UNIT III

DATA COMMUNICATION

INTRODUCTION

Data Communication is a process of exchanging data or information In case of computer

networks this exchange is done between two devices over a transmission medium. This process

involves a communication system which is made up of hardware and software. The hardware

part involves the sender and receiver devices and the intermediate devices through which the

data passes. The software part involves certain rules which specify what is to be communicated,

how it is to be communicated and when. It is also called as a Protocol. The following sections

describe the fundamental characteristics that are important for the effective working of data

communication process and is followed by the components that make up a data communications

system.

BASIC TERMS AND CONCEPTS

Delivery: The data should be delivered to the correct destination and correct user.

2. Accuracy: The communication system should deliver the data accurately, without introducing

any errors. The data may get corrupted during transmission affecting the accuracy of the

delivered data.

3. Timeliness: Audio and Video data has to be delivered in a timely manner without any delay;

such a data delivery is called real time transmission of data.

4. Jitter: It is the variation in the packet arrival time. Uneven Jitter may affect the timeliness of

data being transmitted.

Components of a Data Communication System

1. Message Message is the information to be communicated by the sender to the receiver.

2. Sender The sender is any device that is capable of sending the data (message).

3. Receiver The receiver is a device that the sender wants to communicate the data (message).

4. Transmission Medium It is the path by which the message travels from sender to receiver. It

can be wired or wireless and many subtypes in both.

5. Protocol It is an agreed upon set or rules used by the sender and receiver to communicate data.

A protocol is a set of rules that governs data communication. A Protocol is a necessity in data

communications without which the communicating entities are like two persons trying to talk to

each other in a different language without know the other language.

TOPOLOGY

The transmission medium layout used to link devices is the physical topology of the network.

For conductive or fiber optical mediums, this refers to the layout of cabling, the locations of

nodes, and the links between the nodes and the cabling.The physical topology of a network is

determined by the capabilities of the network access devices and media, the level of control or

fault tolerance desired, and the cost associated with cabling or telecommunication circuits.

In contrast, logical topology is the way that the signals act on the network media, or the

way that the data passes through the network from one device to the next without regard to the

physical interconnection of the devices. A network's logical topology is not necessarily the same

as its physical topology. For example, the original twisted pair Ethernet using repeater hubs was

a logical bus topology carried on a physical star topology. Token ring is a logical ring topology,

but is wired as a physical star from the media access unit. Physically, AFDX can be a cascaded

star topology of multiple dual redundant Ethernet switches; however, the AFDX Virtual links are

modeled as time-switched single-transmitter bus connections, thus following the safety model of

a single-transmitter bus topology previously used in aircraft. Logical topologies are often closely

associated with media access control methods and protocols. Some networks are able to

dynamically change their logical topology through configuration changes to their routers and

switches.

Wired Technology

The orders of the following wired technologies are, roughly, from slowest to fastest transmission

speed.

Coaxial cable is widely used for cable television systems, office buildings, and other work-

sites for local area networks. The cables consist of copper or aluminum wire surrounded by

an insulating layer (typically a flexible material with a high dielectric constant), which itself

is surrounded by a conductive layer. The insulation helps minimize interference and

distortion. Transmission speed ranges from 200 million bits per second to more than 500

million bits per second.

ITU-T G.hn technology uses existing home wiring (coaxial cable, phone lines and power

lines) to create a high-speed (up to 1 Gigabit/s) local area network.

Signal traces on printed circuit boards are common for board-level serial communication,

particularly between certain types integrated circuits, a common example being SPI.

Ribbon cable (untwisted and possibly unshielded) has been a cost-effective media for serial

protocols, especially within metallic enclosures or rolled within copper braid or foil, over

short distances, or at lower data rates. Several serial network protocols can be deployed

without shielded or twisted pair cabling, that is, with "flat" or "ribbon" cable, or a hybrid

flat/twisted ribbon cable, should EMC, length, and bandwidth constraints permit.

Twisted pair wire is the most widely used medium for all telecommunication. Twisted-pair

cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires consist

of two insulated copper wires twisted into pairs. Computer network cabling

(wired Ethernet as defined by IEEE 802.3) consists of 4 pairs of copper cabling that can be

utilized for both voice and data transmission. The use of two wires twisted together helps to

reduce crosstalk and electromagnetic induction. The transmission speed ranges from 2

million bits per second to 10 billion bits per second. Twisted pair cabling comes in two

forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in

several category ratings, designed for use in various scenarios.

An optical fiber is a glass fiber. It carries pulses of light that represent data. Some advantages

of optical fibers over metal wires are very low transmission loss and immunity from

electrical interference. Optical fibers can simultaneously carry multiple wavelengths of light,

which greatly increases the rate that data can be sent, and helps enable data rates of up to

trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high

data rates, and are used for undersea cables to interconnect continents.

Price is a main factor distinguishing wired- and wireless-technology options in a business.

Wireless options command a price premium that can make purchasing wired computers, printers

and other devices a financial benefit. Before making the decision to purchase hard-wired

technology products, a review of the restrictions and limitations of the selections is necessary.

Business and employee needs may override any cost considerations

Wireless Technology

Terrestrial microwave – Terrestrial microwave communication uses Earth-based transmitters

and receivers resembling satellite dishes. Terrestrial microwaves are in the low gigahertz

range, which limits all communications to line-of-sight. Relay stations are spaced

approximately 50 km (30 mi) apart.

Communications satellites – Satellites communicate via microwave radio waves, which are

not deflected by the Earth's atmosphere. The satellites are stationed in space, typically

in geostationary orbit 35,786 km (22,236 mi) above the equator. These Earth-orbiting

systems are capable of receiving and relaying voice, data, and TV signals.

Cellular and PCS systems use several radio communications technologies. The systems

divide the region covered into multiple geographic areas. Each area has a low-power

transmitter or radio relay antenna device to relay calls from one area to the next area.

Radio and spread spectrum technologies – Wireless local area networks use a high-frequency

radio technology similar to digital cellular and a low-frequency radio technology. Wireless

LANs use spread spectrum technology to enable communication between multiple devices in

a limited area. IEEE 802.11 defines a common flavor of open-standards wireless radio-wave

technology known as Wi-Fi.

Free-space optical communication uses visible or invisible light for communications. In most

cases, line-of-sight propagation is used, which limits the physical positioning of

communicating devices.

The study of network topology recognizes eight basic topologies: point-to-point, bus, star, ring or

circular, mesh, tree, hybrid, or daisy chain..

Point-to-point

The simplest topology with a dedicated link between two endpoints. Easiest to understand, of the

variations of point-to-point topology, is a point-to-point communication channel that appears, to

the user, to be permanently associated with the two endpoints. A child's tin can telephone is one

example of a physical dedicated channel.

Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up

dynamically and dropped when no longer needed. Switched point-to-point topologies are the

basic model of conventional telephony.

The value of a permanent point-to-point network is unimpeded communications between the two

endpoints. The value of an on-demand point-to-point connection is proportional to the number of

potential pairs of subscribers and has been expressed as Metcalfe's Law.

Daisy chain

Daisy chaining is accomplished by connecting each computer in series to the next. If a message

is intended for a computer partway down the line, each system bounces it along in sequence until

it reaches the destination. A daisy-chained network can take two basic forms: linear and ring.

A linear topology puts a two-way link between one computer and the next. However, this

was expensive in the early days of computing, since each computer (except for the ones at

each end) required two receivers and two transmitters.

By connecting the computers at each end of the chain, a ring topology can be formed. When

a node sends a message, the message is processed by each computer in the ring. An

advantage of the ring is that the number of transmitters and receivers can be cut in half. Since

a message will eventually loop all of the way around, transmission does not need to go both

directions. Alternatively, the ring can be used to improve fault tolerance. If the ring breaks at

a particular link then the transmission can be sent via the reverse path thereby ensuring that

all nodes are always connected in the case of a single failure.

Bus

Bus network topology

In local area networks using bus topology, each node is connected by interface connectors to a

single central cable. This is the 'bus', also referred to as the backbone, or trunk) –

all data transmitted between nodes in the network is transmitted over this common transmission

medium and is able to be received by all nodes in the network simultaneously.

A signal containing the address of the intended receiving machine travels from a source machine

in both directions to all machines connected to the bus until it finds the intended recipient, which

then accepts the data. If the machine address does not match the intended address for the data,

the data portion of the signal is ignored. Since the bus topology consists of only one wire it is

less expensive to implement than other topologies, but the savings are offset by the higher cost of

managing the network. Additionally, since the network is dependent on the single cable, it can be

the single point of failure of the network. In this topology data being transferred may be accessed

by any node.

Linear bus

In a linear bus network, all of the nodes of the network are connected to a common transmission

medium which has just two endpoints. When the electrical signal reaches the end of the bus, the

signal is reflected back down the line, causing unwanted interference. To prevent this, the two

endpoints of the bus are normally terminated with a device called a terminator.

Distributed bus

In a distributed bus network, all of the nodes of the network are connected to a common

transmission medium with more than two endpoints, created by adding branches to the main

section of the transmission medium – the physical distributed bus topology functions in exactly

the same fashion as the physical linear bus topology because all nodes share a common

transmission medium.

Star

Star network topology

In star topology, every peripheral node (computer workstation or any other peripheral) is

connected to a central node called a hub or switch. The hub is the server and the peripherals are

the clients. The network does not necessarily have to resemble a star to be classified as a star

network, but all of the peripheral nodes on the network must be connected to one central hub. All

traffic that traverses the network passes through the central hub, which acts as a signal repeater.

The star topology is considered the easiest topology to design and implement. One advantage of

the star topology is the simplicity of adding additional nodes. The primary disadvantage of the

star topology is that the hub represents a single point of failure. Also, since all peripheral

communication must flow through the central hub, the aggregate central bandwidth forms a

network bottleneck for large clusters.

Extended star

The extended star network topology extends a physical star topology by one or more repeaters

between the central node and the peripheral (or 'spoke') nodes. The repeaters are used to extend

the maximum transmission distance of the physical layer, the point-to-point distance between the

central node and the peripheral nodes. Repeaters allow greater transmission distance, further than

would be possible using just the transmitting power of the central node. The use of repeaters can

also overcome limitations from the standard upon which the physical layer is based.

A physical extended star topology in which repeaters are replaced with hubs or switches is a type

of hybrid network topology and is referred to as a physical hierarchical star topology, although

some texts make no distinction between the two topologies.

A physical hierarchical star topology can also be referred as a tier-star topology, this topology

differs from a tree topology in the way star networks are connected together. A tier-star topology

uses a central node, while a tree topology uses a central bus and can also be referred as a star-bus

network.

Distribute

A distributed star is a network topology that is composed of individual networks that are based

upon the physical star topology connected in a linear fashion – i.e., 'daisy-chained' – with no

central or top level connection point (e.g., two or more 'stacked' hubs, along with their associated

star connected nodes or 'spokes').

Ring

Ring network topology

A ring topology is a bus topology in a closed loop. Data travels around the ring in one direction.

When one node sends data to another, the data passes through each intermediate node on the ring

until it reaches its destination. The intermediate nodes repeat (re transmit) the data to keep the

signal strong. Every node is a peer; there is no hierarchical relationship of clients and servers. If

one node is unable to re transmit data, it severs communication between the nodes before and

after it in the bus.

Advantages:

When the load on the network increases, its performance is better than bus topology.

There is no need of network server to control the connectivity between workstations.

Disadvantages:

Aggregate network bandwidth is bottlenecked by the weakest link between two nodes.

Mesh

The value of fully meshed networks is proportional to the exponent of the number of subscribers,

assuming that communicating groups of any two endpoints, up to and including all the endpoints,

is approximated by Reed's Law.

Fully connected network

Fully connected mesh topology

In a fully connected network, all nodes are interconnected. (In graph theory this is called

a complete graph.) The simplest fully connected network is a two-node network. A fully

connected network doesn't need to use packet switching or broadcasting. However, since the

number of connections grows quadratically with the number of nodes:

This makes it impractical for large networks. This kind of topology does not trip and affect other

nodes in the network.

Partially connected network

Partially connected mesh topology

In a partially connected network, certain nodes are connected to exactly one other node; but

some nodes are connected to two or more other nodes with a point-to-point link. This makes it

possible to make use of some of the redundancy of mesh topology that is physically fully

connected, without the expense and complexity required for a connection between every node in

the network.

Hybrid

Hybrid topology is also known as hybrid network. Hybrid networks combine two or more

topologies in such a way that the resulting network does not exhibit one of the standard

topologies (e.g., bus, star, ring, etc.). For example, a tree network (or star-bus network) is a

hybrid topology in which star networks are interconnected via bus networks. However, a tree

network connected to another tree network is still topologically a tree network, not a distinct

network type. A hybrid topology is always produced when two different basic network

topologies are connected.

A star-ring network consists of two or more ring networks connected using a multistation access

unit (MAU) as a centralized hub.

Snowflake topology is a star network of star networks.[Two other hybrid network types

are hybrid mesh and hierarchical star.

MODEM STANDARDS AND TYPES:

The devices communicate with each other by sending and receiving data. The data can flow

between the two devices in the following ways.

1. Simplex

2. Half Duplex

3. Full Duplex

Simplex

In Simplex, communication is unidirectional Only one of the devices sends the data and the other

one only receives the data. Example: in the above diagram: a cpu send data while a monitor only

receives data.

Half Duplex

In half duplex both the stations can transmit as well as receive but not at the same time. When one

device is sending other can only receive and viceversa (as shown in figure above.) Example: A

walkie-talkie.

Full Duplex

In Full duplex mode, both stations can transmit and receive at the same time. Example: mobile

phones

BASEBAND TRANSMISSION

The signal is transmitted without making any change to it (ie. Without modulation)

In baseband transmission, the bandwidth of the signal to be transmitted has to be less than the

bandwidth of the channel. Ex. Consider a Baseband channel with lower frequency 0Hz and higher

frequency 100Hz, hence its bandwidth is 100 (Bandwidth is calculated by getting the difference

between the highest and lowest frequency). We can easily transmit a signal with frequency below

100Hz, such a channel whose bandwidth is more than the bandwidth of the signal is called

Wideband channel Logically a signal with frequency say 120Hz will be blocked resulting in loss

of information, such a channel whose bandwidth is less than the bandwidth of the signal is called

Narrowband channel.

BROAD BAND TRANSMISSION

Given a bandpass channel, a digital signal cannot be transmitted directly through it In broadband

transmission we use modulation, i.e we change the signal to analog signal before transmitting it.

The digital signal is first converted to an analog signal, since we have a bandpass channel we

cannot directly send this signal through the available channel. Ex. Consider the bandpass channel

with lower frequency 50Hz and higher frequency 80Hz, and the signal to be transmitted has

frequency 10Hz. To pass the analog signal through the bandpass channel, the signal is modulated

using a carrier frequency. Ex. The analog signal (10Hz) is modulated by a carrier frequency of

50Hz resulting in an signal of frequency 60Hz which can pass through our bandpass channel. The

signal is demodulated and again converted into an digital signal at the other end as shown in the

figure below.

ENCODING AND MODULATING

The most fundamental digital modulation techniques are based on keying:

PSK (phase-shift keying): a finite number of phases are used.

FSK (frequency-shift keying): a finite number of frequencies are used.

ASK (amplitude-shift keying): a finite number of amplitudes are used.

QAM (quadrature amplitude modulation): a finite number of at least two phases and at least

two amplitudes are used.

In QAM, an in-phase signal (or I, with one example being a cosine waveform) and a quadrature

phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite

number of amplitudes and then summed. It can be seen as a two-channel system, each channel

using ASK. The resulting signal is equivalent to a combination of PSK and ASK.

In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique

pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of

bits. This number of bits comprises the symbol that is represented by the particular phase,

frequency or amplitude.

If the alphabet consists of  alternative symbols, each symbol represents a message

consisting of N bits. If the symbol rate (also known as the baud rate) is symbols/second (or baud),

the data rate is bit/second.

For example, with an alphabet consisting of 16 alternative symbols, each symbol

represents 4 bits. Thus, the data rate is four times the baud rate.

In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is

constant, the modulation alphabet is often conveniently represented on a constellation diagram,

showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis,

for each symbol.

PSK and ASK, and sometimes also FSK, are often generated and detected using the

principle of QAM. The I and Q signals can be combined into a complex-

valued signal I+jQ (where j is the imaginary unit). The resulting so called equivalent lowpass

signal or equivalent baseband signal is a complex-valued representation of the real-

valued modulated physical signal (the so-called passband signal or RF signal).

These are the general steps used by the modulator to transmit data:

1. Group the incoming data bits into codewords, one for each symbol that will be transmitted.

2. Map the codewords to attributes, for example, amplitudes of the I and Q signals (the

equivalent low pass signal), or frequency or phase values.

3. Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum

of the equivalent low pass signal, typically using digital signal processing.

4. Perform digital to analog conversion (DAC) of the I and Q signals (since today all of the

above is normally achieved using digital signal processing, DSP).

5. Generate a high-frequency sine carrier waveform, and perhaps also a cosine quadrature

component. Carry out the modulation, for example by multiplying the sine and cosine

waveform with the I and Q signals, resulting in the equivalent low pass signal being

frequency shifted to the modulated passband signal or RF signal. Sometimes this is

achieved using DSP technology, for example direct digital synthesis using a waveform

table, instead of analog signal processing. In that case, the above DAC step should be

done after this step.

6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic

spectrum.

At the receiver side, the demodulator typically performs:

1. Bandpass filtering.

2. Automatic gain control, AGC (to compensate for attenuation, for example fading).

3. Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an

intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator

sine wave and cosine wave frequency (see the superheterodyne receiver principle).

4. Sampling and analog-to-digital conversion (ADC) (sometimes before or instead of the

above point, for example by means of undersampling).

5. Equalization filtering, for example, a matched filter, compensation for multipath

propagation, time spreading, phase distortion and frequency selective fading, to

avoid intersymbol interference and symbol distortion.

6. Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF

signal.

7. Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol

values.

8. Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups).

9. Parallel-to-serial conversion of the codewords into a bit stream.

10.Pass the resultant bit stream on for further processing such as removal of any error-

correcting codes.

As is common to all digital communication systems, the design of both the modulator and

demodulator must be done simultaneously. Digital modulation schemes are possible because the

transmitter-receiver pair has prior knowledge of how data is encoded and represented in the

communications system. In all digital communication systems, both the modulator at the

transmitter and the demodulator at the receiver are structured so that they perform inverse

operations.

Asynchronous methods do not require a receiver reference clock signal that is phase

synchronized with the sender carrier signal. In this case, modulation symbols (rather than bits,

characters, or data packets) are asynchronously transferred. The opposite is synchronous

modulation.

TRANSMISSION IMPAIRMENTS

In communication system, analog signals travel through transmission media, which tends

to deteriorate the quality of analog signal. This imperfection causes signal impairment. This

means that received signal is not same as the signal that was send.

Attenuation – It means loss of energy. The strength of signal decreases with increasing distance

which causes loss of energy in overcoming resistance of medium. This is also known as attenuated

signal. Amplifiers are used to amplify the attenuated signal which gives the original signal back.

Attenuation is measured in decibels(dB). It measures the relative strengths of two signals or

one signal at two different point.

Attenuation(dB) = 10log10(P2/P1)

P1 is power at sending end and P2 is power at receiving end.

Distortion – It means change in the shape of signal. This is generally seen in composite

signals with different frequencies. Each frequency component has its own propagation speed

travelling through a medium. Every component arrive at different time which leads to delay

distortion. Therefore, they have different phases at receiver end from what they had at

senders end.

Noise – The random or unwanted signal that mixes up with the original signal is called

noise. There are several types of noise such as induced noise, crosstalk noise, thermal noise

and impulse noise which may corrupt the signal.

Induced noise comes from sources such as motors and appliances. These devices act as

sending antenna and transmission medium act as receiving antenna. Thermal noise is

movement of electrons in wire which creates an extra signal. Crosstalk noise is when one

wire affects the other wire. Impulse noise is a signal with high energy that comes from

lightning or power lines.

Multiplexing:

In telecommunications and computer networks, multiplexing (sometimes

contracted to mixing) is a method by which multiple analog message signals or digital data

streams are combined into one signal over a shared medium. The aim is to share an expensive

resource. For example, in telecommunications, several telephone calls may be carried using one

wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in

communications. In telephony, George Owen Squire is credited with the development of

telephone carrier multiplexing in 1910.

The multiplexed signal is transmitted over a communication channel, which may be a

physical transmission medium (e.g. a cable). The multiplexing divides the capacity of the high-

level communication channel into several low-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.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that

performs the reverse process is called a demultiplexer (DEMUX or DMX). Inverse

multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into

several streams, transfer them simultaneously over several communication channels, and recreate

the original data stream.

Multiplexing technologies may be divided into several types, all of which have significant

variations: space-division multiplexing (SDM), frequency-division multiplexing (FDM), time-

division multiplexing (TDM), and code division multiplexing(CDM).

Multiple variable bit rate digital bit streams may be transferred efficiently over a single

fixed bandwidth channel by means of statistical multiplexing, for example packet

mode communication. Packet mode communication is an asynchronous mode time-domain

multiplexing which resembles time-division multiplexing.

Digital bit streams can be transferred over an analog channel by means of code-division

multiplexing techniques such as frequency-hopping spread spectrum (FHSS) and direct-sequence

spread spectrum (DSSS).

In wireless communications, multiplexing can also be accomplished through

alternating polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent

channel and satellite, or through phased multi-antenna array combined with a multiple-input

multiple-output communications (MIMO) scheme.

Space-division multiplexing

In wired communication, space-division multiplexing simply implies different point-to-

point wires for different channels. Examples include an analogue stereo audio cable, with one pair

of wires for the left channel and another for the right channel, and a multipart  telephone cable.

Another example is a switched star network such as the analog telephone access network

(although inside the telephone exchange or between the exchanges, other multiplexing techniques

are typically employed) or a switched Ethernet network. A third example is a mesh network.

Wired space-division multiplexing is typically not considered as multiplexing.

In wireless communication, space-division multiplexing is achieved by multiple antenna

elements forming a phased array antenna. Examples are multiple-input and multiple-

output (MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output

(MISO) multiplexing. For example, an IEEE 802.11n wireless router with k number of antennas

makes it in principle possible to communicate with k multiplexed channels, each with a peak bit

rate of 54 Mbit/s, thus increasing the total peak bit rate with a factor k. Different antennas would

give different multi-path propagation (echo) signatures, making it possible for digital signal

processing techniques to separate different signals from each other. These techniques may also be

utilized for space diversity (improved robustness to fading) or beam forming (improved

selectivity) rather than multiplexing.

Frequency-division multiplexing

Frequency-division multiplexing (FDM): The spectrum of each input signal is shifted to a distinct

frequency range.

Frequency-division multiplexing (FDM) is inherently an analog technology. FDM

achieves the combining of several signals into one medium by sending signals in several distinct

frequency ranges over a single medium.

One of FDM's most common applications is the old traditional radio and television

broadcasting from terrestrial, mobile or satellite stations, using the natural atmosphere of Earth, or

the cable television. Only one cable reaches a customer's residential area, but the service provider

can send multiple television channels or signals simultaneously over that cable to all subscribers

without interference. Receivers must tune to the appropriate frequency (channel) to access the

desired signal.

FREQUENCY DIVISION MULTIPLEXING

In telecommunications, frequency-division multiplexing (FDM) is a technique by which

the total bandwidth available in a communication medium is divided into a series of non-

overlapping frequency sub-bands, each of which is used to carry a separate signal. These sub-

bands can be used independently with completely different information streams, or used

dependently in the case of information sent in a parallel stream. This allows a single transmission

medium such as the radio spectrum, a cable or optical fiber to be shared by multiple separate

signals.

The most natural example of frequency-division multiplexing is radio and

television broadcasting, in which multiple radio signals at different frequencies pass through the

air at the same time. Another example is cable television, in which many television channels are

carried simultaneously on a single cable. FDM is also used by telephone systems to transmit

multiple telephone calls through high capacity trunk lines, communications satellites to transmit

multiple channels of data on uplink and downlink radio beams, and broadband DSL modems to

transmit large amounts of computer data through twisted pair telephone lines, among many other

uses.

An analogous technique called wavelength division multiplexing is used in fiber optic

communication, in which multiple channels of data are transmitted over a single optical

fiber using different wavelengths (frequencies) of light.

In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology

which multiplexes a number of optical carrier signals onto a single optical fiber by using

different wavelengths (i.e., colors) of laser light. This technique

enables bidirectional communications over one strand of fiber, as well as multiplication of

capacity.

The term wavelength-division multiplexing is commonly applied to an optical carrier

(which is typically described by its wavelength), whereas frequency-division

multiplexing typically applies to a radio carrier (which is more often described by frequency).

Since wavelength and frequency are tied together through a simple directly inverse relationship, in

which the product of frequency and wavelength equals c (the propagation speed of light), the two

terms actually describe the same concept.

A WDM system uses a multiplexer at the transmitter to join the signals together, and

a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have

a device that does both simultaneously, and can function as an optical add-drop multiplexer. The

optical filtering devices used have conventionally been etalons (stable solid-state single-

frequency Fabry–Pérot interferometers in the form of thin-film-coated optical glass).

The concept was first published in 1978, and by 1980 WDM systems were being realized

in the laboratory. The first WDM systems combined only two signals. Modern systems can handle

up to 160 signals and can thus expand a basic 10Gbit/s system over a single fiber pair to over

1.6 Tbit/s.

WDM systems are popular with telecommunications companies because they allow them

to expand the capacity of the network without laying more fiber. By using WDM and optical

amplifiers, they can accommodate several generations of technology development in their optical

infrastructure without having to overhaul the backbone network. Capacity of a given link can be

expanded simply by upgrading the multiplexers and demultiplexers at each end.

TDM

Time-division multiplexing (TDM) is a method of transmitting and receiving independent

signals over a common signal path by means of synchronized switches at each end of the

transmission line so that each signal appears on the line only a fraction of time in an alternating

pattern. 

Time-division multiplexing is used primarily for digital signals, but may be applied

in analog multiplexing in which two or more signals or bit streams are transferred appearing

simultaneously as sub-channels in one communication channel, but are physically taking turns on

the channel. The time domain is divided into several recurrent time slots of fixed length, one for

each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time slot 1,

sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel

plus a synchronization channel and sometimes error correction channel before the

synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts

all over again with a new frame, starting with the second sample, byte or data.

In telecommunication and radio communication, spread-spectrum techniques are

methods by which a signal (e.g. an electrical, electromagnetic, or acoustic signal) generated with a

particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a

wider bandwidth. These techniques are used for a variety of reasons, including the establishment

of secure communications, increasing resistance to natural interference, noise and jamming, to

prevent detection, and to limit power flux density (e.g. in satellite downlinks).

Spread-spectrum telecommunications: This is a technique in which a

telecommunication signal is transmitted on a bandwidth considerably larger than

the frequency content of the original information. Frequency hopping is a basic modulation

technique used in spread spectrum signal transmission.

Spread-spectrum telecommunications is a signal structuring technique that employs direct

sequence, frequency hopping, or a hybrid of these, which can be used for multiple access and/or

multiple functions. This technique decreases the potential interference to other receivers while

achieving privacy. Spread spectrum generally makes use of a sequential noise-like signal structure

to spread the normally narrowband information signal over a relatively wideband (radio) band of

frequencies. The receiver correlates the received signals to retrieve the original information signal.

Originally there were two motivations: either to resist enemy efforts to jam the communications

(anti-jam, or AJ), or to hide the fact that communication was even taking place, sometimes

called low probability of intercept (LPI).

Frequency-hopping spread spectrum (FHSS), direct-sequence spread

spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and

combinations of these techniques are forms of spread spectrum. Each of these techniques employs

pseudorandom number sequences — created using pseudorandom number generators — to

determine and control the spreading pattern of the signal across the allocated bandwidth. Ultra-

wideband (UWB) is another modulation technique that accomplishes the same purpose, based on

transmitting short duration pulses. Wireless standard IEEE 802.11 uses either FHSS or DSSS in

its radio interface.

ERROR DETECTION AND CONTROL

Network is responsible for transmission of data from one device to another device. The end to end

transfer of data from a transmitting application to a receiving application involves many steps,

each subject to error. With the error control process, we can be confident that the transmitted and

received data are identical. Data can be corrupted during transmission. For reliable

communication, error must be detected and corrected.

Error control is the process of detecting and correcting both the bit level and packet level errors.

Types of Errors

Single Bit Error

The term single bit error means that only one bit of the data unit was changed from 1 to 0 and 0 to

1.

Burst Error

In term burst error means that two or more bits in the data unit were changed. Burst error is also

called packet level error, where errors like packet loss, duplication, reordering.

Error Detection

Error detection is the process of detecting the error during the transmission between the sender

and the receiver.

Types of error detection

Parity checking

Cyclic Redundancy Check (CRC)

Checksum

Redundancy

Redundancy allows a receiver to check whether received data was corrupted during transmission.

So that he can request a retransmission. Redundancy is the concept of using extra bits for use in

error detection. As shown in the figure sender adds redundant bits (R) to the data unit and sends to

receiver, when receiver gets bits stream and passes through checking function. If no error then

data portion of the data unit is accepted and redundant bits are discarded. otherwise asks for the

retransmission.

Parity checking

Parity adds a single bit that indicates whether the number of 1 bits in the preceding data is even or

odd. If a single bit is changed in transmission, the message will change parity and the error can be

detected at this point. Parity checking is not very robust, since if the number of bits changed is

even, the check bit will be invalid and the error will not be detected.

1. Single bit parity

2. Two dimension parity

Moreover, parity does not indicate which bit contained the error, even when it can detect it. The

data must be discarded entirely, and re-transmitted from scratch. On a noisy transmission medium

a successful transmission could take a long time, or even never occur. Parity does have the

advantage, however, that it's about the best possible code that uses only a single bit of space.

Cyclic Redundancy Check (CRC)

CRC is a very efficient redundancy checking technique. It is based on binary division of

the data unit, the remainder of which (CRC) is added to the data unit and sent to the receiver. The

Receiver divides data unit by the same divisor. If the remainder is zero then data unit is accepted

and passed up the protocol stack, otherwise it is considered as having been corrupted in transit,

and the packet is dropped.

Sequential steps in CRC are as follows.

Sender follows following steps.

Data unit is composite by number of 0s, which is one less than the divisor.

Then it is divided by the predefined divisor using binary division technique. The remainder is

called CRC. CRC is appended to the data unit and is sent to the receiver.

Receiver follows following steps.

When data unit arrives followed by the CRC it is divided by the same divisor which was used

to find the CRC (remainder).

If the remainder result in this division process is zero then it is error free data, otherwise it is

corrupted.

Checksum

Check sum is the third method for error detection mechanism. Checksum is used in the upper

layers, while Parity checking and CRC is used in the physical layer. Checksum is also on the

concept of redundancy.

In the checksum mechanism two operations to perform.

Checksum generator

Sender uses checksum generator mechanism. First data unit is divided into equal segments of n

bits. Then all segments are added together using 1’s complement. Then it is complemented ones

again. It becomes Checksum and sends along with data unit.

Exp:

If 16 bits 10001010 00100011 is to be sent to receiver.

So the checksum is added to the data unit and sends to the receiver. Final data unit is 10001010

00100011 01010000.

Checksum checker

Receiver receives the data unit and divides into segments of equal size of segments. All segments

are added using 1’s complement. The result is complemented once again. If the result is zero, data

will be accepted, otherwise rejected.