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)

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

Documents

different operating

software slide

edition4 modem hardware

data pump hardware

modem command

protocol slide

modem protocol standards

modulationdemodulation