msis 4523 ch6.bandwidthutil
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Data Communications SystemsCh 6: Bandwidth Utilization
JinKyu Lee, Ph.D.
Include the course code (MSIS4523) in every email subject!!
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Topics
Multiplexing Techniques
Commonly Used Forms of Multiplexing
Frequency Division Multiplexing
Time Division Multiplexing Synchronous vs. Statistical Wavelength Division Multiplexing
Discrete Multi-tone for DSL
Code Division Multiplexing
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What is Multiplexing
Sharing a common transmission medium simultaneouslybetween several data sources
Line sharing saves transmission costs
Allows efficient utilization of higher data rates
Means more cost-effective transmissions
Takes advantage of the fact that most individual datasources require relatively low data rates
Commonly Used
Forms of Multiplexing
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Frequency Division Multiplexing (FDM)
FDM is an analog multiplexing
technique that combines signals
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Frequency Division Multiplexing
Requires analog signaling & transmission Total bandwidth = sum of input bandwidths +
guard bands
Modulates signals so that each occupies a
different frequency band
Standard for radio broadcasting, analogtelephone network, cell phones, and television
(broadcast, cable, & satellite)
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The FDM Multiplexing Process
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The FDM Demultiplexing Process
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Assume that a voice channel occupies a bandwidth of 4KHz. We need to combine three voice channels into alink with a bandwidth of 12 KHz, from 20 to 32 KHz.Show the configuration using the frequency domainwithout the use of guard bands.
Shift (modulate) each of the three voice channels to adifferent bandwidth, as shown on the next chart
Solution
FDM Example 1
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Example 1 Solution
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Five channels, each with a 100-KHz bandwidth, are tobe multiplexed together. What is the minimumbandwidth of the link if there is a need for a guard bandof 10 KHz between the channels to prevent
interference?
Example 2
Solution
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Example 2 Solution
5 x 100 + 4 x 10 = 540 KHz
For five channels, we need at least four guard bands.This means that the required bandwidth is at least
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Four data channels (digital), each transmitting at 1Mbps, use a satellite channel 1 MHz wide. Design anappropriate configuration using FDM
Example 3
Solution
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Example 3 Solution
The satellite channel is analog. We divide it into fourchannels, each channel having a 250-KHz bandwidth. Eachdigital channel of 1 Mbps is modulated such that each 4 bitsare modulated to 1 Hz. One solution is 16-QAM modulation.
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Analog FDM Hierarchy
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ADSL Technology
ADSL Uses discrete multitone (DMT) to exploit the 1-
MHz capacity of existing twisted pair loops
There are three elements of the ADSL strategy
Reserve lowest 25 kHz for voice, known as POTS (Plain oldtelephone service)
Use echo cancellation or FDM to allocate a small upstream bandand a larger downstream band
Use FDM/DMT within the upstream and downstream bands,using DMT signaling
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Discrete Multitone (DMT)
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The Discrete Multitone (DMT) Method
Multiple carrier signals at different frequencies
Sends some of the bits on each channel
Transmission band (upstream or downstream) is dividedinto 4-kHz channels
Each channel is capable of carrying 60 kbps
256 downstream channels = 15.26 Mbps capacity
Provides 1.5 to 9 Mbps in practice
Test signals determine noise at each channel
Sends less bits over noisy channels
It then assigns more bits to better quality channels and fewer bitsto poorer quality channels
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ADSL/DMT Bandwidth Allocation
Each channel is 4 KHz wide, carryinga maximum of 60 Kbps. Only the
least noisy channels are used
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Wave Division Multiplexing (WDM)
WDM is an analog multiplexing technique tocombine optical signals. Optical prisms are
used for the multiplexing function
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Synchronous Time-Division Multiplexing(TDM)
Used in digital transmission systems
Requires data rate of the medium to exceeddata rate of signals to be transmitted
Signals take turns over medium
Slices of data are organized into frames
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Time Division Multiplexing (TDM)
TDM is a digital multiplexingtechnique to combine data
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TDM Frame Structures
In a TDM, the data rate of the link is sum of thecomponent input data streams, plus any overhead
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Four channels are multiplexed using TDM. If eachchannel sends 100 bytes/s and we multiplex 1 byte perchannel, show the frame traveling on the link, the sizeof the frame, the duration of a frame, the frame rate,and the bit rate for the link.
Example 6
Solutionsize of the frame size of the frame
Frame rate = # of frames transmitted per sec.
Bit rate = bps
duration of a frame = 1/# of Frame
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A multiplexer combines four 100-Kbps channels using atime slot of 2 bits. Show the output with four arbitraryinputs. What is the frame rate? What is the frameduration? What is the bit rate? What is the bit duration?
Example 7
Solution
msize of the frame
duration of a frame = 1/# of Frames per sec
Frame rate = # of frames transmitted per sec.
Bit rate = bps
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Synchronous TDM and the PSTN
Basis of modern carrier telecommunications systems
US, Canada, Japan: DS-1(T-1), DS-3(T-3), ...
Europe, elsewhere: E1, E3, ...
ATM (DL Std) and SONET (MUX Std) everywhere
DS-1: Data rate of 1.544 Mbps
Original TDM system designed for voice
Uses PCM to digitize voice transmission at 8000 times/sec with 8bits/sample = 64kbps
24 channels x 8 bits/channel + 1 frame bit = 193 bits/frame with8000 frames/sec
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T-1 Frame Structure
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Multiplexing Telephone Lines on a T-1 Line
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North American Digital Signal (DS) Hierarchy
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DS and T Lines Rates
Service LineRate
(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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European E Line Rates
E LineRate
(Mbps)Voice
Channels
E-1 2.048 30
E-2 8.448 120
E-3 34.368 480
E-4 139.264 1920
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Statistical Time Division Multiplexing(STDM)
Intelligent TDM
Data rate capacity required is well below the
sum of connected capacity Digital only, because it requires more complex
framing of data
Frame slots only to devices with full buffers
Uses memory buffers to avoid loss of data
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Statistical Time Division Multiplexing
Statistical multiplexor - transmits only the data from active
workstationsAccepts incoming data streams
Creates a frame containing only the data to be transmitted
If a workstation is not active, no space is wasted on the multiplexed stream
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Statistical Time Division Multiplexing (cont.)
To identify each piece of data, an address is included
If data is of variable size, length is also included
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Statistical Time Division Multiplexing (cont.)
More precisely, the transmitted frame contains a
collection of data groups
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Comparison of Multiplexing Techniques