Digital Transmission EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL TRANSMISSION Multiplexing...

Digital Transmission

EKT 231 : COMMUNICATION SYSTEMEKT 231 : COMMUNICATION SYSTEM

CHAPTER 4 : DIGITAL TRANSMISSIONCHAPTER 4 : DIGITAL TRANSMISSION

MultiplexingMultiplexing FDM TDM

WDM

Chapter reviewChapter review

Chapter revisionChapter revision

Outlines:-

What is multiplexing and What is multiplexing and demultiplexing?demultiplexing? Multiplexing – the process of combining

together many separate signals to send them over the same transmission media.

Demultiplexing – reverses the multiplexing at the receiver



Cont’d...Cont’d... Multiplexing allows several analog

signals to be processed by one analog-to-digital converter (ADC) E.g. Several phone calls may be transferred

using one wire.

The multiplexed signal is transmitted over a communication channel which may be a physical transmission medium.



Cont’d...Cont’d... The multiplexing divides the capacity of

the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred.

A reverse process, known as demultiplexing, can extract the original channels on the receiver side.



Cont’d...Cont’d... A multiplexing technique may be further

extended into a multiple access method or channel access method. E.g. Time Division Multiple Access (TDMA)

Multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity.



Cont’d...Cont’d... Concept of multiplexing



Why needs multiplexing?Why needs multiplexing? Make the communications very

economical by transmitting thousands of independent signals over a single transmission line.

Three predominant ways to multiplex1. Frequency Division Multiplexing (FDM)2. Time Division Multiplexing (TDM)3. Wavelength Division Multiplexing (WDM)



Frequency Division Frequency Division Multiplexing (FDM)Multiplexing (FDM) Predominantly used in analog

communications. Widely used in FM stereo broadcast. Compatibility with mono-receivers. Requires only a slight increase in BW. Great advantage in increasing system

capacity.



Cont’d...Cont’d... Transmitting end of an FDM System



Cont’d...Cont’d... Process

Each signal assigned s different carrier frequency.

Modulated carrier frequencies are combined for transmission over a single line by MUX.

At Rx, a DEMUX separates channels by their frequencies & routes them to the proper end users.



Time Division Time Division Multiplexing (TDM)Multiplexing (TDM) Offers greater system improvement. Capable of combining various protocols &

different types of signals onto a single high speed transmission link. E.g. voice and data

More efficient than FDM



Cont’d...Cont’d... Process

The transmission must be digital in nature. To transmit telephone conversations (voice is

analog signal) Analog signal is converted to digital and

transmitted Reconverted into analog at the Rx telephone.



Cont’d...Cont’d... Drawbacks

Greater complexity. Greater transmission BW required.



Wavelength Division Wavelength Division Multiplexing (WDM)Multiplexing (WDM)

Cost effective way to increase the capacity of fiber optic communications.

Make use of the optical fiber’s available BW by multiplexing many wavelengths along a single mode optical fiber channel.

Each wavelength of light can transmit encoded information at the optimum data rate.



REVISIONREVISION1) Define M-ary Encoding2) Find the number of voltage levels which can

represent an analog signal with a) 8 bit per sampleb) 12 bit per sample

3) Calculate the rate of serial data flow, given the one bit time of 2μs.

4) A modulator transmits symbols, each of which has 64 different possible states, 10K times per second. Determine the bit rate.

REVISION (Cont’d)REVISION (Cont’d)5) If an audio frequency signal is band limited to a

range of 300 Hz to 3.4 kHz, find a) The minimum frequency required to sample the audio signal in order to be transmitted without aliasing. b) The Nyquist interval

6) Digital modulation can be grouped as Amplitude-shift keying (ASK), Frequency-shift keying (FSK) and Phase-shift keying (PSK). Briefly explain each of them with your own words

7) What is a constellation diagram?

REVISION (Cont’d)REVISION (Cont’d) Constellation diagram

Graphical representation of the complex envelope of each possible symbol state.

The x-axis represents the in-phase component and the y-axis represents the quadrature component of the complex envelope.

The distance between signals on a constellation diagram relates to how different the modulation waveforms are and how easily a receiver can differentiate between them.

REVISION (Cont’d)REVISION (Cont’d)8) A typical dial-up telephone connection has a bandwidth of 3

kHz and a signal to noise ratio of 30 dB. Calculate the Shannon limit.

9) For the digital message 1111 0100 1001, sketch the waveform for the following:a. ASKb. FSKc. PSKd. QAM

10)For 16-PSK system, operating with an information bit rate of 32 kbps, determine:a. Baudb. Minimum bandwidthc. Bandwidth efficiency

REVISION (Cont’d)REVISION (Cont’d)11)List down the factors in choosing the digital modulation

technique for a specific application.

a) Error performanceb) Bandwidth efficiencyc) Implementation complexity and costd) Effects of fading, interference and channele) Transmitted power

12)What is the sampling process?

A process of taking samples of information signal at a rate of Nyquist’s sampling frequency.

The original information signal can be reconstructed at the receiver with minimal distortion if the sampling rate follows the Nyquist.

REVISION (Cont’d)REVISION (Cont’d)13)Differentiate between Natural Sampling & Flat top

sampling.

14)Sketch the waveform of pulse modulation forPulse Amplitude Modulation (PAM)Pulse Width Modulation (PWM)Pulse Position Modulation (PPM)

15) A CD audio laser disk has a frequency bandwidth of 20 Hz to 20 kHz. What is the minimum sample rate required to satisfy the Nyquist sampling rate?

REVISION (Cont’d)REVISION (Cont’d)16) What is meant by quantization?

17) Sketch the diagram that shown the quantization error.

18) Explain the companding process.

19) A PCM system requires 72 dB of dynamic range. The input frequency is 10 kHz. (a) Determine the number of sample bits

required.(b) Specify the minimum sample frequency to

satisfy the Nyquist sampling frequency.

Digital Transmission EKT 231 : COMMUNICATION SYSTEM CHAPTER 4 : DIGITAL TRANSMISSION Multiplexing...

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