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  • At the center of each cell is a base station to which all the telephones in the cell transmit. The base station consists of a computer and transmitter/receiver connected to an antenna. In a small system, all the base stations are connected to a single device called an MTSO (Mobile Telephone Switching Office) or MSC (Mobile Switching Center). In a large one, several MTSOs may be needed, all of which are connected to a second-level MTSO, and so on. The MTSOs are essentially end offices in the telephone system, and are, in fact, connected to at least one telephone system end office. The MTSOs communicate with the base stations, each other, and the PSTN using packet-switching network.

Chapter 2 Chapter 2 Presentation Transcript

  • CSCI-370: Introduction to Computer Networks Chapter 2: The Physical Layer By Dr. Tao Zhang School of Engineering and Computing Sciences
  • Outline
    • Guided transmission media
    • Wireless transmission
    • Communication satellites
    • The public switched telephone network
    • The Mobile telephone system
    • Cable television
  • Guided Transmission Data
    • Magnetic Media
    • Twisted Pair
    • Coaxial Cable
    • Fiber Optics
  • Performance Characteristics
    • Bandwidth
    • Delay
    • Cost
    • Ease of install and maintenance
  • Magnetic Media
    • Tape, DVD
    • Bandwidth can be huge at a low cost and reasonable delay
      • Assume that express delivery is used
    • This is the reason why the video on demand (VoD) services is not practical yet
  • Twisted Pair
    • Category 5 UTP
    • More twists per centimeter
    UTP: Unshielded Twisted Pair Category 3 UTP
  • Twisted Pair
    • UTP: Unshielded Twisted Pair
    • Usage
      • Telephone line
      • Ethernet
    • Bandwidth
      • UTP3: 16MHz
      • UTP5: 100MHz
      • UTP6: 250MHz
      • UTP7: 600MHz
  • Coaxial Cable
    • A coaxial cable
  • Coaxial Cable
    • Usage
      • Telephone network: before fiber optics
      • Cable TV
    • Bandwidth
      • 1GHz
  • Fiber Optics
    • Usage
      • From LAN to WAN
    • Bandwidth
      • Achievable: 50Tbps
      • Practical: > 2Tb/s
  • Fiber Optics
    • (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.
    • (b) Light trapped by total internal reflection.
  • Transmission of Light through Fiber
    • Attenuation of light through fiber in the infrared region.
  • Fiber Cables
    • (a) Side view of a single fiber.
    • (b) End view of a sheath with three fibers.
  • Fiber Cables (2)
    • A comparison of semiconductor diodes and LEDs as light sources.
  • Fiber Optic Networks
    • A fiber optic ring with active repeaters.
  • Fiber Optic Networks (2)
    • A passive star connection in a fiber optics network.
  • Wireless Transmission
    • The Electromagnetic Spectrum
    • Radio Transmission
    • Microwave Transmission
    • Infrared and Millimeter Waves
    • Lightwave Transmission
  • History
    • The movement of electrons can create a electromagnetic wave
    • The electromagnetic wave was first predicted by James Clerk Maxwell in 1865
    • Henrich Hertz first observed the EM wave in 1887
  • Definition
    • Frequency: the number of oscillations per second of a wave
      • Denoted as f and the unit of frequency is Hz (Hertz)
    • Wavelength: The distance between two consecutive maxima (minima)
      • Denoted as 
  • EM Wave
    • The EM wave can be transmitted and received by antenna
      • Omni-directional
      • Directional
      • Smart
    • EM wave travels in vacuum with the speed of light
      • c=3x10^8 m/sec
      • Note: the speed of EM wave in copper and optical fiber is about 2x10^8 m/sec
    • Relationship
      • f x  =c (in vacuum)
  • The Electromagnetic Spectrum
    • The electromagnetic spectrum and its uses for communication.
  • The Electromagnetic Spectrum
    • Band name (defined by ITU)
      • VLF: very low frequency
      • LF: low frequency
      • MF: medium frequency
      • HF: high frequency
      • VHF: very high frequency
      • UHF: ultra high frequency
      • SHF: super high frequency
      • EHF: extreme high frequency
      • THF: tremendous high frequency
  • BAND
    • f x  =c (in vacuum)
      • df/d  = -c/  ^2
      •  f =  x c /  ^2
        •  f: corresponding to bandwidth
        • At high frequency band, it is possible to encode multiple bits per hertz
    • Narrow band:  f/f <<1
    • Wide band:
      • Spread spectrum (CDMA system)
        • Frequency hopping
        • Direct sequence spread spectrum
    • Ultra-wide band (UWB):  f/f > 20% or  f>500Mhz
      • According to FCC (Federal communications commission)
      • FCC allow un-licensed band 3.1GHz~10.6GHz
      • 10dB bandwidth
  • Radio Transmission
    • (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
    • (b) In the HF band, they bounce off the ionosphere.
  • Microwave Transmission
    • If the frequency of a wave is larger than 100MHz, then the transmission is nearly straight lines
      • Energy can be concentrated into a small beam
      • Wave can hardly pass through building
    • Usage
      • Long distance telephone communications
      • Mobile phone
      • Television
  • Infrared and Millimeter Waves
    • Transmission: directional
    • Cost: low
    • Cannot pass object
      • Pros: less interference, security
      • Cons: limited coverage
    • Usage:
      • Remote controller
  • Lightwave Transmission
    • Convection currents can interfere with laser communication systems
    • A bidirectional system with two lasers is illustrated
  • Lightwave Transmission Convection currents can interfere with laser communication systems
  • Politics of the Electromagnetic Spectrum
    • International: ITU-R
    • US: FCC
    • The ISM bands in the United States
      • ISM: Industrial, Scientific, Medical
    802.11b Bluetooth 802.11a
  • Communication Satellites
    • Geostationary Satellites
    • Medium-Earth Orbit Satellites
    • Low-Earth Orbit Satellites
    • Satellites versus Fiber
  • Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.
  • Geostationary Satellite
    • GEO: Geostationary-earth-orbit
    • Altitude: 35,800km
    • Motion of the satellite: none
    • Obit slot assignment
      • ITU
    • Usage
      • Telecomm
      • Television
      • Military
  • Frequency Band for Satellites
    • The principal satellite bands.
  • VSAT: Very Small Aperture Terminals
    • VSATs using a hub.
  • Medium-Earth Orbit Satellites
    • MEO: Medium–earth orbit
    • Motion of the satellite: slow (in longitude)
      • Example: 6 hours a circuit
    • Usage
      • No used in telecommunication
  • Low-Earth Orbit Satellites
    • LEO: Low–earth orbit
    • Motion of the satellite: fast (in longitude)
    • Cons
      • Need more satellite to provide coverage
    • Pros
      • Low power
      • Short delay
    • Usage
      • Iridium
      • Globalstar
  • Iridium
    • Number of satellites: 66
    • Altitude: 750km
    • Orbits: 6
    • Each satellite can cover up to 48 cells
      • Total cells: 1628
    • Each satellite has a capacity of 3840 channels
      • Total channels: 253,440
    • Calls can be switched from one satellite to another
  • Iridium
    • (a) The Iridium satellites from six necklaces around the earth.
    • (b) 1628 moving cells cover the earth.
  • Globalstar
    • Number of satellites: 48
    • Connections cannot be switched between two satellites
  • Globalstar
    • (a) Relaying in space.
    • (b) Relaying on the ground.
  • Public Switched Telephone System
    • Structure of the Telephone System
    • The Politics of Telephones
    • The Local Loop: Modems, ADSL and Wireless
    • Trunks and Multiplexing
    • Switching
  • Structure of the Telephone System
    • (a) Fully-interconnected network.
    • (b) Centralized switch.
    • (c) Two-level hierarchy.
    The total hierarchy is 5 in US
  • Structure of the Telephone System (2)
    • A typical circuit route for a medium-distance call.
    Twisted pair
  • The Politics of Telephones The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it
  • The Politics of Telephones
    • In 1984, the US government forced AT&T to break into AT&T long line and 23 BOCs (Bell Operating Company)
      • BOCs were then grouped in to 7 RBOCs (regional BOCs)
    • Deregulation: 1996
      • Local telephone companies, long-distance companies, mobile operators and cable TV companies can enter one another;s business
  • The Politics of Telephones
    • Terms
      • LATA: local access and transport areas (164 in US)
      • LEC: local exchange carrier
        • Example: BOCs
        • ILEC: Incumbent LEC
        • CLEC: Competitive LEC
      • IXC: inter-exchange carrier
        • AT&T long line, MCI, Sprint, …
  • Major Components of the Telephone System
    • Local loops
      • Analog twisted pairs going to houses and businesses
    • Trunks
      • Digital fiber optics connecting the switching offices
    • Switching offices
      • Where calls are moved from one trunk to another
  • The Local Loop: Modems, ADSL, and Wireless
    • The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and CODECs.
    Last Mile D D A A D D
  • Digital Modulation A binary signal Amplitude modulation Frequency modulation Phase modulation Frequency shift keying
  • Digital Modulation
    • Baud: the number of samples per second
      • Symbol rate
        • Note: one symbol can be used to represent multiple bits
      • Example: suppose baud =2,400
        • One bit (0 or 1) / symbol => bit rate=2,400 bps
        • Two bits (0, 1, 2, 3) / symbol => bit rate=4,800 bps
          • QPSK: Quadrature phase shift keying
    • Digital modulation: mapping digital information to analog symbol
  • Digital Modulation
    • Suppose we have two orthogonal signals f(t) and g(t)
      • f(t)=0, t<0 or t>T
      • g(t)=0, t< 0 or t>T
      • int f 2 (t) dt = 1
      • int g 2 (t) dt = 1
      • int f(t)xg(t) dt = 0
    • Suppose we need to transmit two symbols a and b
      • We can transmit af(t)+bg(t)=s(t)
      • At the receiver side
        • int s(t) f(t) dt = a
        • int s(t) g(t) dt = b
    t f(t) t g(t) T T 1/T 1/T
  • Constellation QPSK QAM-16 QAM-64 QAM: Quadrature amplitude modulation
  • Constellation (a) (b) V.32 for 9600 bps V.32 bis for 14,400 bps TCP: Trellis code modulation (introduce more bits for error correction) 5 bits (4 information and 1 parity) 2^5=32b, symbol rate=2,400/sec 2^6=128
  • Constellation
    • V.32
      • 4 bits/symbol, 9,600bps
    • V.32 bis
      • 6 bits/symbol, 14,400bps
    • V.34
      • 12 bits/symbol, 28,800bps
    • V.34 bis
      • 14 bits/symbol, 33,600bps
  • Modem
    • Date rate can be dynamically adjust
    • Many modem will compress data
      • Source coding
    • Direction
      • Both directions, simultaneously: full duplex
      • Both directions, one at a time: half duplex
      • One direction: simplex
  • Speed Limitation
    • Two loops: 35kbps (Shannon capacity)
    • One loop: 70kbps (Shannon capacity)
      • 56 kbps (4kHz sampling for voice signal, 7 bits available to users I US)
      • V.90: 33.6kbps upstream, 56kbps downstream
        • Upstream is the direction from user to ISP
      • V.92: 48kbps up, 56kbps down
  • Digital Subscriber Lines (DSL)
    • Bandwidth versus distanced over category 3 UTP for DSL.
  • Digital Subscriber Lines (2)
    • Operation of ADSL using discrete multitone modulation (DMT)
      • The modulation scheme is also known as OFDM (orthogonal FDM)
  • Digital Subscriber Lines (3)
    • A typical ADSL equipment configuration.
    Network interface device
  • Wireless Local Loops
    • Architecture of an LMDS system
    • Standardization: IEEE 802.16 WMAN
  • Wireless Local Loops
    • Difference between WLL and mobile phone
      • WLL does not provide mobility
        • Fixed wireless
      • WLL can provide higher data rate
      • WLL may utilize large antenna
    • MMDS: Multi-channel multipoint distributed service
      • Frequency band (by FCC)
        • 2.1GHz and 2.5GHz, total bandwidth 198MHz
    • LMDS: local multipoint distributed service
      • Frequency band (by FCC):
        • 28-31GHz, total bandwidth 1.3GHz
  • Multiplexing
    • Multiplex
      • To merge multiple low data rate links into one high data rate link
    • Technique
      • FDM
      • WDM
      • TDM
  • Frequency Division Multiplexing
    • (a) The original bandwidths.
    • (b) The bandwidths raised in frequency.
    • (c) The multiplexed channel.
  • Wavelength Division Multiplexing
    • Wavelength division multiplexing.
  • Time Division Multiplexing
    • The T1 carrier (1.544 Mbps).
  • Time Division Multiplexing
    • Multiplexing T1 streams into higher carriers.
  • SDH/SONET
    • Two back-to-back SONET frames.
  • Time Division Multiplexing (5)
    • SONET and SDH multiplex rates.
  • Switching
    • Circuit switching
    • Message switching
      • Store-and-forward
    • Packet switching
  • Circuit Switching
    • (a) Circuit switching.
    • (b) Packet switching.
  • Circuit Switching
    • A path must be setup before the data transmission
    • To establish this path, signaling messages must be transmitted in the network
    • There is a connection setup time (delay)
    • After the connection is established, the delay for data is the propagation delay
  • Message Switching Circuit switching Message Switching Packet Switching
  • Packet Switching
    • A comparison of circuit switched and packet-switched networks.
  • The Mobile Telephone System
    • First-Generation Mobile Phones:
      • Analog Voice
    • Second-Generation Mobile Phones:
      • Digital Voice
    • Third-Generation Mobile Phones:
      • Digital Voice and Data
  • Advanced Mobile Phone System (AMPS)
    • (a) Frequencies are not reused in adjacent cells.
    • (b) To add more users, smaller cells can be used.
  • Channel Categories in AMPS
    • Total number of duplex channels: 832
      • 832 channels: 824-849MHz (FDM 30KHz channel)
      • 832 channels: 869-894MHz
    • Four categories:
      • Control (base to mobile) to manage the system
      • Paging (base to mobile) to alert users to calls for them
      • Access (bidirectional) for call setup and channel assignment
      • Data (bidirectional) for voice, fax, or data
  • Second-Generation Mobile Phones
    • D-AMPS
      • Digital Advanced Mobile Phone System
    • GSM
      • Global System for Mobile Communications
    • CDMA
    • PDC
  • D-AMPS
    • Standard: IS-45, IS-136
    • Usage: USA, Japan
    • Band (in addition to 850MHz band)
      • Upstream: 1850-1910MHz
      • Downstream: 1930-1990MHz
    • Channel: 30kHz
      • Compatible to AMPS
    • Slots in a frame: 6
    • Digital voice
      • Compression: vocoder
  • D-AMPS
    • (a) A D-AMPS channel with three users.
    • (b) A D-AMPS channel with six users.
  • GSM
    • Standard
      • ETSI: European Telecommunications Standards Institute
        • An European standard organization in the telecommunication world
    • Usage: world wide (most popular)
    • Channel: 200kHz
    • Slots in a frame: 8
  • GSM
    • GSM uses 124 frequency channels, each of which uses an eight-slot TDM system
  • GSM Frame Structure
    • A portion of the GSM framing structure.
  • CDMA
    • Standard: IS-95
      • Qualcomm
    • Usage: USA
  • CDMA
    • (a) Binary chip sequences for four stations
    • (b) Bipolar chip sequences
    • (c) Six examples of transmissions
    • (d) Recovery of station C’s signal
  • Third-Generation Mobile Phones: Digital Voice and Data
    • Basic services an IMT-2000 network should provide
      • High-quality voice transmission
      • Messaging (replace e-mail, fax, SMS, chat, etc.)
      • Multimedia (music, videos, films, TV, etc.)
      • Internet access (web surfing, w/multimedia.)
    • Options
      • W-CDMA: USA
        • Wideband CDMA
      • UMTS: European Union
        • Universal Mobile Telecommunication Systems
      • TD-SCDMA: China
  • 3G Mobile Phone Systems
    • Not fully deployed
    • Problems
      • 2.5G can provide data communication
        • GPRS: General Packet Radio Service
          • An overlay model
      • WLANs are widely deployed
      • 3G can provide at most 2Mbps
        • Relatively low
    • Future: NOT CLEAR
      • 4G
  • Cable Television
    • Community Antenna Television
    • Internet over Cable
    • Spectrum Allocation
    • Cable Modems
    • ADSL versus Cable
  • Community Antenna Television
    • An early cable television system.
  • Internet over Cable
    • Cable television
  • Internet over Cable (2)
    • The fixed telephone system.
  • Spectrum Allocation
    • Frequency allocation in a typical cable TV system used for Internet access
  • Cable Modems
    • Typical details of the upstream and downstream channels in North America.