9781111306366 ppt ch8

Guide to Operating Systems, 4th ed.

Chapter 8: Modems and Other Communication Devices

Guide to Operating Systems, 4th ed. 2

Objectives

2

• Describe analog modem architecture and where it is still used

• Describe digital modem architecture for high-speed communications through wireless, ISDN, cable, DSL, and satellites

• Describe data communication techniques for flow control, error correction, and data compression

• Configure modem and Internet communications in different operating systems


Analog Modem Architecture

• Analog modems are used for sending faxes, sending data form point-of-sale devices, and in locations where digital connections are not available

• Computers handle information in a digital format as a series of 1s and 0s, represented by the presence (1) or the absence (0) of a voltage

• Analog information is sent over a phone line in analog format – rising and falling of voltage

• Computers can’t understand analog so a modem connects these two incompatible systems and lets them communicate with each other



• Analog modems work over copper or fiber-optic telephone lines– Phone lines are called Plain Old Telephone Service (POTS),

which is the old term, or Public Switched Telephone Network (PSTN)

– Often called dial-up modems and the connections using analog modems are often called dial-up connections



• Analog Modem Hardware Basics– Name comes from modulator/demodulator

• A modem modulates digital signals from the computer into analog signals (to be sent over telephone lines)

• It also demodulates incoming analog signals back into digital signals the computer can understand

– Data pump – component that performs basic modulation/demodulation

– Controller – handles communications tasks and interprets commands

• Where protocols for modulation, error correction, and data compression are stored

• Protocol – an established guideline that specifies how networked data is formatted, transmitted, and interpreted at the receiving end



• Analog Modem Hardware Basics (cont.)– Modem protocol standards are established by the International

Telecommunications Union (ITU)– Latest analog standards, V.90 and V.92, define a 56,000 bits

per second (bps) communications protocol



Modem standards and protocols



• Data Terminal Equipment (DTE) – Example: a computer because it prepares data to be

transmitted

• Data Communications Equipment (DCE)– Example: modem – the speed of the modem is called the DCE

communications rate

• Internal modem – usually built on expansion cards that plug into the computer’s bus (PCI or PCIe)

• External modem – standalone case with its own power supply – usually plugs into a USB port on the computer



• The universal asynchronous receiver-transmitter (UART) is an electronic chip that converts data from the computer into data that can be sent to serial ports

• Computers communicate with external devices in two basic modes:– Asynchronous communication – uses 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– Often referred to as winmodems– Winmodem has a hardware data pump but implements the

controller functions in software (instead of a microchip)– Host Signal Processor (HSP) modems have no controller or

data pump hardware• Use the host’s central processing unit, along with special software

to handle the same jobs

– Disadvantage: requires more memory and processing power



• Hayes AT Command Set for Analog Modems– Command language (invented by Dennis Hayes) used to

configure a general-purpose modem • Also known as Attention commands or AT commands

– Hayes-compatible modem means that the modem supports all or part of the Hayes AT command set

• A Hayes-compatible modem is equipped with software that acts as a command interpreter

– The AT command set is used extensively to set up and control modems so that they are compatible with a variety of host hardware


Digital Modems

• Digital modem is a misnomer - there is no actual modulation or demodulation of analog signals– Performs the same basic functions as an analog – moving data

through a connection and into another computer at a remote location

– Major difference – the data is digital from start to finish– Five types of popular telecommunications networks (each use

different types of digital communications and digital modems)• Wireless

• ISDN

• Cable networks

• DSL

• Satellite


Wireless

• Wireless network adapter – allows you to connect to Wi-Fi hot spots– Wireless connections to the Internet are available in many

places today and almost all portable computers and mobile devices come with a built-in wireless adapter

– Internet connectivity can also be accomplished with an Internet card or wireless modem sold by your cellular phone company

• Provides access anywhere your carrier offers coverage

– To keep your wireless connection secure, choose a strong default password, turn on encryption, disable the Service Set Identifier (SSID) display, filter by MAC address, and enable the firewall feature

ISDN

• Integrated Services Digital Network (ISDN)– Uses digital communications over a telecommunications line

for high-speed computing, videoconferences, Internet connections, etc…

– Good alternative in areas that do not have DSL or cable modem access (it is more expensive than these options)

– Two interfaces are supported in ISDN:• Basic Rate Interface (BRI) – has an aggregate data rate of 144

Kbps

• Primary Rate Interface (PRI) – has an aggregate data rate of up to 1.544 Mbps

• PRI is used for LAN-to-LAN connectivity, Internet service provider (ISP) sites, videoconferencing, and corporate sites that support telecommuters who use ISDN



Cable Modems

• Cable TV providers offer data and voice services to businesses and homes through a cable modem– Cable modems are usually an external device that plugs into a

USB port or NIC in a computer and connects to the coaxial cable used for the cable TV system

– Communicates using upstream (outgoing data) and downstream (incoming data) frequencies or channels

– Data Over Cable Service Interface Specification (DOCSIS) – project to provide a set of standards to govern cable modem communications

• Current standard is DOCSIS 3.0

– If purchasing a cable modem, make sure it is certified to meet at least the DOCSIS 3.0 standard


DSL Modems

• Digital Subscriber Line (DSL) – digital technology that works over copper wire and fiber-optics– To use, you must install an intelligent adapter for your computer

which is connected to the DSL network– One channel is used for outgoing transmissions (upstream) and

another channel is used for incoming transmissions (downstream)

– Maximum upstream transmission rate is 5 Mbps– Downstream communications can reach 85 Mbps– Like a cable modem with a few advantages:

• DSL line is dedicated to a single user (cable modem uses a line shared by other users)

• DSL offers full bandwidth per line, cable modem uses shared bandwidth


DSL Modems

Connecting to DSL


DSL Modems

DSL monitoring and management software

DSL Modems

DSL technologies



Satellite

• Good alternative for areas that do not offer ISDN, cable, wireless, or DSL services

• Connection speeds are currently up to 5.0 Mbps or download and up to 300 Kbps for uploading– Actual speed depends on weather, signal strength and amount

of use experienced on the satellite

• Some satellite providers offer equipment that combines network access with TV access using one dish

Satellite

• User equipment needed:– A satellite dish about 2-3 feet in diameter (usually a little larger

than a dish used for satellite TV)– A digital modem to transmit and receive the signal– Coaxial cables from the modem to the dish– A USB cable from the modems that connects to a USB port on

your computer– Software from the satellite provider to enable the computer

setup

• A limitation of satellite is that they can experience interference during severe rain and snowstorms



Satellite

Satellite communications setup


Data Communications Techniques

• When two devices communicate, they must have a way to halt and resume the flow of data– If not, buffers would fill and overflow, resulting in lost data

• Flow control – prevents buffers from getting too full– Through the use of hardware or software

• Software Flow Control – Xon-Xoff– When the receiving computer needs time to process data it can

send an Xoff request to the remote modem to stop data flow– Once it catches up, it can send an Xon to resume data flow

• Hardware Flow Control – Halts and resumes data flow by changing the voltage on specific

pins in the serial interface



• Hardware flow control is more reliable and permits faster modem performance – always use hardware flow control instead of Xon-Xoff

Flow control options in Windows



• Error Correction– Errors are often introduced into the data stream by telephone

lines or other equipment– Modems check for errors and resend bad blocks of data– Most basic form of error correction involves the start and stop

bits• Each eight-bit byte is framed by a start bit at the beginning and a

stop bit at the end

• Start and stop bits always have the same value, if a bit is added or lost the start and stop bits won’t be in the right place

• Receiving modem will notice this and request that the sending modem resend that block of data

• Modem’s UART is responsible for adding and stripping the start and stop bits



Data byte framing with start and stop bits



• Another form of error checking is parity checking– Ensures data integrity through a system of data bit

comparisons between the sending and receiving computer– In the example: 0100101 adding up the 1s yields 3

• If parity is set to even, then an extra parity bit with a 1 value must be added to make it 01001011

• A data byte that already has an even umber of 1s gets a parity bit of 0 to maintain the even parity check

• The receiving computer checks the 1s in each byte to make sure they add up to an even value – if they are odd, the computer knows that an error occurred

• Most modems use the ITU’s V.42 standard to provide error checking


• Data Compression – efficiency varies by file type– Compression can reduce the size of a TIFF (Tagged Image

Format) file by more than 90 percent– Modems compress data “on the fly” while you send it– Typically accomplished by using the V.42bis standard, which

employs the Lempel-Ziv-Welch (LZW) compression method• Compresses data as it is sent rather than waiting for all of the data

to be prepared in a buffer, compressed, and sent

• Can detect when the data is already compressed (a file that is compressed using PKZIP) and does not attempt to compress it

– Data can be compressed on a 4:1 basis (up to 4 times faster transmission rate)



Modems and the Operating System

• All OSs include a communications component

• Most analog modems come with a DVD/CD-ROM for configuring an Internet connection

• For digital connections, the installation requirements vary by the provider– You typically receive an installation disc tailored to the OS and

the provider– Usually involve steps to provide the user account and

password information


Windows Configuration

• In Windows 7/Server 2008 R2:– Click Start, Control Panel, and Open Network and Sharing

Center– In Change your network settings, click Set up a new connection

or network– Click the option for the device you want to install and follow the

options

• In Windows Vista/Server 2008:– Click Start, Control Panel and double-click Network and

Sharing Center– In the Tasks pane, click Set up a connection or a network– Click the option for the device you want to install and follow

instructions


Accessing network connections in Windows Server 2008 R2



• In Windows XP:– Click Start, Control Panel, Network and Internet Connections– Click Create a connection to the network at your office and

follow the prompts

• In Windows Server 2003/R2:– Click Start, Control Panel, Network Connections– Click the device you want to configure and click Properties– On the General tab, under Connect using, click the device you

want to configure– Click Configure and enter the information about your device

• Configure Windows Firewall to help discourage intruders from accessing your computer


UNIX/Linux Configuration

• In UNIX/Linux an analog modem as two purposes– Dial-out device– Dial-in device – is treated like a terminal connected to the

computer using a USB connection• To support dial-in connectivity, UNIX/Linux uses a daemon (an

internal program) called getty

• You should set the modem to Auto Answer mode – modem automatically answers the phone when there is an incoming call

• Some UNIX/Linux versions include software that can be used to access and configure a modem – called Minicom

– Linux with the GNOME desktop offers the Network Connection tool that enables you to configure a DSL and a mobile broadband connections (see figure on next slide)



Add DSL connection in GNOME desktop in Linux



• The Network Connection tool allows you to configure the following for a DSL connection:– Ability to connect automatically– DSL access information– Information for a wired connection– PPP communication settings (authentication methods for

security and optional data compression)– IP (Internet Protocol) settings, such as IP address

• To open this tool:– Click System in the top Panel, point to Preferences, and click

Network Connection



• Most UNIX/Linux OSs come with Mozilla Firefox as the Web browser– Use Evolution or Mozilla Mail for email

• There are some advanced terminal emulation programs available as third-party add-ons– x3270– Kermit– C-Kermit


Mac OS X Configuration

• Mac OS X Leopard and Snow Leopard offer Network Setup Assistant for configuring network connections

• In some cases, the modem already contains the information needed to perform an automatic setup

• Also support connecting to the Internet through a Bluetooth-compatible cell phone– By configuring a Bluetooth dial-up connection

• Two other tools to set up Internet access:– Internet Connect – System Preferences


Mac OS X Configuration

Mac OS X Snow Leopard Network Setup Assistant


Chapter Summary


Chapter Summary

• Modems open WAN communications to millions of users so they can access the Internet and communicate with servers for information sharing, education, and work

• Analog modems have a three-part architecture consisting of a data pump, controller, and UART

• The most common way to communicate through analog modems is by using the Hayes AT modem command set to control modem settings and establish WAN connections

• Digital modems are not actually modems because they do not perform the modulation/demodulation require for analog devices and are called modems because they perform the same basic function of allowing communications between computers over a WAN connection


Chapter Summary

• Digital modems work with digital telephone systems, cable systems, wireless systems, and satellite systems – most commonly ISDN, CATV networks, DSLs, and wireless land and satellite networks

• Both analog and digital modems communicate by using communications protocols, software flow control, hardware flow control, error correction, and data compression

• Windows enables the configuration of modems and Internet connections through Control Panel utilities built into the OS

• Modem and Internet configuration tools are available for UNIX/Linux systems, such as Minicom and Network Configuration tool and Internet Configuration Wizard


Chapter Summary

• Mac OS X comes with the Internet Connect, System Preferences, and Network Setup Assistant tools for configuring modem and Internet connections


Education

9781111306366 ppt ch8