Fall 2005 Long Distance Communication Carriers, Modulation, And Modems

1. Fall 2005Long Distance CommunicationCarriers, Modulation, And Modems Qutaibah Malluhi Computer Science and Engineering Qatar University

2. Long-Distance Communication • Encoding used by RS-232 cannot work in all situations • For example, can not work over long distances • Signal loss over long distance • Electric current attenuates (becomes weaker) as it travels on wire • Resulting signal loss may prevent accurate decoding of data • Therefore, Encoding bits by voltage levels (like in RS-232) does not work for long distance communication • Different data encoding schemes are needed

3. Long Distance Communication • Important fact: an oscillating signal travels further than direct current • For long-distance communication • Send a sine wave (called a carrier wave) • Change (modulate) the carrier to encode data bits • Extract bits from the modulated wave by a demodulator at the receiving destination

4. Illustration Of A Carrier • Carrier • Usually a sine wave • Oscillates continuously • Frequency of carrier fixed • Carrier can travel over much longer distances than RS-232 signal

5. Characteristics of a Carrier • Amplitude – height of wave • Volts, amps, or watts • Frequency - # of times signals make complete cycle • expressed in hertz (Hz) • Phase – position of waveform

6. Amplitude

7. Frequency and Period • Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency. • Frequency and period are the inverse of each other • Period is measured in seconds while frequency measured in Hertz (HZ) • E.g. period= 1 millisecond  frequency= 1 Khz

8. Phase • Phase describes the position of the waveform relative to time zero.

9. Sign Wave Examples

10. Encoding Data With A Carrier • Called modulation (or Shift Keying) • Modifications to basic carrier encode data for transmission • Modulated carrier technique used for radio and television • Modulation is used with all types of media • copper, fiber, radio, infrared, laser

11. Types of Modulation • Amplitude modulation • Encode (modulate) data by changing the strength, or amplitude of the carrier • Frequency modulation • Encode data by changing the frequency of the carrier • Phase shift modulation • Encode data by changing the timing, or performing phase shifts on the carrier • Example: Two modulation techniques for radio are frequency modulation (FM) and amplitude modulation (AM)

12. Example Of Amplitude Modulation • Strength of signal encodes 0 or 1 • One cycle of wave needed for each bit • Data rate limited by carrier bandwidth • Simple but less efficient • more susceptible to noise errors

13. Example of Frequency Modulation • Frequency variation of signal encodes 0 and 1 • Frequency: # of times signals make complete cycle • Frequency expressed in hertz (Hz) • Does not suffer from sudden noise spikes

14. Phase-Shift Example • Phase – position of waveform • Section of wave is omitted at phase shift • Data bits determine size of omitted section

15. Example of Phase-Shift Modulation ½ cycle shift ½ cycle shift 3/4 cycle shift • Change in phase encodes K bits • Data rate higher than carrier bandwidth • For example, if 4 possible shifts can be detected by hardware, each shift value can encode 2 bits • Bit rate = 2 * baud rate

16. Phase-Shift Modulation with 4 Shift levels

17. Modem • Sending digital data using analog signal requires modulation • Modulator encodes data bits as modulated carrier • Demodulator decodes bits from carrier • Requires a hardware device called modem • modulator/demodulator • Contains separate circuitry for • Modulation of outgoing signal • Demodulation of incoming signal

18. Full Duplex Communication • Bidirectional, or full duplex, transmission is needed • Requires modulator and demodulator at both endpoints • One modem at each end • Modulator on one modem connects to demodulator on other • Separate wires carry signals in each direction • Long-distance connection requires a 4-wire circuit

19. Modem Examples • If external modem, RS-232 can be used to connect computer to modem • If internal modem, system bus is used

20. ISDN modem Cable modem Coax connector for cable and 10Base-T connector for computer Other Types of Modems

21. Operation of Dialup Modems • Receiving modem waits for call in answer mode Other modem, in call mode: • Simulates lifting handset • Listens for dial tone • Sends tones (or pulses) to dial number • Answering modem: • Detects ringing • Simulates lifting handset • Sends carrier • Calling modem: • Sends carrier • Data exchanged

22. Multiplexing • Allow multiple channels/users share link capacity • Fundamental to networking • Multiple signals encoding data can be carried on same medium without interference • Allows multiple simultaneous data streams • Example - Dialup modems can carry full-duplex data on one voice channel • Example - multiple TV stations in air medium • Each separate signal is called a channel

23. Types Of Multiplexing • Time Division Multiplexing (TDM) • Statistical Time Division Multiplexing (STDM) • Frequency Division Multiplexing (FDM) • Spread Spectrum Multiplexing • Wave Division Multiplexing (WDM)

24. Time Division Multiplexing (TDM) • Use a single carrier and sends data streams sequentially • Only one item at a time on shared channel • Each channel allowed to be carried during pre-assigned timeslots only • Basis for most computer networks that use shared media - will give details in later chapters • Pros: fair, simple to implement • Cons: inefficient (i.e., empty slots when user has no data)

25. TDM Illustrated

26. Empty Timeslots in TDM

27. Statistical Time Division Multiplexing (STDM) • Each timeslot is allocated on a demand basis (dynamically). • Example: ATM • Pros: improved performance • Cons: requires buffering when aggregate input load exceeds link capacity

28. Basic Principle behind FDM • Two or more signals that use different carrier frequencies can be transmitted over a single medium simultaneously without interference • Note: this is the same principle that allows a cable TV company to send multiple television signals across a single cable

29. Frequency Division Multiplexing (FDM) • Multiple items transmitted simultaneously • Each channel is allocated a particular carrier frequency (called bands). • Frequencies must be separated to avoid interference • All (modulated) signals are carried simultaneously (as a composite analog signal) • Receiver can "tune" to specific frequency and extract modulation for that one channel

30. FDM Demonstrated

31. Spread Spectrum Multiplexing • Spread spectrum uses multiple carriers concurrently • Single data stream divided up and sent across different carriers • Can be used to bypass interference or avoid wiretapping

32. Wave Division Multiplexing (WDM) • Facts • FDM can be used with any electromagnetic radiation • Light is electromagnetic radiation • When applied to light, FDM is called wave division multiplexing

33. Summary • Various transmission schemes and media available • Electrical current over copper • Light over glass • Electromagnetic waves • Digital encoding used for data • Asynchronous communication • Used for keyboards and serial ports • RS-232 is standard • Sender and receiver agree on baud rate

34. Summary (cont’d) • Modems • Used for long-distance communication • Available for copper, optical fiber, dialup • Transmit modulated carrier • Phase-shift modulation popular • Frequency modulation and amplitude modulation are other examples

35. Summary (cont’d) • Multiplexing • Fundamental concept • Used at many levels • Applied in both hardware and software • Three basic types • Time-division multiplexing (TDM) • Frequency-division multiplexing (FDM) • Statistical time-division multiplexing (STDM) • When applied to light, FDM is called wave-division multiplexing