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The Overlapping

Channel Problem

What Overlapping Channel Problem?

In th UK, th ara o th wirlss spctrum st asid or th us o 802.11b/g wirlss ntworking

dvics is th ISM (Industrial, Scintic and Mdical) band btwn 2.400 GHz and 2.497 GHz. In

th UK this is subdividd into 13 channls o 25MHz. In th US, only th rst 11 o ths channls

ar availabl a act with implications or UK dploymnts (s WAGs Advic blow).

Howvr, this dscription can giv a mislading pictur. Radio signals ar not tightly connd to

a singl point in th spctrum but ar distributd symmtrically around a mid rquncy. This will

b amiliar rom th xprinc o tuning a radio, whr th signal gradually gts strongr as you

turn th dial bor hitting th pak rquncy and thn slowly dis away again. In act, th width o

spctrum across which th signal is distributd is rlatd to th bandwidth (in th ntworking sns)

o th data bing transmittd and th symbol rat usd to ncod it (this rlationship is discussd

in Nyquist/Shannon sampling thory). 802.11b, with a bandwidth o 11Mbit/s and a binary symbol

systm, rquirs 22MHz pr channl (802.11g achivs its gratr data rat in th sam channl

width by incrasing th bits carrid pr symbol). So, a bttr pictur o th channls in th 2.4

GHz spctrum is givn by th ollowing (th our dirntly shadd, solid curvs indicat signals

associatd with th our non-ovrlapping channls: 1, 5, 9 and 13):

Channel Start Frequency Mid Frequency End Frequency

1 2399.5 MHz 2412 MHz 2424.5 MHz

5 2419.5 MHz 2432 MHz 2444.5 MHz

9 2439.5 MHz 2452 MHz 2464.5 MHz

13 2459.5 MHz 2472 MHz 2484.5 MHz

Signicant ovrlap can b sn btwn adjacnt bands. APs (accss points) with physically

ovrlapping broadcast clls congurd to th sam or nar-adjacnt channls will b trying to us

substantially th sam rquncis to transmit and rciv data.

Whilst a problm, ovrlap dos not compltly prclud wirlss communications. Th Mdia

Accss Control (MAC) spcication or 802.11b ntworks implmnts a CSMA/CA (Carrir Sns

Multipl Accss Collision Avoidanc) mchanism, which ctivly mans that ach AP listns on

its channl bor attmpting a transmission. In th ovrlap scnario dscribd abov, th multiplAPs would nd up sharing th availabl channl rsourc (with som urthr rduction in throughput

du to collisions). WiFi was designed to support many intrring, ovrlapping ntworks, handling

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F A C T S H e e T

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th rsultant packt collisions with minimum uss. So rathr than ntirly disrupting a wirlss LAN,

ovrlapping channl assignmnts mrly gratly rduc its cincy. This is clarly not dsirabl, and so

th standard advic in wirlss ntworking circls in th UK is to slct on o th our non-ovrlapping

channls or adjacnt clls, allowing all APs to oprat at thir pak throughput. Howvr, this raiss a

nw problm ...

The Four Colour Problem

Th Four Colour Problm is a mathmatical curiosity dating back to 1852, whn Francis Guthri, trying to

colour in a map o countis o england, noticd that our colours sucd to nsur that adjacnt rgions

(i.. thos sharing a common boundary sgmnt, not just a point) rcivd dirnt colours. Howvr,

a rigorous mathmatical proo or this conjctur was not orthcoming until that by Appl and Hakn in

1976,1 itsl controvrsial or its us o computrs in proo.

It is not a grat lap to substitut th pattrn o adjacnt clls o a wirlss LAN in a building or th

countis map, and th availabl rquncy bands or th colourists paints. So, mathmatics sms to b

tlling us that whatvr th pattrn o adjacnt wirlss clls on a plan surac, thy can always b

congurd in a way that compltly avoids channl intrrnc, using only th our non-ovrlapping

channls and avoiding adjacnt clls using th sam channl. (In act, in typical situations, vn thr

channls can b nough ortunatly or th Amricans, who only hav 11 channls in thir corrsponding

ISM band and thus only thr non-ovrlapping channls.)

Unortunatly, w sldom dploy wirlss on just a singl foor o a building, and adding th third

dimnsion into th problm maks it much hardr to nd a non-ovrlapping solution. In most buildings,

wirlss ntworking signals pntrat cilings and foors as radily as thy do walls (most omnidirctional

antnna on APs ar wav whips, which gnrat a fattnd torus o signal distribution cntrd on thir

antnna; PCMICA clint radios tnd to mit in a sphrical pattrn), so our colouring problm channl

conguration dsign has to addrss cll boundaris both abov and blow as wll as on th sam lvl

as th AP.

Somhow w nd to nd mor colours or our paltt.

A Solution?

An analysis by Mitch Burton2 rminds us that whilst using mor closly spacd channls dos rsult in

intrrnc, th actual loss in throughput is minimal i w slct v widly spacd channls instad o

th usual our (or our rathr than thr, in th Stats). According to Burtons analysis, whn two bur

channls sparat ach o th 802.11b channls in us (i.. channls 1, 4, 8, 11 and 13), any givn AP will

only xprinc about 4% mor intrrnc. In comparison, th normally-rcommndd thr channl

bur (i.. using 1, 5, 9, 13) rsults in

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Another Solution?

Most ntrpris-class APs or th capacity to assign an appropriat channl automatically. This is

typically dtrmind during th boot procss by a simpl poll o ach channl at a randomisd intrval, so

that groups o APs booting togthr do not all slct th sam band. Th channl assignmnt rom ths

polling rsults is mad by a vry basic algorithm, picking ithr th channl arthst rom all thos alrady

in us, or i all ar usd thn picking at random.

Som manuacturrs also or wirlss managmnt dvics that control a group o APs and, among

othr unctions, mak channl and transmission powr assignmnts taking into account th group as a

whol. Ths systms can also monitor th spctrum and dynamically adjust th channl assignmnts

in rspons to changs such as nw intrrnc sourcs or AP ailurs (.g. by xtnding rang to

compnsat as much as possibl).

WAGs Advice

Firstly, th JANeT WAG (Wirlss Accss Group) would advis against using automatic channl

assignmnt options, ithr at th AP lvl or as implmntd by a wirlss managmnt dvic. For th

ormr, th primitiv algorithm is barly sucint to avoid initial intrrnc on booting, and is othrwis

littl bttr than a guss. Managmnt dvics mak a bttr stab at dsigning a plan, but tnd to

b biasd towards (a) US channl assignmnts and (b) using rducd output powr to crat a viabl

pattrn. This rsults in a gratr numbr o APs bing rquird to covr an ara than might b rquird

with manually-assignd spctrum managmnt. Most importantly, a manually-assignd dploymnt is

undrstood by its oprators, whilst automatd systms ncourag trating spctrum managmnt as a

black box. A known wirlss nvironmnt acilitats plannd upgrads, snsibl monitoring stratgis

and raction to nw sourcs o intrrnc. That said, th tchnology continus to volv, and this ara

should b monitord as rmwar upgrads bcom availabl in cas a viabl tchnological solution ariss.

Rturning to manual assignmnt, in most WiFi dploymnts in th UK it is sucint to adopt th

convntion o using th our non-ovrlapping channls (in act, in many situations, thr channls ar

nough to achiv a non-ovrlapping topology, so an vn gratr bur btwn adjacnt clls can b

achivd). Howvr, thr ar scnarios whn vn our channls ar insucint:

in highr dnsity dploymnts

across multipl foors o a building

whr xtrnal intrrnc prcluds th us o on or mor o th usual channls.

In ths circumstancs it may prov possibl to us partially-ovrlapping channls at th pric o only

minimal ovrhad in trms o trac collisions. Th rul o thumb is to maximiz th channl sparation

btwn clls as much as is practical, and to b awar that onc th channl sparation drops blow

around 25MHz (thr channls-worth o bur), you will start to xprinc dgradation in prormanc.

Unortunatly, wirlss dploymnt is somthing o an arcan art, and ach cas must b valuatd on its

mrits and tstdin situ to s whthr th impact o th incrasd ovrlap is accptabl.

You should also bar in mind that rducing th powr output o your APs is anothr way to dcras th

numbr o intrring cll boundaris, and that whn all ls ails th 5GHz spctrum usd by th 802.11a

standard is availabl, with an incrasd numbr o non-ovrlapping channls.

In th situation whr an additional AP is rquird within an xisting dploymnt, th cincy o th

WLAN is bst maintaind by rdsigning th ntir channl assignmnt pattrn or all APs in th ara

rathr than taking th asir option o just optimising th band chosn or th AP bing addd in.

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Th potntial dicultis o managing channl assignmnt in wirlss dploymnt also mak it advisabl or

institutions to dvlop a ormal spctrum managmnt plan. This is dsignd to:

prvnt rogu dploymnts occupying channls you nd or utur ocial WiFi srvics

dvlop rlationships with nighbours who might also dploy wirlss (altrnativly, try to gt your kit

dployd rst)

chck that unrlatd tchnologis that also us th ISM band (scurity systms tc.) do not limit your

utur wirlss ntworking options

dvlop ntworking policis and monitoring rgims that lt you polic th radio nvironmnt on

campus to guard your local spctrum rsourc.

Finally, as notd abov, thr ar two mor channls availabl or WiFi hardwar in europ than in th

USA. Givn th dominanc o th US markt or this tchnology, it is prhaps unsurprising that thr

is som obsrvd bias in hardwar prormanc around this point o dirnc: w obsrv that som

hardwar xhibits introprability problms on channls 12 and 13 that it dos not display on th lowr,

common channls 1-11. Whilst ths dicultis otn driv rom a actory dault assumption o US

dploymnt that is radily corrctd through conguration chang or drivr updats, it is worth noting th

potntial or incrasd support load rom dploymnts making us o ths channls whr a common

hardwar platorm is not providd (and as a point to tst whn assssing hardwar or dploymnt).

1. K. Appl, W. Hakn, and J. Koch, evry planar map is our colorabl. II. Rducibility. Illinois J. Math., 21(3):491-567, 1977

2. Mitch Burton, Channl ovrlap calculations or 802.11b Ntworks. TchOnlin, Novmbr 2002.

http://www.tchonlin.com/community/d_rsourc/tch_papr/21668

JANeT is a rgistrd tradmark o thHighr education Funding Councils or england, Scotland and Wals. Th JNT Association is th rgistrd usr

o this tradmark. JANeT, th Unitd Kingdoms ducation and rsarch ntwork, is undd by th Joint Inormation Systms Committ (JISC).

Th JNT Association 2007 PB/INFO/063 (07/05)