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					Physical Layer
Modems
   Problem
       Computer is digital
       Telephone line is analog
       Need translation device called a modem
                                  Analog
                Digital           Signal
                Signal
                          Modem


                                                 2
     Modulation
   A Modem is a Modulator and Demodulator
       Modulation is converting outgoing digital
        device signals into analog transmission line
        signals
       Demodulation is converting incoming analog
        transmission line signals into digital device
        signals      Digital                Analog
                      Signal   Modem       Signal




                                                        3
Modulation
   Example: Frequency Modulation
       1 is a high-frequency vibration
       0 is a low-frequency vibration

                          0               1


                   1             1

                                              4
Modem Forms
   Internal Modem
       On printed circuit board inside PC
       Does not take up desktop space
       Can be complex to install--must open up
        computer




                                                  5
Modem Forms
   External Modem
       Easy to install (just plug into serial port)
       Takes up desk space
       Needs wire to serial port (adds to wiring
        clutter)
       Needs electrical power



                                                       6
Modem Forms
   PC Card Modems
       Fit in PC Card slots on notebooks
       Easy to install
       Do not take up desktop space
       Expensive
       Few desktop PCs have PC Card slots



                                             7
Modem Standards
   Two Modems Must Follow Same Standards
       Speed standards (modulation method)
       Error correction and compression standards
       Facsimile standards
       Modern modem standards are created by the ITU-T
   Training Period
       When two modems first start talking, they negotiate
        standards to use; settle on highest common standards




                                                               8
    Modem Speed Standards
    (ITU-T)
   V.90
       Receive at 56 kbps but send at only 33.6 kbps
       Not all phone lines, ISPs support 56 kbps
       Fall back to 33.6 kbps if cannot support

   V.34
       33.6 kbps send and receive
   V.32 bis
       14.4 kbps send and receive
       bis means second (version of the standard)

                                                        9
    Modem Standards: Error
    Correction and Compression
   ITU-T
       V.42: Error detection and correction
       V.42 bis: Compression (up to 4:1)
       Most newer modems have both

   MNP Level 5
       Proprietary standard of Microcom
       Once widely used
       Provides both error correction and compression
       Now largely superceded by V.42 and V.42 bis
                                                         10
Modem Standards
   Facsimile Modem Standards
       Most modems also act as fax modems

       Facsimile requires different modulation
        than data transmission

       V.14: 14.4 kbps

       V.29: 9.6 kbps

                                                  11
V.90 Modems
   Telephone Bandwidth is Limited
       Bandwidth limits speed
       Limits speed to about 35 kbps
       When you transmit, limited to 35 kbps
       Limits occur when you transmit in analog to the phone
        company
            Analog-to-digital converter (ADC) at the telephone
             company limits you to about 35 kbps

    V.34                         ADC Telephone
PC                                    Network
   modem
      33.6 kbps                                                   12
V.90 Modems
    Receive at 56 kbps
        ISP connects with digital line to the Telco
        Sends at 56 kbps
        No analog-to-digital converter to limit
         speed
                                        Digital
         56 kbps            Telephone    Link
PC                   DAC                  No    ISP
         modem               Network
                                        ADC!

                                        56 kbps       13
V.90 Modems
   Receive at 56 kbps
       56 kbps digital channels inside phone network
       Only local loop is analog today
       Trunk lines and switches support 56 kbps
        transmission
       Telephone company transmits in analog to
        subscriber at 56 kbps
       Digital to audio converter (DAC) does not limit
        speed to less than 56 kbps


                                                      14
    V.90 Modem
   Telephone Company Does Not
    Have to Do Anything Differently
       ADCs and DACs are already in place for
        ordinary voice service
       Lack of change in phone system allowed
        fast implementation
                     ADC                Digital
         56 kbps            Telephone    Link
PC                                                ISP
         modem               Network
                     DAC
                                                        15
    V.90 Modem
   ISP Does Have to Do Things
    Differently
        Digital line to telephone network
        Special equipment at ISP
        Does not use a V.90 modem
        If used V.90 modem, could only send at
         33.6 kbps
                                          Digital
                                           Link
          56 kbps      DAC    Telephone             ISP
PC
          modem                Network       No
                                            V.90
                                           Modem!     16
    Alternatives to Ordinary Phone
    Lines & Modems
   Faster Speeds; Called Broadband Services

   Also Higher Cost
       Translation devices more expensive than modems
       Faster (more expensive) transmission line to ISP
       ISP may charge more because of heavier
        transmission load
       Faster interface than 232 serial port may be needed



                                                           17
Transmission Line Versus ISP
   Transmission line may be provided by a
    transmission carrier, not the ISP

   Users have to pay separate fees to carrier
    and ISP

   Some ISPs also provide transmission line
       But not all do
       Transmission line costs must always be considered

        PC                               ISP
                   Transmission   Line
                                                     18
ISDN
    Integrated Services Digital Network
    Codecs
    DSUs
    ISDN
   Get ISDN line from telephone company
       Multiplexes (mixes) three channels on one UTP wire pair to
        the desktop
   Two B Channels are 64 kbps
       Original idea: one for voice, one for data
                   B                     D Channel: 16 kbps
                 Channel                   control signals

            1101                         2B+D
                              ISDN     Multiplexed
                              Modem     Onto One
                      B                Set of Wires
                    Channel
                                                                     20
    ISDN Costs
   Must install an ISDN line
       Will cost more than telephone line

   Actually, could use your existing phone line
       But then would need one B channel for telephony
   Line is expensive
       $60-$80/mo plus installation fee
       ISP charge is separate; may charge more for ISDN access

   Dial Up: Not always connected
       Do not have to pay for full-time use

   Need “ISDN modem” (expensive)
                                                                  21
    ISDN Modem
   ISDN “Modem” is a Misnomer
       Modem is for for digital device, analog line
       ISDN line is digital

   ISDN modem really contains
       Codec to link analog telephone to digital ISDN line
       Data service unit (DSU) to translate between PC
        digital format and ISDN digital format (voltage
        levels, timing, etc.)


                                                          22
    ISDN Modem
   Codec
       Translates analog device signal into digital signal for
        propagation
       Constantly samples the intensity of the voice analog signal
       Divides each second into 8,000 sampling periods
       Only measures intensity of voice signal in each

                              Sample


         Intensity
           Value
         0110010


                     1/8,000 sec Sampling Period                      23
    ISDN Modem
   Codec
       Measures voice intensity as an 8-bit
        intensity value (0-255)
       Overall, sends 8 bits 8,000 times per second
        (64 kbps)
       This is why telephone channels are 64 kbps
            Designed for digitized voice
            Often steal 8 kbps for supervisory signaling, so
             56 kbps



                                                                24
DSU (Data Service Unit)
   DSU translates between different digital
    formats
       Device and line are both digital, but still must have
        translator
            Different   bit rates
            Different   number of possible states
            Different   voltage levels for the states
            Different   ways to represent ones and zeros

                                                      Digital Line
                              DSU


                                                                     25
    Cable Modem Service
   Service of Cable Television Companies
       Deliver 10 Mbps downstream to the home
       Capacity is shared by multiple subscribers, so real
        speed is more limited
       Limited to about 64 kbps to 256 kbps upstream
       Does not tie up telephone line
       Always available

                    Cable           Cable TV    Also ISP
                    Modem           Network     Functions

                                                              26
    Cable Modem Sharing in
    Perspective
   Sharing is Not as Extreme as it May First
    Appear
       Usually, shared within Blocks of 500 Houses
       Only Some Households in Block will Subscribe
       Only Some Subscribers will be Online at Any
        Moment
       Only Some of Online Subscribers will Send and
        Receive at Any Moment
       Only these will Share the Capacity
   So Sharing Does Not Reduce Speed Unless
    the Adoption Rate is Large
                                                        27
    Cable Modem Service
   Cost is about $50 per Month
       Includes ISP service!
       Installation usually costs $100 to $150 and
        includes a cable modem and a network interface
        card
       Best alternative today beyond V.90 modem
       Cost-competitive with adding a second phone line
        to handle your modem communication


                                                           28
    Digital Subscriber Lines (DSLs)
   Offered by Telephone Companies
       Lines to customer premises are subscriber lines,
        which connect subscribers to the telephone system
       These are digital, hence “digital subscriber line”
       Several types of digital lines for subscribers
            Some are for homes, others for businesses



              DSL       DSL
                                   Telephone         ISP
             Modem                  Network
                                                           29
    Digital Subscriber Lines
   For residential customers, usually multiplexes regular
    phone, high-speed data
       Can use existing phone line coming into house
       Can use Internet without tying up phone
   Most common for home is Asymmetric DSL (ADSL)
       Upstream and downstream speeds are different
       Upstream at 64 kbps or more
       Downstream at 256 kbps to a few Mbps
       Asymmetric speed is good for WWW
       About $50 per month incl. ISP; more for faster service
                        ADSL
     DSL           64   or more   kbps
                                           Telephone
                                                                 ISP
    Modem                                   Network
                     256 kbps-a few Mbps
                                                                       30
Digital Subscriber Lines
   G.Lite (G.992.2) Standard
        New ADSL standard from ITU-T
        Up to 1.5 Mbps downstream speed
        No carrier installation is necessary
     Not being widely adopted by ADSL vendors
      yet
     DSL      ADSL        Telephone
                                                ISP
    Modem 64 or more kbps      Network

              Up to 1.5 Mbps
                                                      31
Digital Subscriber Lines
   Digital Subscriber Line Access
    Multiplexer
    Telco must install a DSLAM at end
    


DSL
    switching office

                      Telephone
    DSL         DSLAM                   ISP
                       Network

     DSL

                                              32
   Digital Subscriber Lines
      Splitting Voice and Data
          Voice and data are split at home by the
           DSL modem
          Voice and data are also split at the
           telephone company’s first switching office
Computer
                                              DSLAM
  Phone         DSL      DSL     DSL
               Modem            Splitter       Voice
                                              Network
                                                     33
    DSLs Versus Cable Modem
    Service
   Both Expected to Sell Well
   Forecast for Worldwide Installations in 2004
       Cable modem lines: 9 million
       DSLs: 10 million
       Source: Insight Research
   Cable is Available Mostly in Residential Areas
   Cable is only widespread in some countries,
    especially the United States


                                                     34
    Broadband Access Lines in
    Perspective
   Limited to Transmission Between
    Customer Premises and the ISP
       ISP Speed can Limit Service Speed
       Internet Backbone Speed can Limit Service Speed
       Server Host Access Line Speed can Limit Service
        Speed
       Server Host Processing Speed can Limit Service
        Speed
   Broadband Access Lines are Not
    Panaceas
                                                          35
Wireless Internet Access
   Reach the Internet by Radio
       Terrestrial (uses earth stations)
       Satellite-based
    ISP




                                            36
     GEO Satellites
   In geosynchronous orbit
       Appear to be stationary in the sky
   Far from the ground
       36,000 km (22,300 miles)
       Need much power to send/receive
       Need dish antennas to concentrate signals
       Must point dish at the satellite
       Impractical for portable computers


                                                    37
    VSATs
   Very Small Aperture Terminals (VSATs)
       Small dishes (1 meter in diameter or less)
       Do not concentrate signal as well as large dishes,
        so inefficient
       Do not focus as precisely as large dishes, so
        satellites cannot be spaced closely together
       However, inexpensive
       Used when there must be many ground stations



                                                             38
                                         Omnidirectional
                                         Antenna

    LEO Satellites
   Low Earth Orbit (LEO) Satellites
    –   Below First Van Allen Radiation band, which peaks
        at 3,000 km (1,800 miles)
    –   A typical orbit is 1,000 km or 600 miles
    –   Need far less power to reach than 36,000 km
        (22,300 mile) geosynchronous satellites
    –   So can get by with omnidirectional antenna
    –   Can use phone of reasonable size, cost
    –   Access anywhere is the world, although local laws
        may limit legality
                                                            39
    MEO Satellites
   Medium Earth Orbit (MEO) Satellites
       Between First and Second Van Allen Radiation
        Band, which peaks at 15,000 km (about 9,000
        miles)
       Typical distance is 6,000 km or 4,000 miles
       Farther away than LEOs, so need more power
       But travel more slowly across sky, so fewer hand-
        offs, fewer satellites needed



                                                            40
    Ports
   Need port fast enough for line
   232 Serial port to 115.2 kbps: only
    V.34, V.90 or ISDN
   USB (universal serial bus)
       12 Mbps
       Available on all new PCs
       Fast enough for DSLs, cable modems
       Faster version coming (USB-2, ~480 Mbps)
                                               41
    Ports
   Firewire (IEEE 1394)
       400 Mbps and faster
       Not available on most new PCs
       Fast enough for DSLs, cable modems

   Ethernet NIC (10 Mbps)
       Network interface card used in PC networks
       Printed circuit board
       Must be installed inside PC systems unit
       Fast enough for DSLs, cable modems

                                                     42

				
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