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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