bit can be flipped
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38. 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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39. 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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40. 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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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
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42. 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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43. Modems and the Operating System
Available in all operating systems
Installation CD-ROMs shipped with most all modems
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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
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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
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46. Configuring Dial-up Connections in UNIX/Linux (continued)
Minicom
create a modem configuration – see Figure 7-8
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47. Configuring Dial-up Connections in UNIX/Linux (continued)
Guide to Operating Systems, Third Edition

48. Configuring Dial-up Connections in UNIX/Linux (continued)
UUCP
UNIX to UNIX Copy Protocol
found on almost all UNIX versions
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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
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50. Configuring Modems and Scripts in NetWare 6.x (continued)
See page
recommended steps for modifying a modem configuration file or PPP login script
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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
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52. Configuring Modems in Mac OS X (continued)
Guide to Operating Systems, Third Edition

53. Configuring Modems in Mac OS X (continued)
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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
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55. Summary (continued)
See Table 7-4 for a review of analog and digital remote access speeds
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56. Summary (continued)
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