Post on 08-Apr-2018
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Lecture
07Data Communication & Networks
Multiplexing & Synchronization
Muhammad Yousaf
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Multiplexing:
Multiplexing:
Combining data from n-inputs then transmitting over asingle higher capacity link
Demultiplexing:
Separating multiplexed data and delivering to theappropriate output lines
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Multiplexing: (Example Scenario)
(Source: http://en.wikipedia.org/wiki/File:Multipexing_demultiplexing_scheme_en.svg)
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Multiplexing Types:
Data can be multiplexed in a variety of mannersMost prominent of them are:
Frequency Division Multiplexing (FDM)Wavelength Division Multiplexing(WDM)
Time Division Multiplexing (TDM)Statistical TDM (STDM)
Also available:
Space Division Multiplexing (SDM)
Code Division Multiplexing (CDM)Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)
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FDM:
Bandwidth available on the medium is much greater ascompared to that required by the individual channel
Want to send number of individual channels on a singlemedium
Each signal is modulated with a different carrier frequencyGuard band is used to avoid intermodulation noise
Channel remains allocated, even if users are not sendingdata
Familiar example is broadcast and cable TV
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FDM – Multiplexing:
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FDM – Spectrum:
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FDM – Demultiplexing:
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TDM:
Available data rate on medium is much greater ascompared to that required by the individual channel
Multiple digital signals are interleaved in time
Time slots are pre-assigned to the sources
Slots remain allocated even if users don’t send data
Time slots are not required to be evenly distributedamongst the sources
Long distance carrier systems often use it
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TDM: cont…
time
Ch-1Ch-2 Ch-3
Ch-1Ch-2
Ch-1
Ch-3
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TDM – Multiplexing:
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TDM – Multiplexing:
(Source: http://en.wikipedia.org/wiki/File:Telephony_multiplexer_system.gif)
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TDM – Demultiplexing:
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Statistical TDM:
In TDM unused slots are wastedNow, idea is not to allocate time slots permanently
Allot time slots on demand only
Multiplexer scans input lines, collects data and
allocate time slots dynamicallyCan accommodate more number of channels thanavailable data rate of medium
More efficient utilization of resources
In peak-hours users can experience the network-busy
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Statistical TDM Frame Formats
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Synchronization
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Synchronization:
Timing problems require a mechanism tosynchronize the transmitter and receiver
Two solutions
Asynchronous Transmission
Synchronous Transmission
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Asynchronous transmission:
Character-wise transmissionData are transmitted one character at a time
So, synchronization is needed to be maintainedwithin each character separately
Start of the byte is indicated with a start-bit
End of the byte is indicated with the stop-bit
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Asynchronous transmission: cont…
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Asynchronous transmission: cont…
In idle state, receiver looks for transition from 1 to 0Then samples next 8-bits for the character value
Then looks for the stop-bit
Voltage level of stop-bit is similar as that for the idlestate
Very simple & cheap scheme, but overhead is high
For every 8-data bits 2-bits are overhead
Overhead is 20%
Normally is used for low data rate I/Ocommunication e.g. keyboard, MIDI devices
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Asynchronous transmission: cont…
Let’s sendA,B,C asynchronously with even parity:
A = 65 = 1000001
B = 66 = 1000010
C = 67 = 1000011
Data sent is:
0100000101 0100001001 01000011111111
0 = start bit
1 = stop bit
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Synchronous transmission:
Data is sent in blocks of data
Block of data bytes is called a frame
Start and end of frame is indicated with apredefined pattern of bits (flag)
Length of flag-bits is different for different protocolsStart of frame is indicated with a preamble
End of frame is indicated with a postambleEthernet maximum frame size is 1500-bytes
Ethernet uses 8-bytes of alternating 1 & 0s for preambleSynchronization overhead is approximately 8/1500 i.e.0.5%
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Synchronous frame format: