SUPPLEMENTARY CHAPTER 3: Communication Channel Technology The Architecture of Computer Hardware and Systems Software: An Information Technology Approach 3rd Edition, Irv Englander John Wiley and Sons  2003 Communication Channel Supplementary Chapter Communication Channel S3-2 Communication Channels: Many Ways to Implement  Signal: specific data transmitted  Diagram shows communication between computer and a wireless laptop  Deceptively simple: phone line carries electrical representation of audio signal  Physically: signal passes through different channel forms including audio, digital, light, radio  Converters between separate physical channels Supplementary Chapter Communication Channel S3-3 Communication Channel  Characterized by  Signaling transmission method  Bandwidth: amount of data transmitted in a fixed amount of time  Direction(s) in which signal can flow  Noise, attenuation, and distortion characteristics  Medium used Supplementary Chapter Communication Channel S3-4 Signaling Transmission Method  Analog: continuous varying waveforms to carry data  Digital:  Two different values of electrical voltage or current or  On/off light source  Frequently preferred because less susceptible to noise and interference Supplementary Chapter Communication Channel S3-5 Channel Organization  Point to point channels  Simplex: channel passes data in one direction only  Half-duplex: transmits data one direction at a time (walkie-talkie)  Full-duplex: transmits data in both directions simultaneously (telephone)  Multipoint: broadcasts messages to all connected receivers Supplementary Chapter Communication Channel S3-6 Multiplexing  Carrying multiple messages over a channel simultaneously  TDM (time division multiplexing) Example: packet switching on the Internet  Use: digital channels   FDM (frequency division multiplexing) Example: Cable TV  Analog channels   Filters separate different data signals at receiving end Supplementary Chapter Communication Channel S3-7 Signaling Technology  Carrier waves  Electrical voltage  Electromagnetic radio wave  Switched light  Data represented by changes in the signal as a function of time  Range of values  Analog: continuous values  Discrete: countable number of possible values  Digital: binary discrete signal Supplementary Chapter Communication Channel S3-8 Waveform  Representation of a signal shown as a function of time Supplementary Chapter Communication Channel S3-9 Communicating between Digital and Analog  Ideally conversion should be reversible  Limited by  Noise: interference from sources like radio waves, electrical wires, and bad connections that alter the data  Attenuation: normal reduction in signal strength during transmission caused by the transmission medium  Distortion: alteration in the data signal caused by the communication channel  Consequences  Error correction required to compensate for transmission limitations  Usually possible to recover original digital data exactly from analog transmission  Small information loss results from converting analog to digital Supplementary Chapter Communication Channel S3-10 Reception Errors  Timing mismatch between sending and receiving computers Supplementary Chapter Communication Channel S3-26 A-to-D Conversion  Digital signals used to represent analog waveforms  Examples: CDs, direct satellite TV, telephone voice mail Supplementary Chapter Communication Channel S3-27 A-to-D: Pulse Code Modulation Analog waveform sampled at regular time intervals  Maximum amplitude divided into intervals  Example: 256 levels requires bits/sample Supplementary Chapter Communication Channel S3-28 A-to-D: Pulse Code Modulation Sample values converted into corresponding number value  Information lost in conversion Supplementary Chapter Communication Channel S3-29 A-to-D: Pulse Code Modulation Number reduced to binary equivalent Supplementary Chapter Communication Channel S3-30 Digital Signal Quality  Subject to noise, attenuation, distortion like analog but  Signal quality less affected because only necessary to distinguish levels  Repeaters  Recreate signals at intervals  Use: transmit signals over long distances  Error correction techniques available Supplementary Chapter Communication Channel S3-31 TDM  Time division multiplexing  Multiple signals share channel Supplementary Chapter Communication Channel S3-32 Bandwidth  Digital signals: sum of sine waves of different frequencies  Higher frequencies: higher data rates  Channel with wider bandwidth has higher data rates  Data rates usually measured in bits per second Supplementary Chapter Communication Channel S3-33 Modems and Codecs  Modem (modulator/demodulator)  Convert digital signals to analog and back  Use: home to service provider via phone line or cable  Speed: baud rate or bits per second (bps)  Baud rate: signaling elements per second  At slow speeds bit encoded per electrical signal  Higher speed transmissions usually measured in bits per second rather than baud rate  High speed modem:  28.8 Kbps access with ASK, FSK and PSK  56 Kbps download with wider bandwidth at telephone switching office Supplementary Chapter Communication Channel S3-34 Codecs  Codec (coder/decoder)  Use: DSL (Digital Subscriber Line) via digital phone lines or cable  Ethernet for connection between the codec and the computer  Speed: 1Mbps or higher Supplementary Chapter Communication Channel S3-35 Transmission Media  Means used to carry signal  Characterized by  Physical properties  Signaling method(s) Bandwidth Sensitivity to noise  Guided media: confine signal physically to some kind of cable  Unguided media: broadcast openly  Signal-to-noise ratio  Higher ratio for given bandwidth increases data capacity of the channel Supplementary Chapter Communication Channel S3-36 Electrical Media  Require complete circuit  wires: one to carry the signal, second as a return to complete the circuit  Wired media or just wire  Inexpensive and easy to use  Signals carried as changing electrical voltage or current Supplementary Chapter Communication Channel S3-37 Types of Cable: Copper  Coaxial cable  Wire surrounded by insulation  Copper shield around insulation   Acts as signal return Shields from external noise  High bandwidth: 100 Mbps  Example: analog cable TV with FDM for dozens of channels at MHz  Twisted pair  Some networks and phone lines in buildings  More susceptible to noise than coaxial cable  Used for shorter distances and slower signals Supplementary Chapter Communication Channel S3-38 Types of Cable: Fiber Optic  Fiber optic cable     Consists of glass fiber thinner than human hair Uses light to carry signals Laser or light-emitting diode produces signal Cladding: plastic sheath to protect fibers  Advantages  Light waves: high frequency means high bandwidth  Less susceptible to interference  Lighter than copper cable  Disadvantages  Difficult to use, especially for multipoint connections Supplementary Chapter Communication Channel S3-39 Microwave  Frequencies below light  Unguided medium  Tightly focused for point-to-point use  Highly susceptible to interference  Applications  Large-scale Internet backbone channels  Direct satellite-to-home TV  IEEE 802.11 Wi-Fi Supplementary Chapter Communication Channel S3-40 ... Communication Channel S3-22 Attenuation  Function of the nature of the transmission medium and the physical length of the channel  More difficult to separate the signal from noise at higher transmission... Channel fading and phase shifts vary with the frequency of the signal  Example: If the signal consists of sine waves of frequencies f1 and f2 from different parts of the spectrum, the output of. .. Communication Channel S3 -3 Communication Channel  Characterized by  Signaling transmission method  Bandwidth: amount of data transmitted in a fixed amount of time  Direction(s) in which signal can flow