Modems: 

                           An Introduction

• The middlemen between the computer and the  telephone system
  

• Convert analog data suitable for transmission over a phone line to digital data suitable for a computer (modulator/demodulator)

• Can be either an external device or an internal device that is installed in an expansion slot inside a computer

• A phone line connects to a modem or telephone using either an RJ-11 or older RJ-12 connector

• We want connect one computer to another computer by internet then we can use  the modem
  
  

Modem Types

• Telephone modem
  

• Optical Modem (Optical fiber)
  

• Cable modem
  

• DSL Modem
  

• etc

                   
What is cabal modem->
                                                     A Cable Modem

is a digital modem that uses a coaxial cable                            connection for the data transmission.

A cable modem is used for connecting to the Internet and is       much faster than a typical dial-up modem.

while a 56K modem can receive data at about 53 Kbps, cable modems support data transfer rates of up to 30 Mbps.

That's over 500 times faster. However, most ISPs limit their subscribers' transfer rates to less than 6 Mbps to conserve bandwidth

Cabal  Modam: -

                                    Another important way that a cable modem is different than a dial-up modem is that it doesn't connect to a phone line.

Instead, the cable modem connects to a local cable TV line, hence the term "cable modem."

This allows cable modems to have a continuous connection to the Internet. Therefore, there is no need to dial your ISP every time you want to check your e-mail.

Cable modems, which have a much more complex design than dial-up modems.

But some models can be integrated within a computer.

Instead of connecting to a serial port like a external dial-up modem, cable modems attach to a standard Ethernet port so they can transfer data at the fastest speed possible

HOW DOES IT WORK:-

                                             This data connection is received by a cable modem that decodes the signal into your PC.

 Who Makes Cable Modems?

• Bay Networks -- LANcity
      
• Motorola

• Zenith
  
• General Instruments
  
• Hewlett-Packard
• Scientific Atlanta
  
• Scientific Atlanta
  
• Phasecom

                           
                 Who Doesn’t Make Cable Modems?

• Intel
      Announced, then backed out in 1996
  
  
• DEC
• Resold LANcity modems in 1995, left                   business soon thereafter

Modems:--

1. 
   The middlemen between the computer and the telephone system

2. Convert analog data suitable for transmission over a phone line to digital data suitable for a computer (modulator/demodulator

                   


                   
                                 modem

• Can be either an external device or an internal device that is installed in an expansion slot inside a computer
  
• A phone line connects to a modem or telephone using either an RJ-11 or older RJ-12 connector

How fast is a Cable Modem?

1. Cable modems are up to 10-20Mbps downloads.  

  Typical downloads are over 300Kbps, or close to     600Kbps, but the speed of the cable modem depends on a few things.

• First it depends on how many users are on the system since the cable technology is a "shared" bandwidth.  Too many users using too much throughput can drain this “shared” technology.
• The second factor to cable modem speed is a limit on the cable modem itself.  Some cable providers will limit the upload or download speed on the cable modem, and this could affect your connection speed.
  
• Variable Bit Rate: Bandwidth on Demand

                   Asymmetric Configuration

2.         Downstream Transmission

•         Frequency Range: 50-860 MHz
•         Transmission Speed: up to 36 Mbps  
•         (3~10 Mbps realistic)

3. Upstream Transmission

• Frequency Range: 5-42 MHz
• Transmission Speed: up to 10 Mbps (200 Kbps ~ 2 Mbps realistic)
  
  

 How secure is a Cable Modem?

Cable connections are not 100% secure in any  instance like many other connections on the Internet.

Even though most cable providers block ports 137-139, cable modems are likely to be generated in any case where a user has file and print sharing turned on, or possibly other services like SMTP (Simple mail transfer protocol), Web Servers and Telnet services

A general rule is to keep passwords long and turn off any service that you don't absolutely need running.  A firewall type application should be used to keep a network as secure as possible.

Cable Modem Service

1. 
   On the Internet--Speed Kills

2. Now, low-speed open access by telephone

3. Soon, hi-speed closed access by cable modem

Who makes Cable Modems & what are some Cable Modem Service Providers

There are many cable modem brands, and some of the most common ones are from

• Com21
• Motorola,
• Bay Networks
• RCA,
• Cisco
• Toshiba
• 3Com

There are several popular cable modem providers.       A few of them are:

@Home available at http://www.Home.Com, Mediaone Express available at http://www.MediaoneExpress.com

Road Runner, a service by Time Warner available at http://www.RR.Com.

Modem Types

• Full-duplex modem
  

• 2 way communication
  
• Allows simultaneous transmission in both ways
  
• Needs 4 wires
  

• Half duplex
  

• Can provide 2 way communication but it transmits in only one direction at a time.
  
• It uses only two wires.

Digital Subscriber Lines (DSL) Systems:--

• DSL achieves high-speed bi-directional data   communications over single twisted-pair telephone lines

           There are several varieties of DSL
           Symmetrical - equal upstream and downstream data    
           Rates
         

Modem Selection Criteria:--

     

• Digital or analog signals
  
• Asynchronous or synchronous

• Speed

• Distance
  
• Type of line
  

• Cost
  
• Use compatible modems for both end

Cable Modems Will Have “Gatekeeper”                      Market Power:--
             
Highest Speed

Lowest Cost

Fastest to Market

Telephone Modem:--
                                             Telephone modems employ all three of the modulation techniques:

• FSK
  
• PSK
  
• QAM
  

• Because of the limitations of voice-grade telephone lines.
  
•  These modems are restricted to a bandwidth of about 3 kHz
  
• The trend in modem design has been towards more sophisticated modulation schemes to achieve the maximum bit rate with available bandwidth

FSK Modems:--

                            The first telephone modems   used FSK and this technique is still used in specialized applications such as the transmission of call-display information from the central computer to subscriber telephones

PSK Modems:--

• When faster data rates are needed than available with PSK, phase modulation is often used
  
• Most DPSK (delta phase-shift keying) systems use a four-phase system called quadrature phase-shift keying (QPSK)
  
• The bit rate is twice the baud rate and is referred to as a dibit system
  
• The Bell 212A modem is an example of this type of modulation, capable of data rates up to 1200 bits per second

QAM Modems:--
                           

• Quadrature Amplitude Modulation (QAM) modems   are capable of operating at several different speeds,    depending upon the quality of the connection
  
• Modem speeds using the ITU V.34 standard are capable of 33.6 kb/s, full duplex
  
• V.34 modems monitor line conditions and select the appropriate speeds for a given noise level
  

ADSL:--

•  Asymmetric digital Subscriber   Line:-
                                                         
  ADSL is the current demanding of the phone companies for delivering  advanced digital  Seevices.
  
  It offers the promise of high Speed transmission.

Cable Modems vs. ADSL:--
                                                               

•  There is one major advantage that ADSL has over cable modems.  Cable modems use a shared networking technology where all the cable modems share a single pipe to the Internet
  .
  This pipe speed will fluctuate depending on the number of subscribers on the network.
  
• When ADSL is used, the pipe to the Internet is solely "yours", and is not shared along the way to a central office.  This allows for a more consistent speed, and this speed does not typically fluctuate like cable modem networks.

• Competition for Cable Modems:--
                                                     
  Faster Modems:--
  
• 33.6K modem is about 16% faster than 28.8
  
• 56K modem would be about twice as fast
• Even fast modems are only a fraction of speed of Ethernet
  

• Factor of several hundred
  
• Only in theory!
  
• Ethernet is shared

  Faster Metropolitan Links: ISDN

• An old digital technology
• Provides about 128 kilobits / second
• Costs about $30 to $70 per month
• Rapidly being overshadowed by newer schemes
  

Faster Metropolitan Links: ADSL:--

• Moving 1.5 to 6.1 megabits/second
  
• Over existing copper phone wires
  
• Requires improvements to parts of phone network
  
• Asymmetric: megabits to user, only kilobits from user
  

• URLs are small
• Web pages are big
• Don’t install a Web server on your ADSL link

Modem Selection criteria:--

• Digital and Analog Signal
  
• Speed
  
• Distance
  
• Type of Line
  
  Target Customer:--
  
  Cable MSO (Multiple System Operator), SO (System Operator)
  

     ISP (Internet Service Provider)

    CSP (Content Service Provider)

     Importer/ Exporter

            EXTERNAL MODEM

           INTERNAL MODEM

   Quadrature Amplitude Modulation:--

• The only way to achieve high data rates with a narrowband channel is to increase the number of bits/symbol
  
• The most reliable way to do this is with a combination of amplitude and phase modulation called quadrature amplitude modulation (QAM)
  
• A constellation diagram shows the possibilities for a hypothetical system with 16 amplitude/phase combinations

56K Modems:--

•  The V.90 standards allow data transmissions up to a theoretical limit of 56 kb/s, but because of FCC requirements, the maximum allowable is 54 kb/s
  
• V.90 modems appear to exceed the Shannon Limit
• However, the higher rate is available only in the downstream direction and a maximum of 33.6 kb/s is available in the upstream direction
  
• Upper limits vary greatly according to line noise, distance from a telephone substation.
  
  Error Correction and Data Compression with Modems:--
  
•  High-speed modems incorporate error correction and data compression
  
• Error correction is important at higher speeds because states in the constellation pattern are closer together
  
• Error-correction schemes employed are:

• MNP 2, 3, 4, and 10
  
• CCITT V.42
  
• Link Access Procedure for Modems (LAPM)
  
• All of these schemes use CRC to check each packet
  

•  V.42 bis and MNP5 are the two most popular   compression methods used in modems today

   Modem-to-Computer Connections:--

• The UART is the interface between the parallel and serial transmission of data
  
• There are a number of standards for communication between the computer (DTE) and the modem (DCE), but the most popular is the RS-232C (EIA 232D)
  
• This standards defines the voltages and pin numbers for both data and control lines on a serial port

         
  Fax Modems:--

• Whether a fax machine is stand-alone or part of a microcomputer, the modem used is specialized for facsimile functions
  
• There are four CCITT standards (groups) for fax transmission
  
• Groups one and two are now obsolete
• Group three, the most common, uses digital coding of the document followed by analog transmission using QAM at up to 14.4 kb/s

Cable Modems and Digital Subscriber Lines:--

• Because of the high bandwidth available on CATV systems, they’re ideally suited for data transmission application
  
• In a CATV system, all signals go everywhere via a tree network from a single head end
• 
  Modem Terminology:--
  

•  Data communication equipment (DCE)
  

• A device like a modem that is responsible for managing communication between devices

• Data terminal equipment (DTE)
  

• The computer or terminal that is communicating with another device

• The computer or terminal that is communicating with another device

Cable Modem Standards:--

IEEE 802.14 (in May 1994)

MCNS/DOCSIS (USA mainly, in January 1996)

       Multimedia Cable Network System    (MCNS)

    Data Over Cable Service Interface Specification

DVB/DAVIC (Europe mainly)

 European Digital Video Broadcast (DVB)/   Digital Audio Visual Council (DAVIC), EuroDOCSIS

  Install the Modem and Its Drivers:--

• External modems

• Communicate with the CPU by way of a serial cable

• Internal modems

• Communicates with the CPU using either the PCI or ISA bus on the system board

• Working Verifying that Your Modem Is :--
  
• Can use HyperTerminal, a simple communication utility that makes a phone call with your modem
  
• Can communicate directly with your modem using same set of commands that software uses

                    MODEM

Integrated Services Digital Network (ISDN) and DSL

• Integrated Services Digital Network (ISDN) and DSL:--
  
• ISDN requires two devices:

• NT1 (Network Terminator 1)
• TA (Terminal Adapter)
  

• DSL connection requires:
  

• DSL converter box that connects to the PC by way of a network cable and card

Using a Cable Modem, ISDN, DSL, or Satellite to Connect to the Internet:--

• Regardless of how the connection is made, TCP/IP manages the connection to the ISP in the same way
  
• Cable modem, DSL, and a LAN all use a network card in the PC for the physical connection

• Connecting a PC on a LAN to the Internet:--

There are threee approach:-
            protocol Isolation
           Router security

   Deployment of Cable Modems:--

• Many companies

• @Home service in US (TCI, Comcast, etc)
• Media One, Time Warner in several US locations
• Cox Cable in Phoenix
• Shaw cable in Alberta
• Helsinki TV in Finland
• Marubeni cable in Tokyo
• Television Internacional in Monterey, Mexico
• Many others around the developed world

• Numerous trials beginning in 1995
• Hype exceeded deployment rate in 1996, early 1997

Networks in the Home :-

•  Because more homes now have multiple computers, home networks are gaining in popularity.  Home networks offer the same         advantages to home users as to a business.

• Home networks are typically based on existing telephone or wireless technologies.

• Popular PC operating systems, such as Windows and the Mac OS, provide simple   

Data Communications over Standard

• Telephone Lines :--
  Networks commonly use dedicated media to transmit data. However, the public telephone system (plain old telephone system :POTS) can also be used for data communications.

• Standard phone lines transmit data much more slowly than network media, but devices such as modems make phone lines practical for data transmission over long distances.

• Many people and businesses use modems to         exchange data, and to establish connections with office networks.

   Using Digital Telephone Lines -

• DSL Technologies:--
  
  Digital Subscriber Line (DSL) service is outpacing ISDN services.

• Standard DSL offers speeds of 52 Mbps using standard phone lines.

• Several types of DSL service are available, reaching transmission speeds up to 51.84 Mbps.

Data Communications over Standard

Telephone Lines - Modems :--

• Most telephone lines attached to home and businesses are analog, not digital.

• Because PCs transmit and receive data in digital format, a device called a modem is needed to convert digital data to analog format for transmission over phone lines.

• When receiving data from another computer, the modem converts it from analog format to digital format.
  

• 
  Data Communications over Standard   Telephone Lines - Choosing a Modem :--
  
• fits inside Transmission speed - the speed at which the modem sends data - which is measured in bits per second (bps) .

• Data compression, the technology the modem uses to shrink data so it can be transmitted         faster.

• Error correction, the method the modem         uses to ensure data is sent and received without errors.

• Internal versus external, which describes whether or not the modem the PC case.

• Data Communications over Standard Telephone Lines - Uses for a Modem :--
  
  Modems are primarily used for file transfer, or sending files to a remote computer
  
• Sending a file to another computer is called uploading.

• Receiving a file from another computer is called downloading.

• -  full-duplex  , half-duplex
• Marketing Advantage
  (Why choose D-Link)
  
• Digital Home Completeness
  
• World-wide sales service
  
• Real-world experienced support team
  
• Excellent R&D, manufacture, quality control team

        -  

       Relative Speed Comparisons


Pricing:--

•  Typically starts at ~ $30 / month
  

• Probably slow speed
• 1 IP address
• No Web server allowed
  

• Perhaps $70 per month for 1 IP address and 10 megabit service
  
• Perhaps $300 to $1000 per month for multiple IP
  addresses serving a business
  
• Some systems charge fees  if you exceed monthly bandwidth quota
  

• Market Scale:--
  
• Million, Worldwide

• 2000 : Over 6 Million, Dataquest
• 2004 : 16.445 In-state

• North American

• 2000/03 : 2.7 Million (U.S : 2 Million,Canada : 0.7 Million)
• 2003 : 15.9 Million