1 Fall 2005 Long Distance Communication Carriers, 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 u Electric current attenuates (becomes weaker) as it travels on wire u Resulting signal loss may prevent accurate decoding of data u 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  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 ½ cycle shift 3/4 cycle shift Shift Amount Encoded Bits No shift00 ¼ cycle01 ½ cycle10 ¾ cycle11

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

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