Fall 2005 Long Distance Communication Carriers, Modulation, And Modems

1
Fall 2005Long 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
• 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

• 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
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 (contd)

• 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 (contd)

• 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