Chapter 3: Data Communication Fundamentals 3: Data Communication Fundamentals By: Bhargavi Goswami, Sunshine Group of Institutions, Rajkot. Sub: FON, Email: [email protected]

Chapter 3: Data Communication Fundamentals

By: Bhargavi Goswami, Sunshine Group of Institutions, Rajkot. Sub: FON, Email: [email protected] Page 1

Topic List:

• Topic List, Introduction

• Bandwidth & Data Rate

• Analog & Digital Signalling

• Analog & Digital Transmission

• Coding Mechanism

• Modulation

• Modulation in Practice

A. Bandwidth & Data Rate:

(a) Bandwidth:

Represents the range of frequencies that can pass thru a medium.

More the bandwidth larger the data it can transmit.

Fixed

Eg. Wider the road, larger #Vehicles accommodated.

(b) Data Rate:

Data rate is the actual traffic passing at given time.

Purely depends on bandwidth.

Variable.

Eg: Driver asking for infinite speed (bandwidth) for transmission.

Data Rate depends on:

o Bandwidth utilization

o If Analog, #constellation points

o If Digital, Number of levels

o Receiver’s sensitivity

o Media resistance and temperature

B. Frequency & Band:

• Signal travels thru a medium in range of frequencies.

• This range of frequencies is called frequency band.

• If we try to accommodate more radio stations in a band, they become too noisy.

• Similarly for LANs, limits the #users.

C. Media Effect on Bandwidth

• Distortion: Due to elimination of higher frequency components from signal results to distortion.

• Attenuation: Adding more number of different frequency causes attenuation.

• Swimmer’s lane: Middle lanes provide less resistance than outer lanes. Larger the frequency, more

the noise, smaller the frequency, larger the data transmission rate.

D. Harmonics and composite wave:

• Fourier Component: Components of a composite signal are known as Fourier Component.

• Composite Signals: When a signal contains multiple frequencies, it is called composite signals.

• Fundamental Frequency: Frequency of first harmonic (Fourier Component) is knows as Fundamental

Frequency.

• If first component is f, 2nd

is 3f and 3rd

is 5f.

• Frequency is inversely proportional to amplitude.

• Large amplitude signals cover smaller distance.



E. Properties of Channel:

• More the bandwidth of the media, more the number of harmonics that can pass through the media.

• Higher the data rate, less will be the number of harmonics that can pass through the media.

• Baud Rate: #signals that passes thru media in unit time, usually in seconds. As bandwidth is fixed,

Baud Rate is also fixed.

• Bit Rate: #bits that passes thru a media in unit time, usually in seconds. If 1 signal carry n bits then

Baud Rate is 1 and Bit Rate is n.

• Signal: Combination of zeros and ones.

• Maximum Baud Rate: No of harmonics that can be accommodated in a given bandwidth.

• We can obtain Maximum Data Rate (MDR) if we know bandwidth and sensitivity of receiver

(#harmonics recognized correctly).

• Data rate is doubled when baud rate is quadrupled, tripled when baud rate is 8 times, and so on….

F. Baud Rate & Bit Rate Relation:

Nyquist:

• MDR of a channel = 2 × Bandwidth

• MDR of a channel = 2 × Bandwidth × log2 (signal levels)

Claude Shannon:

• MDR of a channel = Bandwidth × log2 (1 + S/N)

Signal to noise ratio (decibel):

• When signal to noice ratio is equal:

• S/N =10 log (Ps/Pn)

• Where, Ps – Signal Power, Pn – Noise Power

• When Ps/Pn becomes 1

• MDR = Bandwidth × log2 (1+10 × log e 1)

= Bandwidth × log 2 (1+0)

= Bandwidth × log 2 1

= 0 (as log21 = 0)



G. Filtering a specific frequency:

• Filter is a device used for restricting bandwidth of a medium to specified limit.

• Used with telephone lines,

▫ Actual bandwidth = 1.1 MHz

▫ Using filter reduced to = 4 kHz

▫ Remaining channel can be used for other telephone connections data transmission at a time.

▫ Remaining channel can also be used for broadband.

▫ That’s y we feel voice interference in laneline phone when internet is on.

• DSL (Digital Subscriber’s Line) removes the filter from line to use entire bandwidth for internet

access.

• Filter Types:

• 1. Low Pass Filter: Such filters keep only lower part of frequency out of given range.

• 2. Band Pass Filter: keeps middle part of frequency.

• 3. High Pass Filter: eliminates lower parts to keep only higher part of frequency.

• Widely used is low pass filter.

H. Analog Signals:

• Used to transmit video and audio signals.

• Higher error rate.

• Continuous by nature.

• Uses curved wave forms.

• Used when we don’t have large bandwidth.

• High error rate due to sine property.

• Amplifiers are used which gives strength to signal but can’t correct the signals.

• Three different properties:

▫ Amplitude: signal’s max voltage value.

▫ Frequency: #oscillations per second.

▫ Phase: direction of a signal

• Can be used to transmit digital data also.

• Also supports modulation techniques.

• Technique to represent zeros and ones using one or more properties of wave is called modulation.



I. Digital Signals:

• Used when we have large bandwidth.

• Used to transfer (0,1) bits generally for file transfer.

• Low error rate.

• Discrete by nature.

• Square wave forms.

• Repeaters are used to give strength to signal which can also correct signals.

• How to transfer digital data over wire?

▫ Using electromagnetic waves

▫ Using light

• MP3 (Music Player), MPEG (Moving Picture Expert Group) are the types where audio video signals

are converted from analog to digital.

• These signals can be of two types: Periodic and A periodic.

• Periodic Signals: Continuously repeats its shape after a specified interval. Used for data

transmission.

• A-Periodic Signals: does not follow pattern.

Digital signaling and bit rate:

• Baud Rate: #voltage signals we send per second is known as baud rate.

• Bit Rate: #bits sent per second are known as bit rate.

• If we use 2n different voltage signals, then

• bit rate = baud rate × n

• If we use n voltage signals, then

• bit rate = baud rate × log2 n

• n is number of levels.

• Suppose we have 4 different voltage levels, say, 1,2,-1,-2.

• 1v = 10, 2v=11, -1v=01, -2v=00.

• This means, 4(voltage) levels can represent 8(combination of 2) bits of data.

• Suppose it takes 0.1 msec then voltage transmitted are 10 Mbps, but total bits transmitted is 20

Mbps.

Characteristics:

• Square waveforms

• Limited discrete values of voltage levels



• Need more bandwidth

• Noise can be completely removed

Repeater reshapes(Digital), amplifier amplifies(Analog):

Amplification: Noise addition to waves

Analog Signal Transmission Digital Signal Transmission

Used to transmit video and audio signals. Used to transfer (0,1) bits generally for file

transfer.

Used when we don’t have large bandwidth. Used when we have large bandwidth.

Higher error rate due to sine property. Low error rate.

Continuous by nature. Discrete by nature.

Uses curved wave forms. Square wave forms.

Can travel short distance. Can travel long distance.

Amplifiers are used which gives strength to signal

but can’t correct the signals.

Repeaters are used to give strength to signal which

can also correct signals.

Analog signals can be used for digital

transmission. E.g.; Modem sends digital data over

analog telephone lines.

Digital signals can be used for analog

transmission. E.g. You tube, Skype transmitting

audio video signals using digital signals.

J. Modulation:

• Amplitude modulation

• Frequency modulation

• Phase Modulation

• Modulation in Practice

• Modulation in cable TV

• Modulation in ADSL



Amplitude modulation:

5 represent 0 and 10 represents 1.

Easy to implement.

Problems:

o Noisy channel, difficult to correct errors.

o Impossible to detect errors also.

o Effects of temperature, rain, etc.

Distortion in digital signals:

Digital error occurs.

Atmospheric disturbance, electromagnetic disturbance (mobile beside tv),

Faulty circuitry, etc results to problems like crosstalk and noise.

It is possible to reshape the signals after some distance using repeaters.

Frequency modulation

Two different frequency representing 0 and 1.f

0 has more oscillations than 1.



Phase Modulation

Out of phase situation for all three signals.

Done by shifting the phase at regular interval.

See next figure.

Justify the statement: Phase modulation is better than other modulation techniques:

• Unlike frequency modulation, it doesn't require two different frequencies. (So better than frequency

modulation)

• If error occurs and amplitude changes, will not have any adverse effect. (Better than amplitude

modulation).

• In Amplitude modulation, gap between two amplitudes must be high, to transfer that signal, more

power is consumed.

• Phase modulation, less power consumption.

• Thus, Phase modulation is better than other modulation techniques.

Modulation in Practice

(a) Modulation in cable TV

(b) Modulation in ADSL

Constellation pattern for QPSK:

• QPSK: Quadrature Phase Shift Keying.

• Total #bits sent are twice the total #signals. (i.e. while sending x signals, 2x bits sent).

• Only mechanism where, amplitude is not affected.

• Four phase values: 45*, 135*, 225* and 315*.

• Distance from origin indicates amplitude (constant).



QAM-16 constellation pattern

• QAM-16 Quadrature Amplification Modulation.

• Combination of phase and amplitude.

• 16 combinations represented by different combination of phase and amplitude.

• Arrow represents phase.

• Curves represent amplitude.

• 3 amplitude and 3 phase values represent 4 constellation points in one quadrant.

• Such 4 quadrants (4x4) makes it 16.

• Similarly we can have QAM-64 using 16 bits in each quadrant and QAM-256 with 64 bits in each

quadrant.

• Disadvantage: More the constellation points in constellation diagram, more vulnerable to noise.

Vulnerability increases from QPSK, QAM-16, QAM-64, QAM-256...

• Thus, prefer QPSK for noisy channels and prefer QAM-256 for noiseless channel.

(a) Modulation in Cable TV:

Cable TV uses analog signals to transmit.

Thick black TV cables having bandwidth 650 to 700 Mbps has much spare bandwidth other than

utilized by TV channel signals that can be utilized for Internet traffic.

This brought the era of Cable Internet.

See the next figure.

Up-stream uses QPSK, downstream transmission uses QAM-16 or QAM-256.

Frequency bands used in cable TV transmission:



(b) Modulation in ADSL:

Asymmetric Digital Subscriber Line.

Technique that uses telephone lines for voice communication as well as internet connectivity.

Telephone lines use small fraction of bandwidth.

i.e 4KHz out of 1.1 MHz.

Filters are kept to use remaining bandwidth for data transmission over internet.

If no telephone needed, filters are removed and entire bandwidth is used for internet.

Even division of bandwidth used for internet traffic need to be distributed into Upstream and

Downstream data transmission.

When division is equal for Upstream and Downstream, SDSL Symmetric Digital Subscriber Line is

used.

Generally preferred is more downstream and less upstream bandwidth for which A-symmetric

Digital Subscriber Line is used.

See previous figure for distribution.

Q. What is Multiplexing and Demultiplexing?

The process of collecting individual connections at one end and distributing at the other end is known as

multiplexing and demultiplexing.

Examples:

o Proxy Server multiplexes users URL request and sends to next router in chain

o Windows Server , Linux Server, client.

o Remote telnet server

o FTP Client connecting to FTP Server

o HTTP Client talking to Web Server

o Oracle or SQL Client connecting to Remote Database Server.

o Single TCP Process handling multiple Applications

o UDP application running VoIP and video conferencing application.

o A switch relies on multiple desktops in LAN relies on multiplexing to combine all transmission.

Two types:

o Time Division Multiplexing

o Frequency Division Multiplexing

o Time Division Multiplexing: TDM divides available spectrum such that each user gets a specific

time slot to transmit its data. Each sender gets its turn once in a cycle. When specific user’s turn

comes, entire bandwidth is given to him. This multiplexing does not impose any kind of delay.

TDM is useful when the data is transmitted using digital signaling.

o Frequency Division Multiplexing: in FDM, each user is given an exclusive frequency band. Each

channel is given a specific frequency. The entire spectrum available is multiplexed for all these

Stations. While multiplexing, the channel frequencies are separated by guard bands to prevent

interference from adjacent stations. FDM is used with analog signaling for data transmission.

o WDM: A variant of FDM known as WDM (Wavelength Division Multiplexing ) is famous for

fiber optics which provides 100 channels of 10 Gbps each. WDM also provides quality of

service.

o Guard Bands: While multiplexing, the channel frequencies are separated by guard bands to

prevent interference from adjacent station.



Q. Write a note on switching.

Switching is an operation where the decision of where to send the incoming data is instantaneous and

predetermined. In routing, decision is taken dynamically and hence service is little delayed than

switching.

Switching is generally done at second layer.

Layer 4 and Layer 5 are transport and application layer where data are populated.

Layer 4 switch separate UDP and TCP data flows.

Layer 5 switch send mail traffic in one direction and http traffic in another direction.

Layer 3 switches are like routers.

Telephone Lines use switches to route the traffic at physical layer.

Though switching is faster, it is monotonous.

Data does not flow continuously and so, it needs store and forwarding transmission facility.

Types: 1) Circuit Switching 2) Packet Switching 3) Message Switching.

1) Circuit Switching:

In circuit switching, there is a dedicated line that connects the sender and the receiver.

Here, path is reserved for duration of connection.

Circuit Switching

Packet Switching



Eg. Telephonic Conversation.

Advantages of Circuit Switching:

o Guaranteed bandwidth

o Predictable communication performance

o Not “best-effort” delivery with no real guarantees

o Simple abstraction

o Reliable communication channel between hosts

o No worries about lost or out-of-order packets

o Simple forwarding

o Forwarding based on time slot or frequency

o No need to inspect a packet header

o Low per-packet overhead

o Forwarding based on time slot or frequency

o No IP (and TCP/UDP) header on each packet

Disadvantages of Circuit Switching:

o Wasted bandwidth

o Bursty traffic leads to idle connection during silent period

o Unable to achieve gains from statistical multiplexing

o Blocked connections

o Connection refused when resources are not sufficient

o Unable to offer “okay” service to everybody

o Connection set-up delay

o No communication until the connection is set up

o Unable to avoid extra latency for small data transfers

o Network state

o Network nodes must store per-connection information

o Unable to avoid per-connection storage and state



2) Message Switching:

It is possible to have packets that are large enough to hold entire message. Such a mechanism is not in

practice is known as message switching.

Not used nowadays.

Communication is not continuous.

Each packets travel independently.

No physical path is established in advance between sender and receiver.

Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e.,

router) and then forwarded later, one hop at a time.

Each block is received in its entirety, inspected for errors, and then retransmitted.

A network using this technique is called a store-and-forward network.

Used with telegrams.

3) Packet Switching:

Doesn’t reserve entire path in advance.

Reserves only small portion of path.

Data is segregated into pieces and transmitted as packets.

In packet-based networks, the message gets broken into small data packets. These packets are sent out

from the computer and they travel around the network seeking out the most efficient route to travel as

circuits become available. This does not necessarily mean that they seek out the shortest route.



Each packet may go a different route from the others.

Each packet is sent with a ‘header address’. This tells it where its final destination is, so it knows where

to go.

The header address also describes the sequence for reassembly at the destination computer so that the

packets are put back into the correct order.

One packet also contains details of how many packets should be arriving so that the recipient computer

knows if one packet has failed to turn up.

If a packet fails to arrive, the recipient computer sends a message back to the computer which originally

sent the data, asking for the missing packet to be resent.

Advantages:

o Security

o Bandwidth used to full potential

o Devices of different speeds can communicate

o Not affected by line failure (rediverts signal)

o Availability – do not have to wait for a direct connection to become available

o During a crisis or disaster, when the public telephone network might stop working, e-mails and

texts can still be sent via packet switching

Disadvantages

o Under heavy use there can be a delay

o Data packets can get lost or become corrupted

o Protocols are needed for a reliable transfer

o Not so good for some types data streams e.g real-time video streams can lose frames due to the

way packets arrive out of sequence.

Q. Differentiate between Circuit Switching and Packet Switching.



Q. Define Attenuation, Distortion and Noise:

1) Attenuation: Change in shape of a signal due to loss of energy after travelling some distance. Different

fouier components have different attenuation and thus the resultant signal not only gets diminished, but also

acquired signal requires reshaping. This is known as attenuation. Attenuated signals require reshaping.

Attenuation also depends on media.

2) Distortion: A signal is said to be distorted when different frequency components reach at the other end at

different times. This effect is known as distortion. The speed mismatch between different components

changes the shape of the signals. This is known as delayed distortion or distortion in short.

3) Noise: It is the error produced in a signal due to various reasons. Noise is the external interference which

changes the shape of the signal.

Thermal Noise: The disturbance caused due to electrons present in the media. This noise depends on the

temperature. More the temperature, more is the thermal Noise.

Non-Thermal Noise: It is caused due to other external reasons like cross-talking, malfunctioning of devices,

etc.

Noise is also used as a synonym of errors. Eg. An image with noise means an image with errors.

Documents

Chapter 3: Data Communication Fundamentals 3: Data Communication Fundamentals By: Bhargavi Goswami, Sunshine Group of Institutions, Rajkot. Sub: FON, Email: [email protected]