Individual Assignment1

7/26/2019 Individual Assignment1

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1. Discuss the relation of the following terms in datatransmission quality

Channel Capacity,

Data rate,

Bandwidth,

Reasons for Widespread Use of Multipleing

Within recent years wireless communications has increased at a

staggering rate. This has caused much debate on how best to

utilize the available spectrum, and indeed has impelled more

spectrum to be opened for commercial use. It not that easy

just to say lets open up more spectrum giving more wireless

communications. No, the limitations of wireless communications

are very much found outside the engineering domain. afety

regulations limits the power used by the user, commercial

availability, under regulatory constraints, determines the

carrier fre!uency and the bandwidth and finally the nature of

the channels is governed by weather, car traffic, physical

objects and so on... . "ll these constraints will determine

the capacity of the channels.

[Muriel Mdard, MAY 2000,]

Channels

" channel is basically the communication medium or the path

which data ta#es to go from its source to its destination. "channel can comprise of several things, you have wires, free

space, a whole networ#. "s you can imagine not all networ#s

are the same different networ#s contain different

characteristics. $ou could send your data over a wireless

networ# which can then be routed onto an %thernet lan. &rom

here to a '( modem and onto a fibre optic bac#bone. "ll these

different channels have three characteristics in common for

communication to ta#e place. They are latency, data rate and

reliability of the channel.

[https://en.wikibooks.org]

When tal#ing about channels we need to first loo# at some of

the terms about channel capacity.

'ata rate ) this is the rate at which data can be communicated

and is indicated in bytes per second *bps+.

andwidth ) is the bandwidth of the transmitted signal as

constrained by the transmitted signal as constrained by the transmitter and the nature of

the transmission medium. It is measured in -ertz.

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Noise )

hannel apacity ) this is the ma/imum rate at which data can

be transmitted over a given communication

path, or channel, under the given

conditions.

%rror rate ) is the rate at which errors occur.

ignal andwidth ) this is the bandwidth of the transmitted

signal or the range of fre!uencies present

in the signal, as constrained by the

transmitter.

In most cases, it is the changing of values in the signal timethat information is passed on. The direct measure of the rate

of change in values of the signal is the fre!uency of the

signal. o in other words the more the fre!uency of a signal,

the more the data rate or information transfer rate is

attainable. It can be more clearly demonstrated in an e/ample0

If we loo# at transmission line coding such as inary "1,

inary &1 or inary 21 data transmission, it is noted that

if we alter the property of a high fre!uency carrier wave to a

higher value it will reduce the bit interval T*345f+duration.

"s a result more bits per second are transferred.

This is also true about digital transmission techni!ues li#e

N67, 8anchester encoding and so on. These can be modelled as

periodic signals and hence it is composed of an infinite

number of sinusoids, consisting of a fundamental fre!uency

*f+and its harmonics. (i#e binary coding it to has a

fundamental fre!uency *T345f+ resulting in increased data rate

as a result of an increased fundamental rate representing adigital signal with shorter bit interval.

This gives us a notable relationship between data rate and

bandwidth on both analog or digital transmission. If you were

to double the bandwidth you would in turn double the data

rate.

[Muriel Mdard, MAY 2000]

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Widespread use of Multipleing

8ultiple/ors have become an important player in thetelecommunications world. The main reason is to reduce cost byminimizing the number of communications lin#s needed betweentwo points.

ome of the biggest advantages are0

The ability to compress certain data characters withfewer bits freeing up additional capacity for themovement of other information.

'ata integrity and accuracy are maintained througherror detection and correction between two points.

Transmission resources are managed on a dynamic basis.Who gets their share of the bandwidth9 What happens whenlin#s go down9 What is plan for high priority

information in the event of problems9

8ultiple/ors are able to solve such problems.

usinesses can face crippling costs if they need constantcommunication lin#s. The beauty of multiple/ing is that it canhelp to create more intelligent use of your resources bygrouping lines together for more efficient use.

Imagine we have a business which relies on credit cards forpayments. This business is in (ondon *location "+, in fact

there are several business in (ondon. The data centre thatmanages all credit transactions is in %dinburgh *location +.There is a clear need here for a constant communications lin#between the businesses in (ondon and the data centre in%dinburgh. The choice here would be a leased line from eachlocation in (ondon to the data centre in %dinburgh.

[Figure ]

This is going to be very costly having a leased line for eachof the businesses. Nobody else will be able to access their

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bandwidth and the providers cant ma#e use of the lines duringnon:wor#ing hours so this comes at a high premium cost.

This is good for the business because it ma#es them feel li#ethey are in control of the networ# as their traffic will notbe affected by any other traffic in the networ#. They can also

use their own networ# management tools giving a sense ofsecurity. The downside is that leased lines are mileagesensitive. This means the longer the communications lin# thehigher the cost. If these leased lines are not usedefficiently then they become an over#ill and a burden on thebusiness.

[Figure 2]

-ow can this company get the same service but at a lot cheapercost9 Thats where multiple/ors come into play.

The business gets to wor# on it right away. 8ultiple/ors comein pairs so they need one in (ondon and one in %dinburgh. Theyare also symmetrical so if we have five lines *outputs+ in

(ondon we need five *inputs+ in %dinburgh. The #ey to thewhole operation is that you will only be using one *4+ leasedline so there will only be a need for one line between (ondonand %dinburgh. These are referred to a tail circuits. They runfrom a centrally placed multiple/or to each of the individualbusinesses. The outcome is that instead of the businessrenting individual leased lines or each other their businessesthey need only rent one leased line saving money.

[!oleniewski, ", 2002]

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!. Discuss the ad"antages and disad"antages of #oth circuitswitching $etwor% and pac%et switching networ%& then discusshow '(M Benefited from #oth)

Within a wide area networ# where multiple local area networ#sare together networ#ing services and the speed at which they

operate play a significant role in the final decision. Two ofthese services come in the form of switching networ# and apac#et switching networ#.

*ac%et +witching

2ac#et switching is how the data pac#ets are moved overswitched wide area networ#s. The two most common services are;.


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The characteristics of the ;.1 or >?1 ma/ line "lso #nown as variable length pac#et switching

" 2"' decides which circuit the information is going tota#e as part of the virtual circuit concept

2ac#ets usually have 4


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[Figure $]

The disadvantages for &rame relay are that there is a decreasein speed and that it is not private with the advantagesinclude lower cost and less e!uipment to operate it.

[Mi%roso&t, 20]

Circuit +witched $etwor%s

In circuit switched networ#s there is a need for a

communication session to be set up between two communicatingnodes for the duration of the session. The resources need arereserved for the session to ta#e place. The resources are usedon demand and as a conse!uence there may be a waiting time foraccess to the communication lin#. " typical e/ample of this isa telephone networ#. efore you can communicate to the personon the other end a connection must be made and the connectionis between you and the receiver only. This is what is #nown asa circuit. Ance the circuit has been established it reserves aconstant transmission rate in the networ# lin#s.

[Figure ']

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"s we can see above there are two hosts directly connectedwith each other. This is #nown as a dedicated end:to:endconnection. &or this to happen the networ# must first reserveone circuit on each of the two lin#s. "bove we can see thatthere are four lin#s circuits and the second circuit has beenused to form the end:to:end connection. The transmission rate

will be divide between the four lin#s. (ets say it has atransmission of 48bps. This gives each lin#


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(he 0+ Model

(he 'pplication 3ayer

This is #nown as layer D of the AI model. Its purpose is to

serve as a window for application services. It is not theactual application but rather the channel through whichapplications communicate.

$etwor% 'pplications

Web

%mail

'N

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(he *resentation 3ayer

(ayer > is #nown as the 2resentation (ayer. Its mainresponsibility is to deliver and present data to theapplication layer. efore it can do this the data must beformatted in order for the application layer to understand it.

It is responsible for0

%ncryption and decryption of messages

ompression and e/pansion of messages, formattranslation

-andling protocol conversions[!regg. M, et all 200]

When data is passed both down to the 2resentation layer it ischanged into a common format. "t the other end it is changedbac# into a useable format *for e/ample, %'I to "II+which

is understood at the 2resentation layer. onversion from oneprotocol to another for communicating over different platformsis also achieved here. This is also the home of where gatewayservices operate. These are connection points between networ#swhich use different platforms or applications and gatewaysthat cross platforms or file systems.

"nother job for this layer is data compression which lessensthe number of bits to be transmitted over the networ# media.

[(rebauhg. A et all, 200)]

+ession 3ayer

Its job is to allow two applications on two differentcomputers to establish and coordinate a session. While insession it manages the data and once the session is over ittears down the session. 2rotocols that reside at this layerinclude0

62 *6emote 2rocedure all+

F( *tructured Fuery (anguage+[!regg. M, et all 200]

Ance the Transport (ayer has done its job of establishing avirtual connection, the ession layer is counted upon forestablishing, monitoring, and terminating sessions using thevirtual circuits established by the transport layer.

It is here that header information is put into pac#etsindication the source and destination of the pac#et. It thenperforms synchronization between the session layers of thesender and receiver aided with the help of "1s.

The session layer also loo#s at and controls messages whichare either sent as full:duple/ or half:duple/. -alf:duple/means messages can be sent both ways but only one way at a

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time. Whereas full:duple/ messages can be sent both ways atthe same time

The difference between the ession layer and the Transportlayer are that the essions layer establishes a connectionbetween two processes whereas the transport layer establishes

a connection between two machines. [(rebauhg. A et all, 200)]

(ransport 3ayer

The primary concern of the transport layer is to deal withsegments and reassembles data into data streams. It has toma#e a choice weather to send data !uic#ly or reliably byestablishing a logical connection between the sending host andthe destination host. This decision is based upon the

application protocol being used. It is also responsible forend:to:end error recovery and flow control. It providesmechanisms for multiple/ing upper:layer applications,establishing sessions and tearing down virtual circuits. Theconnections can be either connectionless or connection:oriented and the two main protocols found at this layerinclude0

T2 *Transfer ontrol 2rotocol+ this provides a reliableconnection by the use of handsha#ing, ac#nowledgments,error detection, and session teardown.

C'2 *Cser 'atagram 2rotocol+is a connectionless protocol.T offers low speed and low overheads.

low Control

Is needed to ensure data integrity. &low control prevents asending host overflowing the buffers on the receiving hostside. ecause this employs connection:oriented communicationit is able to guarantee0

ent segments are ac#nowledged upon receipt.

egments that are not ac#nowledged are resent. ongestion, overloading and data loss are all achieved

through manageable data flow.

In order to understand the difference between connection andconnectionless thin# about this0

$ou want to send an important letter to someone. The firstthing you do is call them to let them #now you are sending theletter *establishing a connection+. Ance your friend gets ithe call you to let you #now he got it o#ay *"1+. This isconnection:oriented communication.

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In the networ#ing world one device first set up a connection:oriented communication through a session. The process isstarted with the use of the three way handsha#e.

In figure 4 below we can see that both hosts applicationprograms are notifying their operating systems that a

connection is being initiated. 8essages are send over thenetwor# confirming the transfer and both sides are ready tobegin. This setup is called overhead.

%stablishing a connection:oriented session

[Figure ]

In the first stage, a connection agreement is sent as are!uest for synchronization.

The ne/t two stages ac#nowledge the re!uest andestablishes the connection rules.

The final stage is also an ac#nowledgement, ac#nowledginga connection has been agreed and accepted and data cannow be sent.


Windowing

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"n important mechanism for flow control is windowing.Windowing enables T2 to control the inta#e of traffic from aT2 session. It can be related to a traffic policeman. Ifeverything is o#ay it increases the window size but if thingsget a little congested in decreases the window size. It doesthis by as#ing hosts to slow down. If the congestion gets too

much it stops all the traffic entirely. When a window isadvertised as a zero *cho#e pac#et+ its giving a red lightsignal to stop all transmission of data.

"s we see in the ne/t figure host gives host " the greenlight to go ahead and send ?BG> bytes as its window size. -ost" then goes on to send ? frames containing 4B


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[Figure -]

The Transport layers job is also to manage logical addressingof ports. This lets the data #now where it should go. elow isa list of commonly used ports0

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[Figure ]

"t any one given time a computer could be running severalapplications at the same time both sending and receivingre!uests. This mechanism helps to #eep a trac# of what isgoing on. Incoming data pac#ets are guided to the correct

application based on the port number whether it be e:mail orweb browsing.

Ane final important function of the Transport layer is nameresolution. When using the web we #now what we want by itsname i.e. www.google.com but the computer has no idea whatgoogle is9 It converts the name into binary so as to beunderstood. It will read D?.4


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networ#s. In order to do this it assigns logical source anddestination addresses and then wor#s out the best path toroute the data to its destination. The protocols involved dealwith logical address which is distinguished from the physicallayer 8edia "ccess ontrol *8"+ address. These addresses arenot burned into hardware unli#e the 8" address but rather

they are assigned either manually or dynamically.

The protocol involved at this layer include0

6"62, "62, oot2, '-2 *these protocols perform addressresolution or configuration+

I82 *'iagnostics and control protocol+

6I2, IJ62, %IJ62, A&2 and J2 *routing protocols+[!regg. M, et all 200]

(ayer K devices *routers+ can be found here to route thetraffic using routing services. The router will first loo# for

the destination I2 address. If it sees it is not for hisnetwor# he will loo# up his routing table for the address andsend the pac#et out on the right interface. If however thedestination address cannot be found it will drop the pac#et.There are two types of pac#ets at this layer.

'ata pac#ets0 These are used to transport user dataacross the networ#. 6outed protocols are used thatinclude I2 and I2v>.

6oute update pac#ets0 These are used to updateneighbours on routers connected within theinternetwor#. 2rotocols used here include 6I2, 6I2


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"ssumes the function of sending and receiving dataover the wire.

alculates 6 or &

ridges and switches are also found at this layer.[(sterloh. , 2002]

This layer also ta#es care of flow control and errornotification on the 2hysical layer as the 2hysical layer isonly responsible for moving data onto and off the networ#medium. It also manages error messaging related to thephysical delivery of pac#ets.

The 'ata (in# (ayer has two sub:layers the 8" ub:layer andthe (( ub:layer.

(he M'C 3ayer

It is here that control for accessing the transmission mediumis done. It moves data pac#ets from one NI to another acrossa medium such as %thernet or fibre:optic cables.

%very NI has a uni!ue 8" address *physical address+ whichidentifies the device. This 8" address is burned into a read:only memory *6A8+ chip on the NI. It comes in the form of a4< he/adecimal digit *e.g. 4


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levels. It is the 2hysical layer that deals with tangibleitems, that is things you can touch such as cables andrepeaters. The specifications include0

Eoltage changes

The timing of voltage changes

'ata rates 8a/imum transmission distances

The physical connections to the transmission medium

The topology or physical layout of the networ#

There are many decisions to be made at this layer for e/ampledigital vs analog signalling, baseband vs broadbandsignalling, whether data is transmitted synchronously orasynchronously and multiple/ing.


Mapping 0+ to (C*2*

The AI model has been mainly used as a teaching model and hasnever been fully adopted. The foundation of the Internet as we#now it today. The 'epartment of 'efence *'o'+ funded the"dvanced 6esearch 2rojects "gency Networ# *"62"N%T+ toimplement the T25I2 model. "s we have seen the AI consistsof seven layers as opposed to the T25I2 model which only hasfour layers. The figure below demonstrates the relationshipbetween the two.

[!regg. M, et all 200]

[Figure 0]

When loo#ing at the different protocols of both models the AIand 'o' models are interchangeable. o when we tal# about the

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2rocess5"pplication layer on the 'o' model we are tal#ingabout the top three layers of the AI model, "pplication,2resentation and the ession layers.

The 2recess5"pplication layer defines protocols for the node:to:node application communication and also controls user:

interface specifications.

The -ost:to:-ost layer parallels the functions of theTransport layer in the AI model. It uses protocols forsetting up the level of transmission service for applications.It also deals with creating reliable end:to:end communicationand ensuring the error free delivery of data. It does this byhandling pac#et se!uencing and maintains data integrity.

The Internet layer e!uals the Transport layer of the AI. Itsets up the logical transmission of pac#ets. It ta#es care of

the addressing of hosts by giving them an I2 address and dealswith routing the pac#ets along multiple networ#s.

The bottom layer is the Networ# "ccess layer which correspondswith the 'ata (in# and the 2hysical layers of the AI. Anceagain it deals with hardware addressing and defines protocolsfor the physical transmission of data.

In many ways the 'o' and the AI models are very ali#e indesign and concept and have similar functions.

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5. Discuss the spread spectrum ad"antages in wirelesscommunication& Compare pros and cons of 6++ and D+++techniques)

&irst of all I feel there is a need to discuss what &- and

' is.

&- was originally designed as a security protocol during

World War II. It was intended as a secret communication system

with the intention of guiding torpedoes to their target with

interception. omposer Jeorge "ntheil and actress -edy (amarr

originally patented sending radio signals from transmitter to

receiver over multiple fre!uencies in a random pattern. It was

achieved by fitting the sender and receiver with identicalpaper rolls perforated with a pseudo:random pattern utilizing

@@ different fre!uencies. eacon frames and probe responses

contain fre!uency hopping parameter set elements necessary for

synchronicity *'well Time, -op et, -op 2atetrn, -op Inde/+.

[aelt, 1, 200]

There are two types of spread spectrum0

&re!uency hopping spread spectrum *&-+

'irect se!uence spread spectrum *'+

In todays W("Ns they only use ' and &- is used by

cordless phones and luetooth. oth of them use more than just

a single pea# fre!uency which allows them to spread their

signal over a larger part of the spectrum.

6++

&re!uency -opping pread pectrum *&-+ is one of two basic

modulation techni!ues used in the spread spectrum signal

transmission. This enables a signal to be transmitted across a

fre!uency band that is much wider than the minimum bandwidth

re!uired by the information signal. The energy is spread more

fre!uencies on a wider electromagnetic spectrum. This results

in improved privacy, decreased narrowband interference, and

increased signal capacity. (ets loo# at it a bit more in:

depth.

[http://sear%hnetworking.te%htarget.%o*]

&- changes fre!uency from @ to K< times each second it does

not spread its signal energy out but rather rapidly shifts the

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energy from fre!uency to fre!uency. " narrow band &- signal

is transmitted first on one narrow *4:8-z+ channel and then

!uic#ly shifted to another channel, then another. There are

two types allowed in the


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are often affected by e/ternal factors li#e noise or

interference which can garble data. To counter this, a

wireless transmitter performs several functions.

crambler0 'ata is first scrambled in a predetermined

manner and is randomized in Bs and 4s. oder0 %ach bit is then converted into multiple bits

of information that contain carefully crafted patterns

to protect against errors due to noise or

interference.

Interleaver0 The coded data stream of symbols is

spread into separate bloc#s so that bursts of

interference might affect one bloc# but not the other.

8odulator0 The bits are used to alter or modulate the

phase of the carrier signal. This enables the 6&

signal to carry the binary data bit values.

[u%ab4, 5, 20$]

"ll that being said what are the pros and cons of each9

&-0: &re!uency -opping pread pectrum. In &- the

transmitted signal changes the fre!uency randomly. In short

that means that a user may start out on channel 4 but at a

random time the fre!uency will change. It could switch to

channel K or > it is completely random. This is great becauseit is hard for a hac#er to trace or intercept because he does

not #now which channel to go to.

'0: 'irect e!uence pread pectrum is different because

the channel is not switched but rather the pseudo:random code

is and never repeats itself. This ma#es it very difficult to

decipher as the code changes from time to time.

The good news is that both of these are used for security

purposes and lead to secure communications.

"lthough &- is easy to implement because it is narrow:band,

its security measures are not as good as '. ' provides

ma/imum security but can be a little bit difficult to

implement due to its wideband nature and more complicates

synchronising circuitry.

ecurity is not everything though there are other factors toconsider. The pace of technology is fast moving and it is not

all that difficult to trac# and decode the information these

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days it just needs the right software and the #nowhow to do

it.

Ather factors to consider are that both are very good at

avoiding or even operating in the presence of noise or other

signals *narrow:band interference+. With &- it avoids noise

by simply hopping to another channel but data can be lost if

the channel is jammed. Whereas, ' has the ability to

attenuate everything apart from the desired signal. It is

possible for ' to operate below the noise floor.

"nother big pro is that they can both be used in the same band

as other devices. The


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&igure &rame relay cloud

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Wiley

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