CHAPTER Modems. Chapter Objectives Discuss basic modem related issues –Standards, Hayes...

CHAPTER CHAPTER

ModemsModems

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 communication link using the fiber-optic interface

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 END OF CHAPTER INTRODUCTIONINTRODUCTION

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

Software Intel Micro Modem

Intel Machine Code

Hayes AT Command

Modem Features

• Auto-dial• Auto-answer• Reverse calling• All the above features are standard

in today’s Modems

END OF MODULEEND 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 Com2Port

To PhoneJack 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

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

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 Settings

Control Panel Modems

Modem Icon in Control Panel

Double Click

Select the Modem

Selecting the Modem

Load the driver from the diskIf the modem is not found in the List.

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

Modem General Properti

esIncrease

the speed to the

maximum.

1

2

3

Connection

Properties

1

2

3 4

Port Settings

1

2

Advanced Connection Settings

1 2

3

4

Dialing Propertie

s1

2

3

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/Synchronou

s 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/Synchronou

s 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

Mainframe

Front EndProcessor

ClusterController

Terminal

SynchronousModem

Leased Line

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)

ComputerModem with

BondingCapability

Phone Lines

Bonding of two phone lines can provide atotal bandwidth of 112K bps.

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 fiber-optic link

Fiber-optic Connection for RS-232C

• 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

Host

Fiber Line Drivers Terminal

2.5 miles

RS 232

Fiber-optic Line Drivers

T1

Building 1 Building 2

Fiber-optic Line Driver

Router,Bridge.(Support Videoconferencing)

Practical Fiber Optics Connection

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

Null Modem

Send Send

Receive Receive

Comp 1 Comp 2

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

Null Modem

Comp 1 Comp 2Cable 1 Cable 2

Comp 1 Null Modem Cable Comp 2

Past

Present

Other Ways to Connect Two Computers (Network)

Comp 1

RJ 45Cross-over Cable

Comp 2

NIC

Other Ways to Connect Two Computers (Network)

Comp 1 Comp 2

Infrared Connection

Infrared Port Device

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

Short-Haul ModemRack

UNIX Computer

SHM Term.

SHMSHM Term.

SHM Term.

SHM Term.

Practical Implementation

Source: Black Box

In Summary

• Traditional modems– Perform modulation

• Nontraditional modems– Do not perform any modulation

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)

Cable Modem

Computer

Coaxial Cable

UTP (Cat 3 – 5)

NIC

Sharing Single IP Address with Multiple Computers

(10base2)

1

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.

Source: Tim Higgins Site

2


(10baseT)

1

1


Source: Tim Higgins Site

2

Two Computers Sharing One IP without a Hub

2


Source: Tim Higgins Site2

1

2

3

Computer

Cable Modem LAN Connection (Multiple IP

Addresses)

Cable Modem Hub

Coaxial Cable

NICComputer

NICComputer

NIC

UTP (Cat 3 – 5)Crossover Cable?

UTP (Cat 3 – 5)

Cable Modem Customer Connections

Customer Customer Customer

Cable Service

Provider

CM CM CM

CMTS

CM: Cable ModemCMTS: Cable Modem Termination System

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


Upstream Communication Characteristics


OSI-ISO Reference Model and Cable Modem

Communication


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 cable-modems.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 cable-modems.org

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)

DSL Modem

Computer

POTS line

UTP (Cat 3 – 5)

NIC

10/100 Mbps(*Cat 5 is recommended)


(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


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


Upstream Communication Characteristics


OSI-ISO Reference Model and Cable Modem

Communication


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/

homeconnect/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 MODULE

END OF CHAPTER END OF CHAPTER

CHAPTER Modems. Chapter Objectives Discuss basic modem related issues –Standards, Hayes...

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