Guide to Operating Systems, 4 th ed. Chapter 8: Modems and Other Communication Devices

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 from point-of-sale devices, and in locations where digital connections are not available or are too costly.

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

• Analog information is sent over a phone line in analog format.– The rising and falling sounds produced when you speak into a

handset.



• Computers can’t recognize analog data and the phone system can’t recognize digital data.

• A modem is a piece of hardware and associated software that connects these two incompatible systems in a way that lets them communicate with each other.

• Analog modems work over copper or fiber-optic telephone lines.– Plain Old Telephone Service (POTS)– 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– An analog modem consists of three basic electronic hardware

or software components:• The data pump

• The controller

• The Universal Receiver-Transmitter (UART)

– A modem is a modulator/demodulator.• A modem modulates digital signals from the computer into analog

signals that can be sent over telephone lines.

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

– The data pump is the component that performs the basic modulation/demodulation.



• Analog Modem Hardware Basics– Controller handles communications tasks and interprets

commands.• This is where protocols for modulation, error correction, and data

compression are stored.– A protocol is an established guideline that specifies how networked

data, including data sent over a telephone network, is formatted, transmitted, and interpreted at the receiving end.

• Also interprets commands to configure and operate the modem using a special command set (Attention commands (AT).

– Modem protocols define some of the basic operational parameters of the modem and determine how compatible it is with other modems with which it communicates.



• 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



• When a computer is connected to a modem, the data transfer speed is the Data Terminal Equipment (DTE) communications rate.– Example: a computer because it prepares data to be

transmitted.

• The modem is called the Data Communications Equipment (DCE) and its speed is the DCE rate.– 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 expansion (the Peripheral Component Interconnect (PCI) or the Peripheral Component Interconnect Express (PCIe) bus.

• External modems are circuit boards that are placed inside a standalone case with its own power supply.– Usually plugs into a USB port on the computer.



• An external USB modem is an excellent choice:– The USB port is a high-speed port that can supply power to

peripheral devices (eliminates the need for external power supplies).

– USB devices are self-configuring.• When you plug in a USB modem, The OS recognizes the

presence of the new device and automatically launches a configuration utility.

– An external modem is a universal device that can be used with Windows, UNIX/Linux, or Mac OS.



• 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.– The UART reads in one byte of data at the computer’s bus

speed.– Adds a start bit at the beginning.– A stop bit at the end.– Generates an interrupt.– Feeds the bits to the port.



• Computers communicate with external devices in two basic modes:– Refers to the method used to keep the data streams on the

local and remote devices aligned so proper data transfer can occur.

• 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

– Usually more efficient, but requires more processing at both ends of the link.


Analog Modem Architecture• Software-Based Modems

– All modems need the functions of the data pump, controller, and UART.

– Some modems do not implement the controller functions in a microchip, but instead implement them in software functions handled by the PC.

– Software modems are often referred to as winmodems.• A controllerless modem that retains a hardware data pump (a

Digital Signal Processor (DSP)) but implements the controller functions in software.

• 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

• The name Digital modem is a misnomer.

• There is no actual modulation or demodulation of analog signals.– Performs the same basic functions as an analog modem.

• Moves data out of a computer, across a telephone line or cable connection, or through the airwaves, and into another computer at a remote location.

– Major difference – the data is digital from start to finish.– Digital modems are digital devices that use digital

transmission media.


Digital Modems

• 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 communications, videoconferences, Internet connections, etc…

– Generally more expensive than DSL and cable modems.– A good alternative in areas that do not have DSL or cable

modem access (it is more expensive than these options).– With ISDN it is possible to connect multiple digital devices (eight)

to one incoming line.– ISDN uses standard copper telephone line pairs with digital

equipment on either end of the connection to encode and transmit the information.

• An ISDN router (to route the transmission) and a Terminal Adapter (TA) – a type of digital modem.


ISDN• Integrated Services Digital Network (ISDN)

– ISDN routers and TAs typically include analog jacks so you can plug in a conventional telephone or modem for use over the digital line.

• You get separate channels for computer data and analog telephone lines.

• You can use one analog line and one data line simultaneously, or two digital lines, or two analog lines.

– 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

– The 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.

• The actual data speed is partially dependent on how much bandwidth many of your neighbors are using through their cable modems at the same time.

• Also, a cable service provider may establish a limit on your bandwidth so that the provider can give more users access to the cable network.


Cable Modems

• Cable TV providers offer data and voice services to businesses and homes through a cable modem.– The cable modem industry has been working to provide a set

of standards to govern cable modem communications in a project called the Certified Cable Modem Project.

• More commonly referred to as Data Over Cable Service Interface Specification (DOCSIS).

– The 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.– Two pairs of wires are connected to the adapter and then out to

the telephone pole.– Communication over the copper wire is simplex.

• 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


DSL Modems

• Digital Subscriber Line (DSL) – Digital technology that works over copper wire and fiber-optics– 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 is connected by means of a combined DSL adapter and router.

• A router is a device that can be used to direct traffic network and create a firewall so that only authorized users can access network services.

– Enables multiple users to access one DSL line.– Protects the network from intruders.


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 for 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

Satellite• Older communications satellites orbit in the

atmosphere at approximately 22,300 miles above the Earth’s surface.

• The extreme distance and high atmospheric disturbances can cause transmission delays that are unacceptable for time-sensitive communications involving data transfers and multimedia.

• Several companies currently have newer low Earth orbiting (LEO) satellites which orbit at a distance of between 485 to 878 miles above the earth’s surface, resulting in faster two-way communications.


Satellite• Satellite networks are used for:

– Broadband (high-speed) Internet communications;– Around the world video conferencing;– Classroom and educational communications;– Other communications involving voice, video, and data.



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.– Accomplished through the use of hardware or software.

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

the buffers, perform I/O, etc., 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.• Not as commonly used today in communications .



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

specific pins in the serial interface.– Controlling data flow with hardware eliminates the problem of

the modem confusing data with control signals.



• 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 to

ensure that the receiving modem gets the information exactly as it was transmitted.

– Modems transfer bits (ones or zeroes).– Three possible errors can occur:

• A bit can be lost;

• An extraneous bit can be introduced;

• A bit can be flipped.



• Error Correction– The 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.

• The 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.– The receiving modem will notice this and request that the sending

modem resend that block of data.

• The modem’s UART is responsible for adding and stripping the start and stop bits.



Data byte framing with start and stop bits



• There are always eight bits between the start and stop bit, even when communicating with systems that only require seven bits.

• This makes it possible to use the eighth bit for another form of error checking.

• Parity checking ensures data integrity through a system of data bit comparisons between the sending and receiving computer.

• Parity can either be even or odd (or none, if parity checking is turned off).



• Parity checking – 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 number of 1s gets a parity bit of 0 to maintain the even parity check (01001010).

• 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.– All versions of V.42 enable the communicating modems to

detect the presence of noise on the communication line. Which can lead to errors.

– When noise is detected through using V.42, the communicating modems can decide to transmit at a slower speed so that fewer retransmissions are required.


• Data Compression– Modems usually compress the data they send.– A data compression routine could study a picture and see that

there’s a lot of repetitive blue in a picture. • The compressed representation of the picture would show a blue

dot and a number that represents the number of times the blue dot is repeated.

• This takes less room than physically representing each blue dot.

– 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.

• Data compression is one way in which modem manufacturers are able to achieve some of the high-speed data transfers expected.



• Data Compression– Compression is typically accomplished by using the V.42bis

standard, which employs the Lempel-Ziv-Welch (LZW) compression method which compresses data in two ways:

• It compresses data as it is sent rather than waiting for all of the data to be prepared in a buffer, compressed, and sent.

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

– In an ideal situation, data can be compressed on a 4:1 basis (up to 4 times faster transmission rate).

• As long as there are no transmission errors because of line noise.

– The V.44 standard is about 15 percent faster than the V.42 bis standard and data can be compressed on a 6:1 basis.



Modems and the Operating System

• All OSs include a communications component.– Such as for communicating through modems.

• Data communications is one of the most basic OS duties.

• 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 the

instructions.

Windows Configuration

Accessing network connections in Windows Server 2008 R2


Windows Configuration• In Windows XP:

– Click Start, Control Panel.– Double-click 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.– Double-click 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.


Windows Configuration• You can configure Windows Firewall to help

discourage intruders from accessing your computer and to filter possible viruses from coming in over the Internet or through FTP downloads.

• You should configure this option if you are connecting to the Internet with any type of device.


UNIX/Linux Configuration

• In UNIX/Linux, and in some configurations of Windows and Mac OS X, an analog modem as two purposes.– Dial-in device – The modem is treated like a terminal

connected to the computer using a USB connection.• The terminal is referred to as a teletype.

• The device port used for a modem is referred to as a TTY port.– To support dial-in connectivity on one of these TTYs, UNIX/Linux

uses a daemon (an internal, automatically running program) called getty.

– You should set the modem to Auto Answer mode for use with getty.» The modem automatically answers the phone when there is an

incoming call and makes the connection to the other modem.



• Some UNIX/Linux versions include software that can be used to access and configure a modem.– Minicom is one of the most popular.

• If Minicom is not available in your version of UNIX/Linux, it can be downloaded.

• Linux with the GNOME desktop offers the Network Connection tool that enables you to configure a DSL connection and a mobile broadband connections.



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 the IP address.



• The Network Connection tool also supports a mobile broadband connection, such as through a cellular 3 G network.

• The information you would need to configure such a connection includes:– Broadband provider– Billing plan name– Billing plan access point name (APN)

• 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

• Dial-out device can be done with the same modem, but, in general, a different device name is used.– The initialization string for most dial-out applications is very

similar to that for dial-in applications.


Mac OS X Configuration

• Mac OS X Leopard and Snow Leopard offer Network Setup Assistant for configuring an analog modem, DSL connection, cable modem, and other network connections.

• The Network Setup Assistant guides you through the setup, depending on the type of connection you wish to configure.

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



Mac OS X Snow Leopard Network Setup Assistant



• If the modem is not preprogrammed with the configuration information, you will need to provide:– Account name– Password for the account– PPPoE (Point-to-Point over Ethernet) service name– (When automatic address configuration is not used through

DHCP, you’ll need to specify IP address, subnet mask, router address, and the addresses for the DNS hosts.

• Leopard and Snow Leopard also support connecting to the Internet through a Bluetooth compatible cell phone by configuring a Bluetooth dial-up network (DUN) connection.


Mac OS X Configuration• The general steps for configuring a Bluetooth DUN

connection are:– Ensure the cell phone is turned on.

– Open System Preferences from the Dock.

– Click Network in System Preferences.

– Click Bluetooth DUN in the left pane of the Network window.• Displayed if your Mac has Bluetooth capability.

– Enter the dial-up information as specified by the cell phone provider.

• Telephone number• Account name• Account password

– Click the Connect button to test the connection

– Click the Network window


Mac OS X Configuration• Two other tools to set up Internet access: Internet

Connect and System Preferences.

• System Preferences can be used to configure a more complete range of parameters.

• To configure using System Preferences:– Click the System Preferences icon on the dock.– Click the Network icon.– Select the Internal Modem.– Display the TCCP/IP tab and configure to use settings from the

ISP (choose Using PPP) or configure hard-coded IP address information (a Manual configuration).

• If requested by the ISP, provide DNS information and/or information connecting to a specific domain.


Mac OS X Configuration• To configure using System Preferences (contid):

– Click the PPP tab and configure the dial-up information (telephone number, user name, password).

– Click the Proxy tab and complete any proxy information as required by the ISP or the Wilderness Supply.

– Click the Modem tab.– Make sure that the modem is properly configured for the

situation in which it will be used.– Close all open windows.


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


Documents

Guide to Operating Systems, 4 th ed. Chapter 8: Modems and Other Communication Devices