CHAPTER
Modems
Chapter Objectives
• Discuss basic modem related issues
– Standards, Hayes compatibility etc.
• Describe the different types of practical
modems available
– Internal, external, null, short-haul
modems etc.
• Present the profile of sample practical
modems
– Asynchronous and synchronous modems
• Show the establishment of a fiber-optic
Chapter Modules
•
•
•
•
•
•
•
•
•
•
Modem Functional Considerations
Internal, External and Portable Modems
A Modem Profile
Modem Software Installation
A Practical Asynchronous/Synchronous
Modem
Faster Modem Standards
Fiber-optics Connection
Null Modem and Short-haul Modem
Cable Modem
DSL Modem
END OF CHAPTER
INTRODUCTION
MODULE
Modem Functional
Considerations
Module Objectives
• List the two types of modem standards
• Describe the features that are
standardized in modem communication
– Modulation, data compression etc.
• Explain the relevance of standards to
speed
• Discuss Hayes compatibility
• List a few common modem features
Major Modem Standards
• ITU standard
– Universal standard
– Currently in use
• Bell standard
– Earlier low speed modem standard
– No more in use
Standards in Modem
Communication
• Modulation standard
• Data compression standard
• Error correction Standard
Speed and Modulation Standards
• Each speed is associated with a
modulation standard
• V.34 is an example standard for the
communication speed of 28,800 bps
• Higher speed modems can operate at
lower speeds as well
– E.G 28,800 bps modem can communicate at
14,400 bps
Data Compression Standards in
Modem
• MNP 5
• ITU V.42 bis
Error Correction Standard
• MNP 2-4
Understanding Hayes
Compatibility
• IBM is to micros what Hayes is to
modems
• Hayes compatibility
– Ensured by adherence to the Hayes
command set known as the AT command
set
– Ensures wide availability of software
Command Translation for PCs
and Modems
• Commands for the PC
– Translated into machine language and
executed
• E.G. Identifying a file for transmission
• Commands for the modem
– Generated in Hayes AT command set form
and transmitted to the modem for
execution
• E.G. Command instructing the modem to dial
Command Translation
Intel Machine Code
Software
Intel Micro
Modem
Hayes AT Command
Modem Features
• Auto-dial
• Auto-answer
• Reverse calling
• All the above features are standard in
today’s Modems
END OF MODULE
MODULE
Internal, External and Portable
Modems
Overview of Internal and
External Modems
• Internal modem
– Installed in the expansion slot
– Usually cheaper
– Occupies one expansion slot
• External modem
–
–
–
–
Connects to the serial port of the computer
More expensive
Does not occupy an expansion slot
But, it does utilize one of the available
communication ports
Internal Modems
Source: Black Box
External Modems
Source: Black Box
External Modem Ports and
Connection
To Com2
Port
To Phone
Jack RJ 11
Connection
Modems for the Notebooks or PC
Card Modem
• Credit card size
• Connects to the PCMCIA slot
– Currently, known as PC card slots
• PC cards have undergone major
refinements
– Efficient in their operation
– Support high-speed communication
devices
PC Card Modems
Source: Black Box
Multi-Purpose PC Cards
Source: Black Box
END OF MODULE
MODULE
A Modem Profile
Module Objectives
• Examine the profile of an actual
commercial modem
• Discuss the standards as they relate to
speed, data compression and error
correction
• Describe the FAX related
communication details of the modem
Profile of a Modem : Black Box
Modem 288 Standards
• Standards
– ITU V.92, ITU V.90, ITU V.34, v.32bis, v.23,
v.22bis, v.22, v.21, Bell 101 and Bell 212a
• Protocols
– Asynchronous and synchronous
• Speed
–
–
–
–
V.90 ------ 56Kbps
V.34 -------> 28.8 Kbps
Others up to 14.4 Kbps
Possible up to 115.2 Kbps
Profile: Operation, Interface and
Connectors
• Operation
– 2 wire dial-up
• Interface
– RS-232C
• Connectors
– 1 DB25 female
– 2 RJ11 female
Profile: Speed and Compression
Standards
• V.90
– Supports 56 Kbps
• V.34
– Supports 28.8 Kbps
• V.32
– Supports 14.4 Kbps
• V.42bis
– Data compression standards
– Up to 4 to 1 compression is possible
Profile: Error Correction and
Other Data Compression
Standards
• MNP 2-4 and 10
– Error correction
• MNP 5
– Data compression
Combined Effect of Speed and
Compression Standards
• Combined effect of v.34 and v.42bis
data compression
– 115.2 Kbps
– This is only a theoretical reality
– Practical speeds can be considerably slow
Effect of Compression on File
Transfer
• Highest transfer speeds cannot be
achieved in the case of already
compressed files
– Example: jpg, mp3 etc.
– Because the above files are already
compressed
Profile: Fax Communication
• Fax details
– 14.4 Kbps speed
– Class 1 and 2 fax software support
– Works with class 3 fax machines
END OF MODULE
MODULE
Modem Software Installation
Module Objectives
• Demonstrate the step-by-step
installation of a modem in the Windows
95 operating system
Installation Overview
• The task is to install a Hayes Accura
28.8 modem on Com2 port
• Overview of the Steps
– Open the control panel
– Choose the Modem icon
– Proceed with the installation as instructed
and finally specify the properties of the
installed modem
Demonstration of Modem
Software Installation
Accessing Modem Installation
Icon Through Control Panel
Start
Modems
Settings
Control
Panel
Modem Icon in Control Panel
Double Click
Select the Modem
Load the driver from the disk
If the modem is not found in
the List.
Selecting the Modem
Non-Mainstream Modem
• If the modem is not of a popular type,
then the driver must be loaded from a
location on the disk
• In the above case, select the option that
says “Have Disk”
Selecting the Port
Finishing the Installation
Installed
Modem
1
2
3
1
2
3
Modem
General
Properties
Increase the
speed to the
maximum.
1
2
3
4
Connection
Properties
1
2
Port Settings
Advanced Connection Settings
1
4
3
2
Dialing
Properties
1
2
3
END OF MODULE
MODULE
A Practical
Asynchronous/Synchronous
Modem
Module Objectives
• Examine briefly the profile of an
asynchronous/synchronous modem
• Discuss speed related factors
• Give an illustration of the typical use of
a synchronous modem
Profile of an
Asynchronous/Synchronous
Modem
• Blackbox 32fast modem
• Claimed speed
– 72k bps synchronous
• Uses sync. Data compression to send data
under HDLC/SDLC
– 115.2k bps asynchronous
• Best compression is automatically
chosen
– Between v.42 bis and MNP 5
Profile of
Asynchronous/Synchronous
Modem
• Other features
– Trellis-coded modulation
– MNP 4 error correction
• Suggested lines
– 2 or 4 wired leased lines
– Switched phone lines may not provide
optimum performance
• UART connection
– For effective high-speed transmission
Typical Use of a Synchronous
Modem
Front End
Processor
Mainframe
Leased Line
Terminal
Synchronous
Modem
Cluster
Controller
END OF MODULE
MODULE
Faster Modem Standards
Faster Modem Standards
• The current fastest analog modem
operates at a speed of 56 Kbps
• Standards
– ITU V.90
– X2
– K56flex
• X2 was introduced by US Robotics that
has been acquired by 3 Com
• K56flex standard was introduced by
Rockwell and Lucent Technologies
Practical Speed Considerations
• 56 Kbps has not been achieved in many
tests in practice
• Speed is usually in the region of 45k bps
or less
• FCC restriction on download speed
– Currently the speed limitation is 53 Kbps
56 Kbps /V.90 Information
• Cedric Walravens’ web site is a good
source of information on 56K /V.90
modems and communication
Data Compression Limitations
• High speed modems use compression
to increase the throughput
• Files such as image files are already
compressed
– There is little leeway for further
compression
– Results in a drop in the communication
speed
Quality of the Phone Lines
• Certain phone lines cannot support 56K
bps speed
• Central office that services a particular
area may not have the latest switching
equipment
• Testing the line for 56K bps support
– X2.usr.com/linetest1.html
Bonding of Telephone Lines to
Achieve Higher Speeds (112 Kbps)
Computer
Modem with
Bonding
Capability
Phone Lines
Bonding of two phone lines can provide a
total bandwidth of 112K bps.
END OF MODULE
MODULE
Fiber-Optics Connection
Module Objectives
• Examine a fiber-optic connection used
for extending the length of the
communication link
• Give an illustration of a practical fiberoptic link
Fiber-optic Connection for RS232C
• Extends RS-232C connection to nearly 2
. 5 miles
• Fiber interface (line driver)
specifications
– Transfer mode
• Synchronous and asynchronous
– Speed
• 56 K bps for sync.
• 64 K bps for aysnc.
• Selectable depending on the connection
Fiber Optic Line Driver
Some Fiber-optic
Connections :
Fiber Modems
2.5 miles
Host
RS 232
Fiber Line Drivers
Terminal
Fiber-optic Line Drivers
Building 1
Building 2
T1
Fiber-optic Line Driver
Router,
Bridge.
(Support
Videoconferencing)
Practical Fiber Optics
Connection
END OF MODULE
MODULE
Null Modems and Short-Haul
Modems
Null Modem
• Use for connecting two computers
together
– Communicate through their respective
serial ports
• Digital signals are not modulated
Send
Comp 1
Receive
Send
Null
Modem
Comp 2
Receive
Characteristics of the
Null Modem
• Cross-over connection
– Enables the sending and receiving of signals to
facilitate communication
• Instead of a null-modem, one may also use a
crossover cable
• In the past, null modems were used with
software like lap-link
– The primary purpose was for file transfer
• Null modem is being replaced by a simple
null modem cable
Null Modem Cable Connection
Comp 1
Cable 1
Null
Modem
Cable 2
Comp 2
Past
Comp 1
Null Modem Cable
Present
Comp 2
Other Ways to Connect Two
Computers (Network)
NIC
RJ 45Cross-over Cable
Comp 1
Comp 2
Other Ways to Connect Two
Computers (Network)
Infrared Port Device
Infrared Connection
Comp 1
Comp 2
Crossover Cable Use
• Laplink
– A software that is used for transferring
information between computers
• The Laplink cable that is used to connect
computers together is indeed a crossover
cable
– In this case, the computers are connected either
over the serial, parallel or USB ports
• Nowadays, a popular method of exchanging
information between computers is to use
Infrared
Short-Haul Modem
• Purpose
– Extend the length of a RS-232 serial
connection
• Signals are not modulated
– The term modem is a misnomer
• Signals are boosted so that they may be
carried over a longer distance
Short-Haul Modem
Asynchronous.
Host
UNIX
Computer
ShortHaul
Modem
Rack
SHM
Term.
SHM
Term.
SHM
SHM
Term.
SHM
Term.
Practical Implementation
Source: Black Box
In Summary
• Traditional modems
– Perform modulation
• Nontraditional modems
– Do not perform any modulation
END OF MODULE
MODULE
Cable Modems
Cable Modem
• A modem that is used for connecting a
computer to the cable TV line
• The purpose is to facilitate high speed
access to the Internet
• Cable modem is a broadband
technology device
– The cable carries multiple channels
including TV channels and upstream and
downstream data channels
Typical Cable Modem
Connection (Single IP Address)
Coaxial Cable
Cable
Modem
UTP (Cat 3 – 5)
NIC
Computer
Sharing Single IP Address with
Multiple Computers (10base2)
Source:
Tim Higgins
Site
1
2
1
•A proxy server can be used.
•Windows 2000.
•Windows 98 Second edition supports the sharing of
•one PC for multiple PC access to the Internet.
Sharing Single IP Address with
Multiple Computers (10baseT)
1
Source:
Tim Higgins
Site
2
1
•A proxy server can be used.
•Windows 2000.
•Windows 98 Second edition supports the sharing of
•one PC for multiple PC access to the Internet.
Two Computers Sharing One IP
without a Hub
1
Source:
Tim Higgins
Site
2
3
2
•A proxy server can be used.
•Windows 2000.
•Windows 98 Second edition supports the sharing of
•one PC for multiple PC access to the Internet.
Cable Modem LAN Connection
(Multiple IP Addresses)
Coaxial
Cable
Cable
Modem
UTP (Cat 3 – 5)
Crossover Cable?
Hub
UTP (Cat 3 – 5)
NIC
Computer
NIC
Computer
NIC
Computer
Cable Modem Customer
Connections
Cable Service
CMTS
Provider
CM
Customer
CM
Customer
CM: Cable Modem
CMTS: Cable Modem Termination System
CM
Customer
Cable Modem Limitation
• Each cable drop is shared by a number
of customers
• As the number of customers increases
in a trunk the bandwidth available per
customer decreases
Cable Modem Types
• Internal Modems
– Installed into a PC’s expansion slot
• External Modems
– An external box connected to the
computer’s NIC
• External Web TV Box
– Used with Web TV setups
– A return channel may be provided through
a POTS line
Cable Modem Standards
• The current versions are known as the
second generation modems
• US Standards
– MCNS/DOCSIS 1.0/1.1
– A de facto standard
– IEEE is yet to propose or approve a
standard
• European
– DVB/DAVIC 1.3/1.4/1.5
The Tutorial for Cable Modems
• www.cable-modems.org
– Contains useful information on cable modems
• Access to the tutorial
– http://www.cable-modems.org/tutorial/
• Access to Q and A
– http://www.cable-modems.org/Q&A/
• Appreciation and credit
– Some of the slides used in this chapter are from
the above web site as acknowledged in the
appropriate slides
– Also, visit the site for a description of some of the
technical terms associated with cable modems
Additional Sites
• www.cablemodemdatacom.com
– Extensive information with good graphics
Modulation and Demodulation
Frequencies
Source: http://www.cable-modems.org/tutorial/01.htm
Speed and Bandwidth
• Shannon’s Law
– Speed is proportional to bandwidth
• In most cases, downstream speed is
higher than upstream speed
• Higher upstream speed is desired in
certain cases
– Hosting of a web server
• Symmetric streams are desired for
video conferencing
Maximum Bandwidth
• Upstream
– Lower frequencies are used
– Up to 3Mbps
• Downstream
– Higher frequencies are used
– Between 27-56 Mbps
• Note that communication speed is
proportional to available bandwidth
Downstream Communication
Characteristics
Source: http://www.cable-modems.org/tutorial/08.htm
Upstream Communication
Characteristics
Source: http://www.cable-modems.org/tutorial/09.htm
OSI-ISO Reference Model and
Cable Modem Communication
Source: http://www.cable-modems.org/tutorial/02.htm
Sample Cable Modem Companies
• 3Com
• CISCO Systems
• Toshiba
• Sony
• More DOCSIS compatible Cable
Modems
– A gallery of selected cable modems from
cable-modems.org
Sample
Cable
Modem
Sample Product Demonstration
• 3Com Cable Modem CMX product
demonstration
– Explore the purpose of the different status
lights on the modem
– Try this to see the product feature and
rotate the modem to view the different
sides of the modem
Possible Downside
• Potential downsides to the cable
modem are discussed in this ZDnet
article
Advantages and Disadvantages
• Advantages
– Higher speeds compared to analog connections
– The connection is always on
– Possible assignment of static IP addresses
• Disadvantages
– Speed drops with an increase in the number of
customers on a cable drop
– Higher costs compared to analog connections
FAQs
• A collection of 100 FAQs from cablemodems.org
– Includes FAQs for DSL as well
Other Cable Modem Links
• ZDnet home page on cable modem
• Cable modem help web site
• Cable modem net information site
• Tom Higgins’ cable modem site
– Recommended
• Cable modem books
– A listing of selected books from cablemodems.org
END OF MODULE
MODULE
DSL Modems
DSL Modem
• A modem that is used for connecting a
computer to the Digital Subscriber Line (DSL)
• The purpose is to facilitate high speed access
to the Internet
• DSL modem is a broadband technology
device
– The regular telephone line carries the DSL line as
well
– Therefore, there are multiple channels present on
the POTS line delivering the DSL connection
Simple DSL Line Drop
Source: xDSL Technology Web Site
Another xDSL Line Drop
Soource: XDSL
Typical DSL Modem Connection
(Single IP Address)
POTS line
DSL
Modem
UTP (Cat 3 – 5)
(*Cat 5 is recommended)
10/100 Mbps
NIC
Computer
Sharing Single IP Address with
Multiple Computers (10base2)
Sharing Single IP with Multiple
Computers (10baseT)
Source: xDSL
Sharing Single IP with Two
Computers without a Hub
DSL Modem LAN Connection
(Multiple IP Addresses)
Source: xDSL
Source: DSL Reports
DSL Resources
• DSL Reports itself is a good DSL web
site
• A good reference to DSL Resources
from DSL reports
• A good page that shows graphically the
speed comparison of DSL with other
technologies
Typical ADSL Connection
Source: ADSL Tutorial from adsl.com
Cable Modem Limitation
• Each cable drop is shared by a number
of customers
• As the number of customers increase in
a trunk the bandwidth available per
customer decreases
ASDL Modem Types
• Internal Modems
– Installed into a PC’s modem
• External Modems
– An external box connected to the
computer’s NIC
Cable Modem Standards
• The current versions are known as the
second generation modems
• US Standards
– MCNS/DOCSIS 1.0/1.1
– A de facto standard
– IEEE is yet to propose or approve a
standard
• European
– DVB/DAVIC 1.3/1.4/1.5
The Tutorial for Cable Modems
• www.cable-modems.org
– Contains useful information on cable modems
• Access to the tutorial
– http://www.cable-modems.org/tutorial/
• Access to Q and A
– http://www.cable-modems.org/Q&A/
• Appreciation and credit
– Some of the slides used in this chapter are from
the above web site as acknowledged in the
appropriate slides
– Also, visit the site for a description of some of the
technical terms associated with cable modems
Modulation and Demodulation
Frequencies
Source: http://www.cable-modems.org/tutorial/01.htm
Maximum Bandwidth
• Upstream
– Lower frequencies are used
– Up to 3Mbps
• Downstream
– Higher frequencies are used
– Between 27-56 Mbps
• Note that communication speed is
proportional to available bandwidth
Downstream Communication
Characteristics
Source: http://www.cable-modems.org/tutorial/08.htm
Upstream Communication
Characteristics
Source: http://www.cable-modems.org/tutorial/09.htm
OSI-ISO Reference Model and
Cable Modem Communication
Source: http://www.cable-modems.org/tutorial/02.htm
Sample DSL Modem Companies
• 3Com
• CISCO Systems
• Alcatel
Sample
Cable
Modem
Source: 3Com Modem
Sample Product Specifications
Source: 3Com
Sample Product Demonstration
• 3Com ADSL Modem Demonstration
– http://www.3com.com/client/pcd/home
connect/dsl/images/prod_sidenav.gif
– Explore the purpose of the different status
lights on the modem
– Try this to see the product feature and
rotate the modem to view the different
sides of the modem
Possible Downside
• Potential downsides to ADSL
Advantages and Disadvantages
• Advantages
–
–
–
–
Higher speeds compared to analog connections
The connection is always on
Possible assignment of static IP addresses
Existing POTS line can be used
• Disadvantages
– Higher costs compared to analog connections
– May not be available at all locations
Other DSL Links
• DSL Reports web site contains a good
collection of selected links
END OF MODULE
END OF CHAPTER
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