Transcript Windows Server 2008

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.
2
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
Guide to Operating Systems, 4th ed.
3
Analog Modem Architecture
• 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
Guide to Operating Systems, 4th ed.
4
Analog Modem Architecture
• 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
Guide to Operating Systems, 4th ed.
5
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
Guide to Operating Systems, 4th ed.
6
Analog Modem Architecture
Modem standards and protocols
Guide to Operating Systems, 4th ed.
7
Analog Modem Architecture
• 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
Guide to Operating Systems, 4th ed.
8
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
• 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
Guide to Operating Systems, 4th ed.
9
Analog Modem Architecture
• 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
Guide to Operating Systems, 4th ed.
10
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
Guide to Operating Systems, 4th ed.
11
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
Guide to Operating Systems, 4th ed.
12
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
Guide to Operating Systems, 4th ed.
13
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
Guide to Operating Systems, 4th ed.
14
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
Guide to Operating Systems, 4th ed.
15
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
Guide to Operating Systems, 4th ed.
16
DSL Modems
Connecting to DSL
Guide to Operating Systems, 4th ed.
17
DSL Modems
DSL monitoring and management software
Guide to Operating Systems, 4th ed.
18
DSL Modems
DSL technologies
Guide to Operating Systems, 4th ed.
19
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
Guide to Operating Systems, 4th ed.
20
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
Guide to Operating Systems, 4th ed.
21
Satellite
Satellite communications setup
Guide to Operating Systems, 4th ed.
22
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
Guide to Operating Systems, 4th ed.
23
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
Guide to Operating Systems, 4th ed.
24
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
– 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
Guide to Operating Systems, 4th ed.
25
Data Communications Techniques
Data byte framing with start and stop bits
Guide to Operating Systems, 4th ed.
26
Data Communications Techniques
• 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
Guide to Operating Systems, 4th ed.
27
Data Communications Techniques
• 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)
Guide to Operating Systems, 4th ed.
28
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
Guide to Operating Systems, 4th ed.
29
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
Guide to Operating Systems, 4th ed.
30
Windows Configuration
Accessing network connections in Windows Server
2008 R2
Guide to Operating Systems, 4th ed.
31
Windows Configuration
• 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
Guide to Operating Systems, 4th ed.
32
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)
Guide to Operating Systems, 4th ed.
33
UNIX/Linux Configuration
Add DSL connection in GNOME desktop in Linux
Guide to Operating Systems, 4th ed.
34
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 IP address
• To open this tool:
– Click System in the top Panel, point to Preferences, and click
Network Connection
Guide to Operating Systems, 4th ed.
35
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
Guide to Operating Systems, 4th ed.
36
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
Guide to Operating Systems, 4th ed.
37
Mac OS X Configuration
Mac OS X Snow Leopard Network Setup Assistant
Guide to Operating Systems, 4th ed.
38
Chapter Summary
Guide to Operating Systems, 4th ed.
39
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
Guide to Operating Systems, 4th ed.
40
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
Guide to Operating Systems, 4th ed.
41
Chapter Summary
• Mac OS X comes with the Internet Connect, System
Preferences, and Network Setup Assistant tools for
configuring modem and Internet connections
Guide to Operating Systems, 4th ed.
42