Modems and Other Communications Devices Guide to Operating Systems Third Edition

Modems and Other Communications Devices

Guide to Operating Systems Third Edition

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


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


• 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

Analog Modem Architecture


Analog Modem Hardware Basics

• Components: – data pump, controller, and UART

• Data pump– component that performs modulation/demodulation


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



• International Telecommunications Union (ITU)

– establishes modem protocol standards

– compose the ITU-T standards

• Latest standards define a 56 Kbps to 64 Kbps protocol



• Data terminal equipment (DTE)

– data transfer rate





• 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



• 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


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


Software-Based Modems (continued)

• Host Signal Processor (HSP) modems

– dispense with the controller and data pump hardware entirely


Software-Based Modems (continued)

• Disadvantage

– software takes up memory and processor cycles

• Advantage

– their dependence on particular operating systems

– cost savings and upgradeability


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


Hayes AT Command Set for Analog Modems (continued)


Hayes AT Command Set for Analog Modems (continued)


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


• 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

ISDN


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


ISDN (continued)

• PRI

– used for LAN-to-LAN connectivity

– ISP sites

– videoconferencing

– corporate sites that support telecommuters


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


Cable Modems (continued)

• Data Over Cable Service interface Specifications (DOCSIS)

– DOCSIS 1.0

– DOCSIS 1.1

– DOCSIS 2.0


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


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





• 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)



• 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




Satellite

• Available

– more and more rural areas

– metropolitan areas

• 500 Kbps for downloading speeds

• 50 Kbps for uploading


Satellite


Data Communications Techniques

• Flow control

– way to halt and resume the flow of data


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)


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


Hardware Flow Control (continued)


• Possible errors

– a bit can be lost

– an extraneous bit can be introduced

– a bit can be flipped

Error Correction


Error Correction

• Basic error correction

– start and stop bits

– each eight-bit byte is framed by a start bit and a stop bit


Error Correction (continued)

• Parity checking

– data verification process

– ensures data integrity

– system of data bit comparisons between the sending and receiving computer


• Link Access Protocol for Modems (LAPM)

– used to construct data into discrete frame-like units for transmission over communications lines

Error Correction (continued)


• 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

Data Compression


Data Compression (continued)

• Lempel-Ziv-Welch (LZW) compression method

– compresses data as it is sent

– can detect when the data is already compressed


Modems and the Operating System

• Available in all operating systems

• Installation CD-ROMs shipped with most all modems


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


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


Configuring Dial-up Connections in UNIX/Linux (continued)

• Minicom

– create a modem configuration – see Figure 7-8





• UUCP

– UNIX to UNIX Copy Protocol

– found on almost all UNIX versions


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


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


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


Configuring Modems in Mac OS X (continued)


Configuring Modems in Mac OS X (continued)


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


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

remote access speeds


Summary (continued)

Documents

Modems and Other Communications Devices Guide to Operating Systems Third Edition