Communication and Networks Assignment
Communications and Networks
version 1.0
Diploma in Information Technology
Copyright © 2020 by Singapore Institute of Management Pte Ltd. All rights reserved.
Lesson 14: Multiplexing
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Lesson 14 Learning Outcomes
Explain the concept of multiplexing
Understand the concept of multiplexing and demultiplexing
Describe the concepts behind FDM, TDM and WDM
Describe the variations of FDM and TDM
Describe the framing used in telephone system version of TDM
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Lesson 14 Learning Outcomes
Describe inverse multiplexing and code division multiplexing
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Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing
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Multiplexing
Multiplexing: combination of information streams from multiple sources for transmission over a shared medium
Multiplexor: mechanism that implements multiplexing
Demultiplexing: separation of a combination back into separate information streams
Demultiplexor: mechanism that implements demultiplexing
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Multiplexing Illustration (1/2)
Each sender communicates with a receiver
All pairs share a transmission medium
Multiplexor combines information from senders
Demultiplexor separate information for receivers
Source: Douglas, C (2016) Computer Networks and Internets
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Multiplexing Illustration (2/2)
Main purpose is sharing the medium
Source: Douglas, C (2016) Computer Networks and Internets
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Multiplexing Example
Source: Douglas, C (2016) Computer Networks and Internets
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Types of Multiplexing
Frequency Division Multiplexing (FDM)
Wavelength Division Multiplexing (WDM)
Time Division Multiplexing (TDM)
Code Division Multiplexing (CDM)
TDM and FDM are widely used
WDM is a form of FDM used for optical fiber
CDM is a mathematical approach used in cell phone mechanisms
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Multiple Access and Multiplexing
Source: https://www.youtube.com/watch?v=oYRMYSIVj1o
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Frequency Division Multiplexing (FDM)
Radio stations/TV can transmit electromagnetic signals simultaneously
With little interference by using separate channel (carrier frequency)
Possible to send simultaneously multiple carrier waves over a single copper wire
Demultiplexer applies filters that each extract a small range of frequencies near one of carrier frequencies
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FDM Illustration
Filters used in FDM only examine frequencies
FDM mechanism will separate the frequency from others without modifying the signal
Source: Douglas, C (2016) Computer Networks and Internets
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FDM Advantages & Limitations
Advantages: simultaneous use of a transmission medium by multiple pairs of entities
FDM provides each pair with a private transmission path like separate physical transmission medium
Limitations: If the frequencies of two channels are too close, interference can occur
Demultiplexing hardware that receives combined signal must divide signal into separate carriers
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FDM Guard Band
Source: Douglas, C (2016) Computer Networks and Internets
Designers choose a set of carrier frequencies with a gap between them known as a guard band
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FDM Independent Channels
Source: Douglas, C (2016) Computer Networks and Internets
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FDM Characteristics (1/2)
Long-lived: the idea of dividing the electromagnetic spectrum into channels came from early experiments in radio
Widely used: used in broadcast radio and television, cable television, and some cellular telephone
Versatile: it filters on ranges of frequency without examining other aspects of signals
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FDM Characteristics (2/2)
Analog: multiplexing and demultiplexing hardware accepts and delivers analog signals
Even if a carrier has been modulated to contain digital information, FDM hardware treats the carrier as an analog wave
But makes FDM susceptible to noise and distortion
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Using Range of Frequencies
Most FDM systems assign each sender and receiver a range of frequencies
Can choose how frequencies can be used
Two primary ways:
Increase the data rate
Increase immunity to interference
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Increasing Overall Data Rate
To increase the overall data rate:
Sender divides the frequency range of the channel into K carriers
Sends 1/K of the data over each carrier
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Subchannel and Subdivision
Sender can perform FDM within allocated channel
subchannel allocation refers to subdivision
To increase immunity to interference
Use a technique known as spread spectrum
Godmother of Spread Spectrum: Hedy Lamarr
Source: Douglas, C (2016) Computer Networks and Internets
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Multiplexing with FDM
Basic idea:
divide the range of the channel into K carriers
transmit data spread, using unique patterns, over multiple channels
allow receiver to use a copy compose data that arrives using same spread pattern used by sender
Works well in cases where noise is likely to interfere with some frequencies at a given time
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Hierarchical FDM
If a set of incoming signals all use frequency 0-4 KHz
map first first onto the range 0-4 KHz
map second onto the range 4-8 KHz
map third onto the range 8-12 KHz
Hierarchy in FDM multiplexors is that each maps its inputs to a larger continuous band of frequencies
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Hierarchical FDM in Telephone
Source: Douglas, C (2016) Computer Networks and Internets
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Practice 14.1
Frequency Division Multiplexing (FDM) allocates different frequencies to each pair of communication.
What is the advantage and limitation of such approach?
How can the limitation of FDM be overcome?
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Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing
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Wavelength Division Multiplexing (WDM)
WDM: application of FDM to optical fiber
Dense WDM (DWDM): many wavelengths of light employed
Inputs and outputs of multiplexing are wavelengths of light denoted by the Greek letter λ, and informally called colors
Prisms are used in optical multiplexing and demultiplexing
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WDM Idea
When white light passes through a prism colors of the spectrum are spread out
WDM demultiplexor uses a prism to separate the wavelengths.
If several colored light beams are each directed into a prism at the correct angle, the prism will combine the beams to form a beam of white light
WDM multiplexor accepts beams of light of various wavelengths to combine them into a beam
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WDM Illustration
Source: Douglas, C (2016) Computer Networks and Internets
WDM Multiplexor
WDM Demultiplexor
Each color can be used as a channel
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Practice 14.2
What is the purpose of a prism in WDM for:
A multiplexor
A demultiplexor
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Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing
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Time Division Multiplexing (TDM)
TDM: assigns time slots to each channel repeatedly
Sent in a round-robin fashion
TDM Multiplexing: transmitting an item from one source first before transmitting an item from another source
Source: Douglas, C (2016) Computer Networks and Internets
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Synchronous TDM
Synchronous TDM: when TDM is applied to synchronous networks, no gap occurs between items
Source: Douglas, C (2016) Computer Networks and Internets
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Synchronous TDM in Telephone
Telephone systems use synchronous TDM to multiplex digital streams from multiple phone calls
Demultiplexer is synchronized with multiplexer
A synchronous TDM sends one slot after another without any indication of the output to which a given slot occurs
Slight difference in the clocks used to time bits can cause a demultiplexer to misinterpret the bit stream
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Synchronous TDM Framing
To prevent misinterpretation, phone system includes an extra framing channel as input
Instead of a complete slot, framing inserts a single bit in the stream on each round
Demultiplexor extracts data from the framing channel and checks for alternating 0 and 1 bits
If an error causes a demultiplexor to lose a bit, likely that framing check will detect the error and allow the transmission to be restarted
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Framing Illustration
Source: Douglas, C (2016) Computer Networks and Internets
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Hierarchical TDM
Each successive stage of a TDM hierarchy uses N times the bit rate
In FDM, each successive stage uses N times the frequencies
Additional framing bits are added to the data
The bit rate of each successive layer of hierarchy is slightly greater than the aggregate voice traffic
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Hierarchical TDM Illustration
Source: Douglas, C (2016) Computer Networks and Internets
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Synchronous TDM Unfilled Slots
Synchronous TDM works well if each source produces data at a uniform
Many sources generate data in bursts, with idle time between bursts
In real life, a slot cannot be empty as the underlying system must continue to transmit data
the slot is assigned a value (such as zero) with an extra bit set to indicate that the value is invalid
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Unfilled Slots Illustration
Sources on the left produce data items at random
Synchronous multiplexor will leave a slot unfilled if the corresponding source has not produced an item by the time, the slot must be sent
Source: Douglas, C (2016) Computer Networks and Internets
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Statistical TDM
One technique to increase utilisation of shared medium is statistical TDM or statistical multiplexing
Instead of leaving a slot unfilled, skip any source that does not have data ready
By eliminating unused slots, statistical TDM takes less time to send the same amount of data
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Statistical TDM Illustration
Use only 8 slots instead of 12
Source: Douglas, C (2016) Computer Networks and Internets
Synchronous TDM
Statistical TDM
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Statistical TDM Overheads
Statistical multiplexing incurs extra overhead
Synchronous TDM system: every Nth slot corresponds to which receiver
Statistical TDM: any slot can correspond to any receiver
Hence, each slot must contain the identification of the receiver to which the data is being sent
Overhead: extra information sent with data
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Inverse Multiplexing
On the Internet, a connection between two points can consists of multiple transmission media
but no single medium has a bit rate that is sufficient as service providers need higher bit rates than what is available
Multiplexing can be used in reverse
Spread a high-speed digital input over multiple lower-speed circuits for transmission and combine the results at the receiving end
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Inverse Multiplexing Illustration
Source: Douglas, C (2016) Computer Networks and Internets
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Inverse Multiplexor Design
Inverse multiplexor cannot be constructed by connecting the pieces of a conventional multiplexor backward
Hardware must be designed so that sender and receiver agree on how data arriving from the input will be distributed over the lower-speed connections
To ensure ordered delivery, it must be designed to handle cases like different latency on different lower-speed connections
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Practice 14.3
How does synchronous TDM handle cases where a sender has nothing to send?
What technique can be used with TDM to better utilise the shared medium?
On the Internet, explain why is multiplexing not such a suitable approach for multiple pairs of communication? Explain an alternative that can be used.
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Lesson 14 Outline
Frequency Division Multiplexing
Wavelength Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing
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Code Division Multiplexing (CDM)
CDM is used in parts of the cellular telephone system and for some satellite communication
Version of CDM used in cell phones is known as Code Division Multi-Access (CDMA)
CDM does not rely on physical properties
such as frequency or time
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CDM Idea
CDM relies on values from orthogonal vector spaces can be combined and separated without interference
Each sender is assigned a unique binary code Ci, known as a chip sequence
Chip sequences are selected to be orthogonal vectors
the dot product of any two chip sequences is zero
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CDM Steps
Each sender’s value to transmit is Vi
Each ith senders transmit (Ci) x (Vi)
Each sender transmit at the same time and the values are added together
To extract value Vi, receiver multiplies the sum by Ci
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CDM Example (1/5)
Consider an example
Use a chip sequence that is only 2-bit long and data values that are 4-bit long
think of the chip sequence as a vector
Source: Douglas, C (2016) Computer Networks and Internets
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CDM Example (2/5)
The first step consists of converting binary values into vectors that use -1 to represent 0
Source: Douglas, C (2016) Computer Networks and Internets
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CDM Example (3/5)
Resulting values is a sequence of signal strengths to be transmitted at the same time
Resulting signal is the sum of two signals
Source: Douglas, C (2016) Computer Networks and Internets
Normal arithmetic addition
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CDM Example (4/5)
A receiver treats sequence as a vector
Computes dot product of vector and chip sequence
Treats result as sequence, converts result to binary by interpreting positive values as binary 1 and negative values as 0
Thus, receiver 1 computes:
Source: Douglas, C (2016) Computer Networks and Internets
C1
Received data
(1(0) + (-1)(-2))
(1(2) + (-1)(0))
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CDM Example (5/5)
Interpreting the result as a sequence produces:
(2 -2 2 -2)
which becomes the binary value: (1 0 1 0)
1010 is the correct value of V1
Receiver 2 will extract V2 from the same transmission using C2
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 11
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End of Lesson
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