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Guide to Operating Systems,
4th ed.
Chapter 8: Modems and Other
Communication Devices
Objectives
• 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
Guide to Operating Systems, 4th ed.
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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.
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
Modem standards and protocols
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
• 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.
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Analog Modem Architecture
• The universal asynchronous receivertransmitter (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.
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Analog Modem Architecture
• 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.
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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.
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Analog Modem Architecture
• 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
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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.
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Digital Modems
• Five types of popular telecommunications networks
(each use different types of digital communications
and digital modems):
–
–
–
–
–
Wireless
ISDN
Cable networks
DSL
Satellite
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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.
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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.
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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.
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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.
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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
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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
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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.
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DSL Modems
Connecting to DSL
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DSL Modems
DSL monitoring and management software
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DSL Modems
DSL technologies
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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.
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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.
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Satellite
Satellite communications setup
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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.
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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.
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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 .
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Data Communications Techniques
• 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.
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Data Communications Techniques
• 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
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Data Communications Techniques
• 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.
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Data Communications Techniques
• 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.
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Data Communications Techniques
Data byte framing with start and stop bits
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Data Communications Techniques
• 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).
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Data Communications Techniques
• 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.
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Data Communications Techniques
• 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.
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Data Communications Techniques
• 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.
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Data Communications Techniques
• 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.
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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.
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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.
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Windows Configuration
Accessing network connections in Windows Server
2008 R2
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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.
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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.
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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.
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UNIX/Linux Configuration
• 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.
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UNIX/Linux Configuration
Add DSL connection in GNOME desktop in Linux
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UNIX/Linux Configuration
• 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.
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UNIX/Linux Configuration
• 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.
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UNIX/Linux Configuration
• 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.
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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.
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Mac OS X Configuration
Mac OS X Snow Leopard Network Setup Assistant
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Mac OS X Configuration
• 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.
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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
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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.
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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.
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Chapter Summary
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61
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
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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
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Chapter Summary
• Mac OS X comes with the Internet Connect, System
Preferences, and Network Setup Assistant tools for
configuring modem and Internet connections
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