Transcript Long Distance Communication: Carriers and Modems

Long Distance Communication: Carriers
and Modems
NETD411 – Network Technology and Architecture
Long-distance communication
 Encoding used by RS-232 cannot work in all situations
– Over long distances
– Using existing systems like telephone
 Different encoding strategies needed
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Sending signals long distances
 Electric current becomes weaker as it travels on wire
 Resulting signal loss may prevent accurate decoding of data
 Signal loss prevents use of RS-232 over long distances
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Oscillating signals
 Continuous, oscillating signal will propagate farther than electric current
 Long distance communication uses such a signal, called a carrier
 Waveform for carrier looks like:
 Carrier can be detected over much longer distances than RS-232 signal
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Encoding data with a carrier
 Modifications to basic carrier encode data for transmission
 Technique called modulation
 Same idea as in radio, television transmission
 Carrier modulation used with all types of media - copper, fiber, radio,
infrared, laser
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Types of modulation
 Amplitude modulation
– strength, or amplitude of carrier is modulated to encode data
– a carrier wave is modulated in proportion to the strength of a signal
– carrier rises and fall instantaneously with each high and low of the conversation
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Types of modulation
 Frequency modulation
– frequency of carrier is modulated to encode data strength, or amplitude of
carrier is modulated to encode data
– low distortion, little static, good voice quality and immunity from electrical and
atmospheric interference
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Types of modulation
 Phase shift modulation
– changes in timing, or phase shifts encode data
– natural flow of the alternating current waveform is delayed temporarily
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Hardware for data transmission
 Modulator
– encodes data bits as modulated carrier
 Demodulator
– decodes bits from carrier
 Data transmission requires modulator at source and demodulator at
destination
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Full duplex communication
 Most systems provide for simultaneous bidirectional, or full duplex,
transmission
 Requires modulator and demodulator at both endpoints:
 Long-distance connection is called 4-wire circuit
 Modulator and demodulator typically in single device called
a modem(modulator/demodulator)
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Modems
– most important and enduring devices for data communications
– converts digital signals into analog signals so that data can be transmitted over
telephone lines
– If external to computer, RS-232 can be used between modem and computer
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Modems
– If internal, direct bus connection used
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Modems
– Can also be rack-mounted
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Other types of modems
 Integrated Services Digital Network (ISDN) modem
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Other types of modems
 Cable modem (front)
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Other types of modems
 Cable modem (rear) with coax connector for cable and 10Base-T
connector
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Leased serial data circuits
 Organizations often include 4-wire circuits in network
 Within a site - on a campus - organization can install its own 4-wire
circuits
 Telephone company supplies off-campus wires
– Telephone cables have extra wires (circuits) for expansion
– Telephone company lease right to use wires to organization
– Organization uses modems for data transfer
 Called serial data circuit or serial line
 Operates in parallel with (but not connected to) telephone circuits
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Optical, radio and dialup modems
 Modems used with other media in addition to dedicated data circuits
 Special form of encoding/decoding transducers that use modulation for
data encoding
– Glass - data encoded as modulated light beam
– Radio - data encoded as modulated radio signal
– Dialup - data encoded as modulated sound
 Dialup modem connects to ordinary phone line
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Carrier frequencies and multiplexing
 Multiple signals with data can be carried on same medium without
interference
– Allows multiple simultaneous data streams
– 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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Multiplexing
 Carrying multiple signals on one medium is called multiplexing
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Multiplexing
 Frequency division multiplexing (FDM) achieves multiplexing by using
different carrier frequencies
 Frequency division multiplexing (FDM) means that the total bandwidth
available to the system is divided into a series of non-overlapping
frequency sub-bands that are then assigned to each communicating
source and user pair.
 Examples: stereo FM, television, and DSL.
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Multiplexing
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Multiplexing
 Time division multiplexing (TDM) uses a single carrier and sends data
streams sequentially
 Transmitter/receiver pairs share single channel
 Time division multiplexing is the process of dividing up one
communication time slot into smaller time slots.
 Examples: T-1 and ISDN telephone lines and some cellular telephone
systems like GSM Phone
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Multiplexing
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Multiplexing
 Statistical Time Division Multiplexing (STDM) is a system developed to
overcome some inefficiencies of standard time division multiplexing,
where time slices are still allocated to channels, even if they have no
information to transmit.
 Statistical Time Division Multiplexing uses a variable time slot length,
allowing channels to compete for any free slot space.
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Multiplexing
 Wavelength Division Multiplexing (WDM) is a method of combining
multiple signals on laser beams at various infared (IR) wavelengths for
transmission along fiber optic media.
 Wavelength Division Multiplexing is similar to frequency-division
multiplexing. But instead of taking place at radio frequencies (RF), WDM
is done in the IR portion of the electromagnetic (EM) spectrum.
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Thank You…
END OF PRESENTATION
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