wireless lesson

8/9/2019 Wireless Lesson

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Self-help book for WLAN installers/providers

Introduction

To meet the needs of many people interested in building WLAN networks, we have decided to

collect in one article a bit of theory and a lot of practical information on quick and effective

implementation of wireless networks working in 2! "#$ and % "#$ bands &'(2))*

WLAN &Wireless Local Area Network* means technology that allows to build wireless data

networks with satisfactory parameters and quite large ranges of operation at a comparatively lowcost Additional advantage of this technology is short time needed for its implementation

The potential of WLANs and its use

• wireless access to a local network in home, office, business etc

• wireless access to the Net in public space, eg in airports, stations, cafes etc &hot+spot*

• wireless point+to+point links &connecting LAN networks, telemetry, remote control,

remote monitoring*

• wireless access to the nternet &both in cities and in the country*

• emergency communications links &wireless backup of wired networks*

WLAN standards

We will describe some solutions compliant to the following three standards-

• '(2))a + in % "#$ band- %)%( + %.%( "#$ and %!/( + %/2% "#$, transfer rate up to%! 0bps1

• '(2))b + in 2! "#$ band- 2! + 2!'. "#$, transfer rate up to )) 0bps1

• '(2))g + in 2! "#$ band- 2! + 2!'. "#$, transfer rate up to %! 0bps1

#owever, other standards are used as well-

• '(2))f + A + nter Access oint rotocol + for cooperation between access points1

• '(2))i + standard defining new security methods in wireless networks1

• '(2))n + standard for transmitting multimedia in homes using 003 technology, up to

.(( 0bps1

• '(2))e + standard defining 4o5 + support for high quality of services1


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• '(2)6 + Wi0a7 standard for backbone networks of high capacity

Wireless network range

t should be reali$ed that the range of a wireless network depends on many factors1 we can havean influence on some of them and the rest is unknown The range of a wireless network depends

on-

) 8actors related to the devices used-

• output power &it has been decided by the manufacturer*,

• cable attenuation &depends on the cable and its length*,

• gain of the antennas &given by the manufacturer*,

• sensitivity of the devices &given by the manufacturer*

2 97ternal factors-

• attenuation between antennas &can be estimated basing on 85L model*1

• interferences from other devices &can:t be predicted + some additional margin of safety

needs to be provided for their compensation*,

• influence of physical barriers &walls, floors, trees etc*

5o, if we want to know what would be the effective range of our network we have to gather the

information mentioned above and carry out simple calculations showed in the further part of thisself+help book

Propagation of radio waves

resnel !one

8resnel $one is one of the most important concepts connected with propagation of

electromagnetic waves, which is indispensable to assess parameters of any wireless link t is thearea actively participating in transmission of radio signal energy 5hape of this area is an ellipse

in longitudinal section, and circle in cross+section ;adius of this circle is a function of the ratio

of distances of the cross+section to the antennas + it has the ma7imum value in the middle of the

link The importance of the first 8resnel $one comes from the fact that almost all energy of thesignal is conveyed via this space


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The shape of resnel !one" # $ is the radius of the I !one"

d)km?d2km, is the distance between masts in km

•d)km + distance from the first antenna in km

• d2km + distance from the second antenna in km

Wrongly made installation. The installer didn't ensure mutual visibility of antennas. The radio

link does not work.


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Another example of wrongly made installation. Presence of barriers in the first Fresnel zone

causes that radio link still doesn't work properly.

Installation made correctly. Visibility of antennas and no barriers in the first Fresnel zone. The

link has been set up properly.

n practice, no obstacles in the central %&' of the I resnel !one guarantee quite minimal power

loss

;elationship of the 8resnel $one radius as the function of the radio link length,

for systems working in 2! "#$ and % "#$ band &table*

;adio link length


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. %' !(

! 6/ !6

% /% %2

6 '2 %/

/ ' 6)

' % 66

)() /(

)( )(6 /.

(urvature of the )arth

n the case of distances reaching a few kilometers and more, it is needed to include curvature of

the ground 8or the distance equal % km the height of barriers in the middle of link raises by )m

&let:s represent the quantity as curvature factor* and for )(km distance + yet ! m Antennas should be situated at or slightly above the minimum height fulfilling the condition-

Height of antennas = elevation of the highest barrier on the route !." #$ curvature factor

At longer distances, more precise calculations should be performed, based on hypsometric

profile of terrain and methods including effects of beam refraction and multiple reflections

Attenuation of gases and rain

These phenomena are well known and recogni$ed as disadvantageous for proper work of radio

systems1 in practice they are harmless for 2! "#$ and % "#$ WLAN systemsSL *odel and attenuation in free space

The basic problem is to estimate attenuation between transmitter and receiver When we design

outdoor link we can use for this purpose 85L model t is propagation model of free space, whichassumes that-

• there is no barrier between transmitter and receiver,


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• reflected waves don:t influence the receiver,

• the fist 8resnel $one isn:t covered,

• there aren:t taken into consideration outer interferences and fading

Attenuation of free space is defined as the loss of signal caused by spherical dispersion of radio

waves in space

85L for the frequency of 2! "#$ is determined by the pattern-

Lp )(( ? 2(log)( C, where C + distance

85L for frequency %! "#$ is determined by the pattern-

Lp )(6 ? 2(log)( C, where C + distance

Attenuation of free space and %d+ rule

;adio signal will weaken during its propagation in space, as it moves away from transmission

antenna Cetermination of attenuation of radio signal is the ne7t step in the designing process


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6 dB rule says that double increment in distance causes increase of signal attenuation by 6 dB

and double reduction of distance causes increase of signal level by 6dB 5implicity of this ruleallows for easy memori$ation of the relation t is enough to remember that in ," .! band the

attenuation at the distance of $ k* is $&& d+"

5o, using the 6 dB rule, we will get for distances 2, !, ' km attenuation values of )(6, ))2, ))'dB respectively 8or distances %((, 2%(, )2% m, the attenuation will be !, '', '2 dB adequately

The 6 dB rule is also applicable for % "#$ band and other bands, however, the attenuation in the

% "#$ band for a ) km distance will be )(6 dB, so it means that the 6 dB rule is also applicablein the frequency domain

0ther propagation *odels

8or professional applications engineers use highly sophisticated models, developed for specificconditions and environments, such as-

• propagation model with covered 8resnel $one

• propagation model including attenuation of walls inside buildings

t is not possible to use such models in amateur calculations

#SL calculations


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The basis for range calculation is creation of the radio link balance in order to obtain the ;5L

value &received signal level*

%lements of energetic balance of a link&

• T5L


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;5L80 means the minimum level of the received signal &when a fading occurs* 8or e7ample, if

we want to reach ;5L80 > +'( dBm, it is required that the radio link has to obtain ;5L > +/(

dBm

3ur goal is to choose such antennas and equipment that would guarantee the required level of

signal &+'( dBm* for most of the time 0ost of wireless WLAN devices ensure highest possible

speed then

Selection of devices - an e1a*ple

Antennas for 2! "#$ band have usually gain between / dBi and 2! dBi 8or this band thecommonly used cables are #+)%% 9))/(, with attenuation !6 dBE)(( m, and #+)((( 9))2

with attenuation 2)% dBE)(( m

#owever, there are already available latest cables up to 6 "#$ These cables are recommendedfor use in new installations instead of above mentioned cables + Tri+Lan 2!( &9))/)* and Tri+Lan

!(( WLL 9))/.

0ore about cables used with WLAN equipment you can find in the article-

Fse of coa7ial cables in WLAN systems

n % "#$ band, antennas reach energetic gain from )( dBi to .2 dBi 5o the gain is generally alittle higher in comparison to 2! "#$ band

As an e7ample + we want to set up radio link over 2 km distance and achieve the best possible parameters of the connection We use devices with )' dBm output power The length of the cable

connecting antenna with WLAN device is / m for both sides of the link We can read from the

table that for these parameters the sum of "T and "; gain shouldn:t be less than 2)6% dB 8rom

the ne7t table we know that we should use ATG' A/)2( antennas

(aution" 5ome manufacturers, for marketing purposes, intentionally overestimate energetic gain

of antennas t may cause poor work of radio links using such antennas, drop in transmission

speed and even momentary loss of connection 5o, the best solution is to use antennas whichhave been tested in laboratory and which have relevant documents proving these tests Besides

that, e7istence of a number of neighbor wireless networks may cause degradation of our signal

Therefore it is sometimes better to increase the criterion for 80 and assume 80>2( dB

Transmitter

power


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)% 2.2' 22' .%2' .'/' !)2' !!'' !/.' !2' %2/'

#+)(((

. / )%/ 2)/ 2%2 2// .). ..' .%/ .2

/ ))6% )/6% 2.6% 2/)% 26% ..2% .%/% ./6% !))%

)% )%.6 2).6 2/.6 .('6 ...6 .66 .!6 !).6 !!'6

)'

#+)%%

. .' )%.' 2).' 2!'' 2/.' .(' ..!' .%.' .'''

/ )..! ).! 2%.! 2''! .).! .!! ./!! ..! !2'!

)% 2)2' 2/2' ..2' .6/' .2' !2'' !%.' !/2' %(/'

#+)(((

. // )./ )/ 2.2 2%/ 2. .)' ../ ./2

/ 6% )%6% 2)6% 2%)% 2/6% .)2% ../% .%6% .)%

)% )..6 ).6 2%.6 2''6 .).6 .!6 ./!6 ..6 !2'6

2(

#+)%%

. /.' )..' ).' 22'' 2%.' 2'' .)!' ...' .6''

/ )).! )/.! 2..! 26'! 2.! .2! .%!! ./.! !('!

)% )2' 2%2' .)2' .!/' ./2' !('' !..' !%2' !'/'

#+)((( . %/ ))/ )// 2)2 2./ 2/. 2' .)/ .%2


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/ /6% ).6% )6% 2.)% 2%6% 22% .)/% ..6% ./)%

)% )).6 )/.6 2..6 26'6 2.6 .26 .%!6 ./.6 !('6

Table indicating re0uired gain of radio link when there are given& length of the link1 transmitter

power1 type and total length of the used cable

Total required gain

of radio link

)!

22

26

2'

..

!'

Above values are theoretical rather, the practical range of links working in 2! "#$ bandwouldn:t e7ceed 2 km The reason is limitation of radiated power, ma7 )(( mW 9; &2( dBm*,

and usually congested band, which requires to adopt high 80 value As a rule, it is more

advantageous to use lower power transmitter and antenna with higher gain than the other way

round

)I#P and choice of devices

Will we break the law using transmitting antenna with very high energetic gainH t should beremarked that regulations don:t inform about limits of gain, which can:t be e7ceeded

5o how is it possible that one person can have an antenna with )% dBi gain, when another breaksthe law installing antenna having )( dBi gainH


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Why some companies indicate, in compliance certificate, antenna with )% dBi gain, when others

recommend antennas with)( dBi gainH

The answer for this questions involves the regulations regarding ma7imal acceptable value of

radiated power + 9; n many countries the ma7imum value of 9; that can be radiated

without a special license is )(( mW &2( dBm* in 2! "#$ band, 2(( mW in %)%(+%2%( "#$,and ) W &.( dBm* in %!/+%/2% "#$ band But the same levels of 9; can be achieved in

many ways, according to the formulas-

%2#P)d*+3.4 = Transmitter power )d*m+ , 5attenuation of connectors )d*+ attenuation of

cable )d*+6 gain of antenna )d*i+ 7= 3!d*m

%2#P)d*+8 = Transmitter power )d*m+ , 5attenuation of connectors )d*+ attenuation of cable )d*+6 gain of antenna )d*i+ 7= 9!d*m

n order not to e7ceed ma7imum permissible 9; value, there have to be selected adequate

parameters-

• transmitter power,

• type and length of cables

• gain of antenna

t:s worth stressing again that it is much more advantageous to use lower power transmitter and

the antenna with higher gain than the other way round WhyH 8rom the link balance we see that

desired radiated power level can be achieved in any way, however the base station isn:t only atransmitter, but also a receiver, and then, when it receives signal from a client, no matter the

power has been transmitted, only sensitivity of the receiver and gain of the antenna is important

5o the gain of antennas is important both during transmitting and during receiving

The output power level is an important issue, too Fsually it seems that the higher power the

better results But it:s not the truth There is some optimal power level adIusted for the location of

clients Too high transmitting power means needless transmission of our signal beyond thedesired area We can interfere networks working far away from us We will also be vulnerable to

attacks on our network performed by people that are quite a long way from us and thus difficult

to spot

The gains of client stations should also be selected carefully The client that uses a high gain

antenna close to the base station, although receives strong signal, it may during transmission also

interfere other, even distant networks Besides that, it will JseeJ those networks and what it

implies, they will cause additional noise &the higher noise the larger number of errors and lower transmission speed*, or it will even share with them the transmission medium + which will also

decrease speed 3n the other hand, client stations with lower gain, optimal for the specific

distance, will only see the base station and won:t cause such problems

(onnectors

0ost of WLAN devices are equipped with 50A+; connectors, whilst outdoor antennas have N+type connectors Fsing #+)%% cable it is needed to terminate it with 50A ; connector on


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one side, and adequate male or female connector &depending on the antenna* on the other side f

we don:t have crimping tool we should choose twist+on connectors #owever, crimp+on

connectors are preferred for their reliability

N connectors on #+)%% and Tri+Lan 2!(

N connectors on #+)((( and Tri+Lan !((

50AE; connectors on #+)%% and Tri+Lan

Drimping tools for #+)%% and Tri+Lan 2!(

Drimping tools for #+)((( and Tri+Lan !((

The installer also needs a soldering iron

The %:933! connector on ;,$88 cable

Antenna connectors

The ways of terminating cables can be found here.


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1

$9? $1"1$$? $1 "1 $3? $1 "1 $9.

ractical tests show that mutual influence of two networks working in the same area depends on

the chosen channels and decreases with increasing the space between the channels When both

networks use the same channel, they have up to half of the ma7imum capacity The worst case iswhen they use neighboring channels + their signals make mutually a high level noise that

dramatically reduces the effective transfer rate to about 2(@ of their capacity !+channel spacing

allows for /(@ efficiency Fnfortunately, theoretically independent channels also have certaininfluence on each other

(hoice of polari!ation

There are two popular variants of polari$ation- circle and linear Dircle polari$ation means thatthe end of the vector of electricity field draws a circle in space Dircle polari$ation can be

de7trorotary or levorotatory ;adio systems with de7trorotary polari$ation don:t influence

systems with levorotatory polari$ation and vice versa


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-ircle polarizations& de@trorotary and levorotatory

n the case of linear polari$ation, the electric field vector oscillates only in one plane t is

hori$ontal or vertical plane

;adio systems with hori$ontal polari$ation don:t affect systems with vertical polari$ation, andvice versa, as these polari$ations are orthogonal This feature allows doubling the number of

radio systems in one place

(aution" t is not allowed to use antennas with orthogonal polari$ation ie an antenna withhori$ontal polari$ation at one side of a link and with vertical polari$ation at the other side of the

link When it comes to cooperation of circle polari$ation antennas with linear polari$ation

antennas + it is possible + however with .dB power loss

Noise

n practice, noise is the sum of undesired radio signals, ie interferences Too big level of noise

can spoil parameters of any radio link or even make the link unserviceable 9ven well balancedradio link may appear to be useless for the reason of high noise level The designer has no

influence on the level of ambient noise 5o, how can we protect ourselves against interferencesH

The simplest way to defend our link against them is finding less congested radio channelAnother way is to select antennas with higher gain, to improve signal to noise ratio &5EN*

The throughput of a wireless link depends on the power level of the received signal and S/

ratio 52t is marked as signal strength and signal 0uality in the drawing.6 To reach ma@imal speed 5$$ /bps6 the indicator should be in green field 5%@cellent6. 2f the level of noise increases1 even

a high value of received signal won't protect us from bandwidth loss.

)ffective transfer rate

Because WLAN system is based on D50AEDA techniques and uses transmission with ADG

confirmation, the end user connected to the network via eg )) 0bps link cannot reach real filetransfer higher than half the value, ie about % 0bps )ffective transfer rate of an2 WLAN

link is less than half of the declared bandwidth capacit2 of the radio channel"


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3odes of operation of Access Points

Access oint may work in a few different modes 9ach mode is characteri$ed by capability &or not* of supporting specific devices and the features collected in the table-

A modeLAN support&number of

supportedcomputers*

5upport for clients equipped

with WLANcards

Dooperation with As

Wireless bridge Kes No Wireless Bridge

0ultiple bridge Kes No Wireless Bridge

;epeater No Kes Access oint

Access oint Kes Kes ;elay Node, A Dlient

A Dlient Kes No Access oint

Planning of WLAN cells and the service for clientsThere are a few ways to cover an area with WLAN signal t all depends on desired range and

capability of the network

The wa2s of covering an area with radio signal + sector cells and omnidirectional cell

n the case on the left we have a terrain covered by three As and three sector antennas 9ach A

uses different frequency n the e7ample on the right we use a single A with an omnidirectional


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antenna The first system can cover 6 times bigger area than the second, and can serve . times

more subscribers The cost of connecting a subscriber in each of the systems will depend on the

distance from the subscriber to the base station 5ubscribers who are located closer to the basestation can be equipped with low+gain antennas, which means lower cost

The si$e of a cell should be chosen taking into consideration all features of the base station,

density of population in the area, and estimated degree of saturation of the market

n practical solutions, the si$e of a cell is limited by the shape of the land and various barriers

like trees, chimneys, buildings etc

4evices integrated with antennas

Wireless devices integrating active components &access points* with antennas are still gaining

popularity They are connected with computers directly via twisted pair cables &FTE8T*,instead of traditional coa7ial cables &between wireless modules and passive antennas* with

lengths limited to several meters &due to attenuation of ;8 signal* The FTE8T cables can be

long up to .( m &it depends on the power requirements of the device and capacity of the power supply using o9 option* This solution eliminates the difficult problems of running "#$ coa7ial

cables &low fle7ibility* and their attenuation

!evices for ".# $H% &and'

3utdoor access point TL+WA%2)(" #igh ower 2!

"#$ Wireless data transmission can be performed inA, A ;outer, W5, or W5 Dlient mode The

device is equipped with a high gain antenna, which,

together with the electronic board, is put in a weather+

resistant housing Thanks to the antenna with )2 dBigain, high output power of the transmitter &2/ dBm*

and high sensitivity of the receiver, the device allows to

create long+range, stable and efficient wirelessconnections

TP,(2AB T(,C


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1. Barriers in the I Fresnel zone Use higher masts. change locations of antennas

2.Wrongly calculated energetic balanceof the link. rongly chosen de!ices

Use cables of loer attenuation. e.g. instead of "#1$$use "#1%%%& use antennas ith higher gain

'. Wrong (olarization of antennas )lign antennas to the same (olarization

*. Wrong alignment of antennasUse signal le!el meter during antennas+ installation.Set antennas in (ositions in hich signals ha!e the

highest (oer

$. "igh le!el of interferences or noise

Select radio channel ith the loest noise le!el.change (olarization of the link to the o((osite. use

antennas ith higher energetic gain. )s a last resort #change antennas locations.

Wrong o(eration of radio system ,iagnosis Solution

).-oss of connection and lo bandidth

of radio link-o !alue of S/

(arameter0oints 1#$ of the (re!ious

table

B.-o transfer rate from base station ith

radio link orking at maimal s(eedFreuent collisions

3urn on R3S43Smechanism for clients

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