Communication and Networks Assignment

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Communications and Networks

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Diploma in Information Technology

Lesson 14: Multiplexing

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

Lesson 14 Learning Outcomes

Describe inverse multiplexing and code division multiplexing

Lesson 14 Outline

Frequency Division Multiplexing

Wavelength Division Multiplexing

Time Division Multiplexing

Code Division Multiplexing

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

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

Multiplexing Illustration (2/2)

Main purpose is sharing the medium

Source: Douglas, C (2016) Computer Networks and Internets

Multiplexing Example

Source: Douglas, C (2016) Computer Networks and Internets

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

Multiple Access and Multiplexing

Source: https://www.youtube.com/watch?v=oYRMYSIVj1o

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

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

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

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

FDM Independent Channels

Source: Douglas, C (2016) Computer Networks and Internets

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

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

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

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

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

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

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

Hierarchical FDM in Telephone

Source: Douglas, C (2016) Computer Networks and Internets

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?

Lesson 14 Outline

Frequency Division Multiplexing

Wavelength Division Multiplexing

Time Division Multiplexing

Code Division Multiplexing

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

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

WDM Illustration

Source: Douglas, C (2016) Computer Networks and Internets

WDM Multiplexor

WDM Demultiplexor

Each color can be used as a channel

Practice 14.2

What is the purpose of a prism in WDM for:

A multiplexor

A demultiplexor

Lesson 14 Outline

Frequency Division Multiplexing

Wavelength Division Multiplexing

Time Division Multiplexing

Code Division Multiplexing

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

Synchronous TDM

Synchronous TDM: when TDM is applied to synchronous networks, no gap occurs between items

Source: Douglas, C (2016) Computer Networks and Internets

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

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

Framing Illustration

Source: Douglas, C (2016) Computer Networks and Internets

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

Hierarchical TDM Illustration

Source: Douglas, C (2016) Computer Networks and Internets

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

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

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

Statistical TDM Illustration

Use only 8 slots instead of 12

Source: Douglas, C (2016) Computer Networks and Internets

Synchronous TDM

Statistical TDM

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

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

Inverse Multiplexing Illustration

Source: Douglas, C (2016) Computer Networks and Internets

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

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.

Lesson 14 Outline

Frequency Division Multiplexing

Wavelength Division Multiplexing

Time Division Multiplexing

Code Division Multiplexing

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

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

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

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

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

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

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

Received data

(1(0) + (-1)(-2))

(1(2) + (-1)(0))

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

Reading

Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 11

End of Lesson