business and data communication.ppt
TRANSCRIPT
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Copyright 2007 John Wiley & 7 - 1
Business Data Communicationsand Networking
9th Edition
Jerry Fitzgerald and Alan Dennis
John Wiley & ons! "nc
Virginia F. Kleist, Ph.D.
College of Business and Economics
West Virginia University
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Cha#ter $
Wireless
%ocal Area Networks
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Cha#ter $ 'utline
• ntroduction
• W!"# Com$onents
• W%F
• W&"'
• Bluetooth
• Best $ractice W!"# Design
• m$roving W!"# Performance
• &anagement m$lications
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*rinci#al W%ANs +echnologies
• W%F – IEEE 802.11b
• tandardi7ation started after .22a, *ut finished
*efore, more commonly used than .22a
– IEEE 802.11a
• First attem$t to standardi7ation of W!"#s8 more
com$licated than .22*
– IEEE 802.11g
• W&"'
• Bluetooth
– "lso an EEE standard /01.29
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-ore on the A*s and N"Cs
• : se$arate channels availa*le for /01.22* – "ll devices using an "P must use the same channel
• W!"# functions as a shared media !"#
– educes the interference
– Users can roam from "P to "P• nitially #C selects a channel )thus an "P+
– Based on ;strength of signal< from an "P
• During roaming, if #C sees another "P 5ith astronger signal, attaches itself to this "P
• Usually a set of "Ps installed to $rovidegeogra$hical coverage and meet traffic needs
– #Cs selects a less *usy channel if its current channel*ecomes *usy )too many users+
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W%AN +o#ology
" 5ireless "ccess Point )"P+ connected
into an Ethernet 5itch
ame as Ethernet• Physical star • !ogical *us
Use the same radio
fre(uencies, so
ta4e turns using
the net5or4
Uses a #C that
transmits radio
signals to the "P
20Base%= or
200Base%=
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Antennas used in W%ANs
• -mni directional antennas – =ransmit in all directions simultaneously
– Used on most W!"#s
• Di$ole antenna )ru**er duc4+
– =ransmits in all direction )vertical, hori7ontal, u$,do5n+
• Directional antennas
– Pro>ect signal only in one direction
• Focused area8 stronger signal8 farther ranges
– &ost often used on inside of an e3terior 5all
• =o reduce the security issue
– " $otential $ro*lem 5ith W!"#s
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+y#es o, Antennas
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W%AN .adio Fre/uencies
• W!"#s use radio transmissions to send data*et5een the #C and the "P
• &ost countries use the 1.? @A7 range and the 9
@A7 range
• =he larger the fre(uency range, the greater the
*and5idth, or ca$acity
• t is im$ortant to ensure that V!"# "Ps do not
conflict 5ith each other
• =herefore, each "P is set u$ to transmit on a
different $art of the 1.? or 9 @A7 fre(uency range
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A W%AN 0sing Di,,erent Channels
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W%AN -edia Access Control
• Uses C&"C" – C" collision avoidance
– " station 5aits until another station is finished
transmitting $lus an additional random $eriod of time
*efore sending anything
• &ay use t5o &"C techni(ues simultaneously
– Distri*uted Coordination Function )DCF+
• "lso called ;Physical Carrier ense ðod<
– Point Coordination Function )PCF+• "lso called ;Virtual Carrier ense ðod<
• -$tional )can *e set as ;al5ays
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Distri1uted Coordination Function
• elies on the a*ility of com$uters to $hysicallylisten *efore they transmit
– When a node 5ants to send a message
• First listens to ma4e sure that the transmitting nodehas finished, then
• Waits a $eriod of time longer
• Each frame is sent using sto$%and%5ait "
– By 5aiting, the listening node can detect that thesending node has finished and
– Can then *egin sending its transmission
– "CK#"K sent a short time after a frame is received,
– &essage frames are sent a some5hat longer time after)ensuring that no collision 5ill occur+
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*oint Coordination Function (*CF)
• olves Aidden #ode $ro*lem – =5o com$uters can not detect each others signals
• " com$uter is near the transmission limits of the "Pat one end and another com$uter is near thetransmission limits at the other end of the "Ps range
– Physical carrier sense method 5ill not 5or4
• olution
– First send a e(uest =o end )=+ signal to the "P
• e(uest to reserve the circuit and duration
– "P res$onds 5ith a Clear =o end )C=+ signal,
• "lso indicates duration that the channel is reserved
– Com$uter 5ishing to send *egins transmitting
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"EEE 234566a
• -$erates in a 9 @A7 fre(uency range• =otal *and5idth is :00 &A7
– Faster data rates $ossi*le U$ to 9? &*$s
• , 6, 21, 2/, 1?, :, ?/, and 9? &*$s
• Uses the same to$ology as .22*
• educed range *ecause of higher s$eed
– 90 meters ) 290 feet+
– Aighest s$eed achieva*le 5ithin 29 meter
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"EEE 2345661
• &oderate s$eed net5or4ing in the 1.?@A7range
• =hree channels for indoor use in the U,more or less in other $arts of the 5orld
• Each channel has a ma3imum data rate of22 &*$s, for users close to the center ofthe W!"# %22 &*$s is the norm
• /01.22* suffers less attenuation than/01.22a
• Designed to connect easily to Ethernet
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234566g -edia Access Control
• "lmost the same media and error control$rotocols as .22*
– imilar $ac4et layout, e3ce$t
•
Pream*les and headers transmitted atslo5er s$eeds )u$ to a ma3imum of 22
&*$s+
• Payload transmitted at higher s$eeds )u$ to
a ma3 of 9? &*$s+• Data =ransmission in the Physical !ayer
– ame techni(ues in .22a and .22*
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"EEE 234566n
• tandard under develo$ment
• @oal to $rovide high s$eed 5ireless
net5or4ing
• Uses *oth the 1.? and 9 @A7 fre(uencyranges simultaneously
• Current drafts $ro$ose s$eeds of 200%100
&*$s
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W"7F" as *u1lic "nternet Access
• Wi%Fi 5as intended to *e used for indoormo*ile 5ireless access
• &any $roviders have in air$orts and malls
and other $u*lic $laces• Political issues, not technical, interfere
5ith the large scale $rovision of Wi%Fi
•
Being offered *y some to5ns as 5ell ascarriers
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W"-A8
• Commercial name for family of EEE/01.2 standards
• =5o $rimary ty$es Fi3ed and mo*ile
•
!ogical and $hysical to$ology same as/01.22 and shared Ethernet
• Uses controlled access 5ith a version of/01.22 $oint coordination function
• =5o ty$es
– /01.2d
– /01.2e
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"ssues to consider with W"-A8
• W&"' is a com$etitor to $u*lic accessWi%Fi and cellular $hone service
• W&"' is incom$ati*le 5ith *oth
• Aas an effective range that offers *enefits
• Aas controlled access, version of PDF
• Considera*ly more distance covered 5ith
W&"' over Wi%Fi – /01.2d has a ma3imum real 5orld effective
range of 9 miles, /01.2e is miles
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Bluetooth -edia Access Control• Uses Fre(uency Ao$$ing $read $ectrum )FA+
– "vaila*le fre(uency range )1.?000%1.?/:9+ divided into G6
se$arate 2%&A7 channels
– " data *urst transmitted using one channel, ne3t data *urst
uses the ne3t channel, and so on.
– Channels changed *ased on a se(uence and esta*lished *ythe slave and the master $rior to the data transfers
• 2,00 ho$s or channel changes $er second
– "lso used to minimi7e interference
• " noisy channel avoided eventually
• #ot com$ati*le 5ith /01.22*
– Potential interference $ro*lems )es$ecially if many Bluetooth
devices $resent close to .22* devices+
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E,,ecti:e Data .ates in W%ANs
• &a3imum s$eed in *its the hard5arelayers can $rovide
– De$ends on #ominal data rate, Error rate,
Efficiency of data lin4 layer $rotocol, and
Efficiency of &"C $rotocol
• Error $lays a greater role in W!"#s
– ignificant im$act of interference on
$erformance• Causes fre(uent retransmissions, thus
lo5er data rates
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Data %ink *rotocol E,,iciency
• Factors involved – =y$ical W!"# overhead
• 92%*ytes )5ith a short $ream*le+
– Pac4et si7e
•
Data $ac4ets assume a 2900%*yte for full length• Control $ac4ets "CK#"K $ac4ets
– =ransmission rates
• -verhead *its transmission s$eeds
• Payload transmission s$eeds
• "ssuming a mi3 of short and full length $ac4ets
– /9H average efficiency for /01.22*
– G9H average efficiency for /01.22a and /01.22g
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-AC *rotocol E,,iciency
• Wi%Fi Uses a controlled a$$roach )PCF+ – m$oses more fi3ed delays initially 5hen traffic is lo5
• Due to PCFs $ermission *ased $rocedures
• Users e3$erience fe5 res$onse time delays as longas the total amount of traffic remains *elo5 /9%60Hof ca$acity
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E,,ecti:e .ate Estimates
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Costs
•
/01.22g – #e5er technology, 5ill re$lace /01.22a and *
– Costs 5ill dro$ here over time
• W%F and Wired Ethernet
– =he data rates for W%Fi are similar to the effective ratesfor 5ired Ethernet net5or4s
– Wired 200Base%= $rovides a good tradeoff on cost vs.$erformance
– But, Wi%Fi may add mo*ility feature for less 5iring costin e3isting *uildings
– &any traditional net5or4s are using com*ination of*oth to meet the needs of users
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Best *ractice .ecommendations
• "do$t /01.22g – Will re$lace /01.22* and .22a
– Prices of .22g #Cs and "Ps coming do5n
• Wireless vs. Wired
• imilar data rates for lo5 traffic environment
• When mo*ility im$ortant /01.22g
• Using W!"# as an ;overlay net5or4,< or in
con>unction 5ith a 5ired !"#
– W!"#s installed n addition to 5ired !"#s
– =o $rovide mo*ility for la$to$s
– =o $rovide access in hall5ays, lunch rooms, other sites
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*hysical W%AN Design
• &ore challenging than designing a traditional !"# – Use a tem$orary "P and la$to$ to evaluate $lacement of
"Ps
– !ocations are chosen to $rovide coverage as 5ell as tominimi7e $otential interference
• Begin design 5ith a site survey, used to determine
– Feasi*ility of desired coverage
• &easuring the signal strength from tem$orary "Ps
– Potential sources of interference
• &ost common source #um*er and ty$e of 5alls
– !ocations of 5ired !"# and $o5er sources
– Estimate of num*er of "Ps re(uired
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*hysical W%AN Design
• Begin locating "Ps – Place an "P in one corner
– &ove around measuring the signal strength
– Place another "P to the farthest $oint of coverage
•
"P may *e moved around to find *est $ossi*le s$ot• "lso de$ends on environment and ty$e of antenna
– e$eat these ste$s several times until the corners arecovered
– =hen *egin the em$ty coverage areas in the middle
• "llo5 a*out 29H overla$ in coverage *et5een "Ps
– =o $rovide smooth and trans$arent roaming
• et each "P to transmit on a different channel
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-ultistory W%AN Design
• &ust include – Usual hori7ontal ma$$ing, and
– Vertical ma$$ing to minimi7e interference from "Ps on
different floors
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W%AN ecurity
• Es$ecially im$ortant for 5ireless net5or4 – "nyone 5ithin the range can use the W!"#
• Finding a W!"#
– &ove around 5ith W!"# e(ui$$ed device andtry to $ic4 u$ the signal
– Use s$ecial $ur$ose soft5are tools to learn
a*out W!"# you discovered
• Wardriving I this ty$e reconnaissance
• Warchal4ing I 5riting sym*ols on 5alls to
indicate $resence of an unsecure W!"#
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+y#es o, W%AN ecurity• ervice et dentifier )D+
– e(uired *y all clients to include this in every $ac4et
– ncluded as $lain te3t Easy to *rea4
• Wired E(uivalent Privacy )WEP+ – e(uires that user enter a 4ey manually )to #C and "P+
– Communications encry$ted using this 4ey
– hort 4ey )?0%21/ *its+ Easy to *rea4 *y ;*rute force<
• E3tensi*le "uthentication Protocol )E"P+ – -ne time WEP 4eys created dynamically after login
– e(uires a login )5ith $ass5ord+ to a server
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+y#es o, W%AN ecurity! cont;d
• Wi%Fi Protected "ccess )WP"+ – ne5 standard
– longer 4ey, changed for every $ac4et
•
/01.22i – E"P login used to get session 4ey
– uses "E encry$tion
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"m#ro:ing W%AN *er,ormance
• imilar to im$roving 5ired !"#s – m$roving device $erformance
– m$roving 5ireless circuit ca$acity
–
educing net5or4 demand
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"m#ro:ing W%AN *er,ormance
• imilar to im$roving 5ired !"#s – m$roving device $erformance
• f /01.22g 5idely de$loyed, re$lace /01.22* cards5ith .22g cards )may *e the cause for slo5$erformance
• Buy high%(uality cards and "Ps
– m$roving 5ireless circuit ca$acity
• U$grade to /01.22g
• ee3amine $lacement of "Ps
• Chec4 sources of interference )other 5irelessdevices o$erating in the same fre(uencies++
• Use different ty$e of antennas
– educing net5or4 demand
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"m#ro:ing Wireless Circuit Ca#acity
• U$grade to /01.22g• e$lace "Ps
– Fe5est 5alls *et5een "P and devices
– Ceiling or high mounted to minimi7e o*stacles
– -n halls, not in closets
• emove sources of interference
– -ther 5ireless devices o$erating in the samefre(uencies
•
Bluetooth devices, cordless $hones, etc.• Use different ty$e of antennas
– Directional antennas in smaller range to get strongersignals )faster through$ut+
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.educing W%AN Demand
• #ever $lace a server in a W!"# – Dou*les the traffic *et5een clients and server,
once from the client to the "P, and once from
the "P to the server
– !ocate the server in the 5ired $art of the
net5or4 )ideally 5ith a s5itched !"#+
• Place 5ired !"# >ac4s in commonly used
locations – f W!"# *ecomes a $ro*lem, users can s5itch
to 5ired !"# easily
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"m#lications ,or -anagement
• W!"#s *ecoming common $lace – "ccess to internal data, any time, any $lace
• Better $rotection of cor$orate net5or4s
– Pu*lic access through W!"# hots$ots
• Com$etition and overla$ 5ith cell $hone
technologies – #e5 cell $hone technologies )faster, longer ranges+
– Drastic $rice dro$s of W!"# devices
• Wides$read nternet access via multi$licity ofdevices )PD"s, etc,+
– Develo$ment of ne5 nternet a$$lications
• #e5 com$anies created8 some old ones out of*usiness
– Drastic increase in the amount of data flo5ing around
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