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Data Communications. Chapter 16, Exploring the Digital Domain. In this chapter . . . You will learn about. chief elements of a communications system how data is encoded and transmitted classifying the variety of computer networks getting “connected” at home. Communicating Information.

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data communications

Data Communications

Chapter 16,

Exploring the Digital Domain

in this chapter
In this chapter . . .

You will learn about

  • chief elements of a communications system
  • how data is encoded and transmitted
  • classifying the variety of computer networks
  • getting “connected” at home
communicating information
Communicating Information
  • broadcasting
    • one sender, many receivers
    • one-way communication
  • networking
    • one sender, one receiver pair (multiples)
    • two-way communication
shannon s data communication model1
Shannon’s Data Communication Model
  • an information source generates a message
  • a transmitter encodes the message as a signal
  • the signal is transmitted over a communications channel—a medium that bridges the distance between
  • the receiver extracts a signal from the communications channel and converts it back into the form of a message
  • the destination receives the message
  • a source of noise is usually present in the communication channel—this is a random element that modifies the signal in unpredictable ways
encoding messages
Encoding Messages
  • messages are encoded as a stream of binary numbers (0s and 1s)
  • signals are transmitted as electro-magnetic energy (electrical, optical, or radio waves)
  • rate of reliable transmissions depend on the properties of the channel and the complexity of the message
transmitting digital data
Transmitting Digital Data
  • analog (modulated signals)
    • amplitude
    • frequency
    • phase (calibrated timing)
  • digital

Two types of signals:

  • the bandwidth of a communication channel determines its capacity to transmit data—analogous to the size of plumbing pipes
  • analog signals may contain independent waveforms of various frequencies
  • the number of frequencies supported by a channel determines its bandwidth
limiting factors in data transmissions
Limiting Factors in Data Transmissions
  • bandwidth
  • signal strength
  • noise, i.e., signal-to-noise ratio
characteristics of computer networks
Characteristics ofComputer Networks
  • distance or expanse covered by the network
  • media used for transmission of signals
  • type of signal used for transmission
  • type of switching performed to route the signal
networks classified by distance
Networks Classifiedby Distance
  • LAN (Local Area Network)
  • WAN (Wide Area Network)
    • usually a large area
    • often owned by a single corporation
  • internetwork
networks classified by media
Networks Classified by Media
  • bounded media
    • copper wire
    • optical fiber cable
  • unbounded media
    • “wireless”
    • radio frequency waves
networks classified by signal
Networks Classified by Signal
  • baseband (narrowband)
    • employs entire bandwidth for one signal
  • broadband
    • multiple signals on the same channel simultaneously
    • channel is divided into separate frequency bands, each capable of carrying a signal
frequency division multiplexing
Frequency-Division Multiplexing
  • bandwidth is divided into separate channels
  • each signal occupies a specific portion of the bandwidth
time division multiplexing
Time-Division Multiplexing
  • signal occupies entire bandwidth
  • time divided into frames, slots
  • pieces of the signal are sent in slots
networks classified by switching
Networks Classified by Switching
  • devices in a network called “nodes”
  • arrangement of nodes and links called “topology”
  • point-to-point connectivity vs. shared connectivity
  • shared connectivity requires “switching”
    • routing data over common links
circuit switching
Circuit Switching
  • a continuous connection or circuit is made between transmitter and receiver
  • ordinary telephone connections are made by circuit switching
packet switching
Packet Switching
  • the message is made of separate data packets each addressed to the destination
  • Packets are transmitted over any available connection to the destination
  • receiving node reassembles the message
analog modems
Analog Modems
  • “modem” = modulate-demodulate
cable modems
Cable Modems
  • employ CATV communications
  • upstream, downstream speeds differ
  • downstream bandwidth shared
digital subscriber line dsl
Digital Subscriber Line (DSL)
  • DSL employs telephone communications
  • DSL uses bandwidth not required for voice
  • connection is continuous
  • Asymmetric DSL is common today
  • maximum length (5.4 km)
  • lines must be “DSL-ready”