Data Communications

1. Data Communications Chapter 16, Exploring the Digital Domain

2. 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

3. Communicating Information • broadcasting • one sender, many receivers • one-way communication • networking • one sender, one receiver pair (multiples) • two-way communication

4. Shannon’s Data Communication Model

5. 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

6. 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

7. Transmitting Digital Data • analog (modulated signals) • amplitude • frequency • phase (calibrated timing) • digital Two types of signals:

8. Amplitude Modulation

9. Frequency Modulation

10. Phase Modulation

11. Bandwidth • 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

12. Limiting Factors in Data Transmissions • bandwidth • signal strength • noise, i.e., signal-to-noise ratio

13. 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

14. Networks Classifiedby Distance • LAN (Local Area Network) • WAN (Wide Area Network) • usually a large area • often owned by a single corporation • internetwork

15. Networks Classified by Media • bounded media • copper wire • optical fiber cable • unbounded media • “wireless” • radio frequency waves

16. 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

17. Frequency-Division Multiplexing • bandwidth is divided into separate channels • each signal occupies a specific portion of the bandwidth

18. Time-Division Multiplexing • signal occupies entire bandwidth • time divided into frames, slots • pieces of the signal are sent in slots

19. 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

20. Circuit Switching • a continuous connection or circuit is made between transmitter and receiver • ordinary telephone connections are made by circuit switching

21. 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

22. Packet vs. Circuit Switching

23. Analog Modems • “modem” = modulate-demodulate

24. Cable Modems • employ CATV communications • upstream, downstream speeds differ • downstream bandwidth shared

25. Digital Subscriber Line (DSL) • DSL employs telephone communications • DSL uses bandwidth not required for voice • connection is continuous • Asymmetric DSL is common today

26. ADSL • maximum length (5.4 km) • lines must be “DSL-ready”