ccnet lec 07 multiplexing sync

8/6/2019 Ccnet Lec 07 Multiplexing Sync

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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




TDM: cont…

time

Ch-1Ch-2 Ch-3

Ch-1Ch-2

Ch-1

Ch-3




TDM – Multiplexing:




TDM – Multiplexing:

(Source: http://en.wikipedia.org/wiki/File:Telephony_multiplexer_system.gif)




TDM – Demultiplexing:




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




Statistical TDM Frame Formats




Synchronization




Synchronization:

Timing problems require a mechanism tosynchronize the transmitter and receiver

Two solutions

Asynchronous Transmission

Synchronous Transmission




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




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





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




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%




Synchronous frame format:




Questions ???

ccnet lec 07 multiplexing sync

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