cis 321 – data communications & networking chapter 8 – multiplexing
TRANSCRIPT
CIS 321 –Data Communications & Networking
Chapter 8 – Multiplexing
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Multiplexing
Simultaneous transmission of multiple signals across a single data link
Can utilize higher capacity links without adding additional lines for each device – better utilization of bandwidth
Multiplexer (MUX) on sender’s end; demultiplexer (DEMUX) separates transmission stream and directs signals to intended receiving devices
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Multiplexing vs. no multiplexing
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Multiplexing Techniques
3 basic techniques Frequency-division multiplexing – analog Wave-division multiplexing – analog Time-division multiplexing – digital
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Frequency-Division Multiplexing
Analog technique - when bandwidth of link is greater than combined bandwidth of signals to be transmitted
Signals from each sending device modulate different frequencies
Modulated signals are combined into a single composite signal
Bandwidth ranges are channels through which the signals travel, separated by guard bands
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FDM
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FDM MUX-DEMUX Process
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Analog Hierarchy of Phone Network
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Other Applications of FDM
AM and FM radio broadcasting Each station uses a different carrier frequency,
shifting its signal and multiplexing Receiver filters (tunes) to the frequency desired
Same concept for TV broadcasting and first generation cell phones
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6.2 Wave-Division Multiplexing
Use light signals transmitted through fiber-optic channels
Very narrow bands of light are combined from several sources to make a wider band of light
A prism is used to bend the light beams based on the angle of incidence and frequency
Receiver’s DEMUX separates signals
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WDM
Picture from Cisco Systems, Inc.
MPEG WDM example ©Sydney VisLab
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WDM Applications
Application: SONET network Multiple optical fiber lines are muxed/demuxed
DWDM (dense WDM) allows muxing of large numbers of channels by spacing channels closer to one another to achieve greater efficiency
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6.3 Time-Division Multiplexing (TDM)
Process of combining digital signals from several sources whereby each connection occupies a portion of time in the link
Link is sectioned by time rather than frequency
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TDM
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Time Slots and Frames
Data flow of each connection is divided into units Link combines one unit of each connection to
make a frame n input connections n time slots Data rate of link must be n times the duration of a
time slot to guarantee flow of data Time slots are grouped into frames; one complete
cycle of time slots; each slot dedicated to one device
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TDM
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Interleaving
Process of taking a specific amount of data from each device in a regular order
May be done by bit, byte, or any other data unit
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Synchronizing
Framing bits are used to alleviate timing inconsistencies that may be introduced
Usually one or two synchronization bits are added to beginning and end of each frame that allows the DEMUX to synchronize with the incoming stream so it can separate time slots accurately
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Framing Bits example
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Bit Padding
Used when differing speeds of devices are not integer multiples of each other (e.g. device A may be five and a half times as fast as device B)
MUX adds extra bits to a device’s source stream to force speed relationships between devices into integer multiples of each other
Extra bits are then discarded at the DEMUX
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Digital Signal (DS) Service
Hierarchy of digital signals DS-0 – single channel of 64 Kbps DS-1 – single service or 24 DS-0 channels multiplexed to
yield 1.544 Mbps DS-2 – single service or 4 DS-1 channels or 96 DS-0
channels to yield 6.312 Mbps DS-3 – single service, 7 DS-2 channels, 28 DS-1
channels, or 672 DS-0 channels to yield 44.376 Mbps DS-4 – 6 DS-3 channels, 42 DS-2 channels, 168 DS-1
channels, 4032 DS-0 channels to yield 274.176 Mbps
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DS Hierarchy
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T Lines
Capacity lines matching data rates of DS-1 to DS-4 services
Service Line Rate (Mbps) Voice Channels
DS-1 T-1 1.544 24
DS-2 T-2 6.312 96
DS-3 T-3 44.736 672
DS-4 T-4 274.176 4032
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T Lines
Digital lines designed for digital data, voice, or audio May be used for regular analog if sampled then
multiplexed using TDM
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T-1 line for multiplexing telephone lines
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T-1 frame structure
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Inverse TDM
Takes data stream from one high-speed line and breaks into portions and sends over several lower-speed lines simultaneously
Used in bandwidth-on-demand where channels can be used for several applications requiring different transmission rates (i.e. voice, data, video)
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Inverse Multiplexing
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More TDM Applications
Second-generation cell phone companies Divides into bands and uses FDM to combine Then uses TDM for sharing