Multiplexing of Multiple Communications on the Shared...
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Multiplexing of Multiple Communications on the
Shared Medium
Computer Networks
Lecture 2http://goo.gl/pZE5o8
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Multiplexing
If the medium provides the wider bandwidth than is required by a single channel, multiple (sub)channels may share the same medium
Frequency-Division Multiplexing (FDM)Time-Division Multiplexing (TDM)Statistical Multiplexing (cells/packets)
The medium may be shared either as a point-to-point link or by multiple transmitters and receivers (multiple access)
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Frequency Division Multiplexing (FDM)
modulation, (filtering), transmission, filtering, demodulationmodulation, (filtering), transmission, filtering, demodulation
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Characteristics of the FDM
The number of subchannels and utilized frequencies are fixed and highly dependent on a circuitry design (capabilities of the analog part)
The frequency band is not fully utilized as we need to maintain gaps between subbands
or to solve problems related to partial overlap of subchannels bands
FDM is efficient for a fixed number of users that fully utilize a dedicated band all the time
e.g. bundling of voice channel on POTS trunks
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Wave Division Multiplexing (WDM)
Special case of FDM
May be implemented by a purely passive (and thus reliable) system (prism)
Used in high-speed WANs and FTTC optical networks
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Time Division Multiplex (TDM)
Timeslots are organized into repeating framesMechanisms for bit synchronization and frame synchronization have to be implementedThe technical implementation may be rather flexibleThe frame structure (e.g. the number of timeslots) may be configurable in software without a need to change the circuitry
CH 1 CH 2 CH 3 CH 1 CH 2 CH 3
frame
timeslot
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Statistical Multiplexing (1)
The most of data transfers between computer systems are bursty
The peak-to-average ratios of 1000:1 are common
It is inefficient to dedicate a guaranteed capacity for individual communications
The disadvantage of TDM is the fixed assignment of timeslots to subchannels
that does not reflect the bursty nature of the subchannels' traffic
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Statistical Multiplexing (2)The problem may be solved using a statistical multiplex scheme, in which data are transmitted in packets/cells with headers identifying the source/destination pair
Either the variable-length packets or fixed-length cells may be used
the latter is more predictable so QoS implementation is easier
Individual packets/cells may be multiplexed arbitrarily on the medium at the transmitter side
The demultiplexer on the receiving side may differentiate data of individual subchannels using headers
The link may be reasonably oversubscribed
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An Example of Statistical Multiplexing:Asynchronous Transfer Mode Networks
Channel 1
Channel 2
Channel 3
MUX DEMUX
2 13 1 3 3
cell
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Combination of the Multiplexing Methods
GSM: FDM + TDMAEach GSM cell serves multiple channels on various frequenciesEvery channel is further divided using TDM
WiFiFDM + TDMA
Optical WDM Internet links (IP)FDM + Statistical multiplex