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Fall 2005
Long Distance Communication
Carriers, Modulation, And Modems
Qutaibah MalluhiComputer Science and Engineering
Qatar University
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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
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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
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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
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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
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Amplitude
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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
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Phase
Phase describes the position of the waveform relative to time zero.
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Sign Wave Examples
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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
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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)
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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
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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
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Phase-Shift Example
Phase – position of waveform Section of wave is omitted at phase shift Data bits determine size of omitted
section
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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
½ cycle shift
3/4 cycle shift
Shift Amount
Encoded Bits
No shift
00
¼ cycle 01
½ cycle 10
¾ cycle 11
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Phase-Shift Modulation with 4 Shift levels
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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
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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
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Modem Examples
If external modem, RS-232 can be used to connect computer to modem
If internal modem, system bus is used
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Other Types of Modems
ISDN modem
Cable modem– Coax connector for cable
and 10Base-T connector for computer
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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
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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
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Types Of Multiplexing
Time Division Multiplexing (TDM) Statistical Time Division Multiplexing (STDM) Frequency Division Multiplexing (FDM) Spread Spectrum Multiplexing Wave Division Multiplexing (WDM)
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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)
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TDM Illustrated
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Empty Timeslots in TDM
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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
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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
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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
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FDM Demonstrated
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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
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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
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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
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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
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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