Anatomy of a Radio LAN

8/12/2019 Anatomy of a Radio LAN

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3 Anatomy of a radio LAN

A radio networkis a collection of nodes communicating together through radio devices,

using radio waves to carry the information exchanged (obvious, isn't it ?). It is sometimecalled a radio Ethernet, by analogy of the wired technology. Most radio devicesare a

card (IA, !cmcia) to "lug in a !# (or wor$station), and interact directly with thestandard networ$ing stac$ on it (no need of !!! or any s"ecific "rotocol stac$).

3.1 The radio modem

A radio device is com"osed of two main "arts. %he first is the radio modem. %his is the

"art transmitting (modulating) the data onto the fre&uency and receiving othertransmissions. It is com"osed of antenna(s), amplificators,frequency synthesisers,filters

and other bits of magic. %hese are mainly analog "arts, and a bit of digital (in an AI#,

theBaseband).

sually, you can't see all those analog bits (and the cleverness of the board layout)because all the modem is enca"sulated in a metal shield to "rotect your !# from those

high fre&uency radiations.

%he modem main characteristics are thefrequency band, thesignalling rate, the

modulationand the transmitted power. !eo"le building modems are also tal$ing a lot of and d*...

3.2 The MAC controller

%he second "art of the radio device is the MAC controller, res"onsible to run the MA#

"rotocol. %his is im"lemented mainly in an AI# and+or a microcontroler on the card, but

some functionalities of the MA# may be as well in the driver on the !#. %he card also

includes some memory for the MA# controller to store incoming and outgoing "ac$ets(buffers) and other data (configuration, statistics).

Most of the time the few most time critical "arts are handled in the radio modem AI#

(the baseband), the bul$ of the MA# in a microcontroller and only some management

functionality in the driver. *ut, the different manufacturers "lace the boundary betweenthe different functionalities differently (cost+"erformance tradeoff), and some have

im"lemented driver only MA#s for lower cost.

%he main characteristics of the MA# are thepacket format(sie, headers), the channel

access mechanismsand the network managementfeatures. %he amount of on-board

memory is also im"ortant, because the MA# may need a significant number of buffers to

com"ensate the !# and interface latencies.

Functional diagram of a Wireless device :


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3.3 The host interface

%he card interfaceto the !# through one of its buses (!"# $%# $cmcia...) or

communication "orts (serial# parallel# &!BorEthernet). %his interface allows thesoftware (mostly the driver) to communicate with the MA# controller and most of the

time directly to the on board memory (the software writes "ac$ets to a s"ecific locationof it, then the controller reads them and sends them).

%he main characteristic of the interface is mainly the s"eed (i+o, shared memory or MA)

and the ability to "rocess re&uests in "arallel. %he flexibility and functionality of it are

usually more a concern for the "erson writing the driver /-)

3.4 The driver

0ith all modern o"erating systems, the end a""lication doesn't access directly thehardware but use a standard A!I. %he o"erating system needs a driverto interface thehardware to the networ$ stac$ ('%$$# etBeui# $*...). %he main function of the driver

is to manage the hardware and to answer its re&uest (to service interru"ts). In most of the

0ireless 1As, the driver also im"lements some "arts of the MA# "rotocol.

%he main characteristic of the driver is the bugs /-(

3.5 ireless LAN or not

0ireless 1As are not the only devices to ma$e use of wireless technology, and it's easy

to get confused between the different "roducts (es"ecially that sometimes they callthemselves incorrectly wireless networ$s). ome exam"le are wireless bridges, wireless

distribution systemsand cable replacement, and they are &uite different from local area

networ$ing. %here is also wide area wirelessnetwor$ "roducts, which are again &uitedifferent from 1As.

ireless !rid"esare used to connect two different 1A segments via radio, for exam"le

between two buildings across the street. ireless distri#$tion systemsis what are used


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by I! to connect multi"le inde"endant customers to a base station, li$e houses in a

neighbourhood. Ca#le re%lacementis mostly li$e IrA (Infrared data lin$) to transfer

data between two com"uters without a serial or "arallel cable.

ometimes those "roducts use standard 0ireless 1A modules, and most of the time

they are based on the same technologies as 0ireless 1As but with restrictedfunctionality (li$e no broadcasting) and only allow a set of "oint to "oint lin$s (so, no

native %#!+I! to"ology). %hey interface to the serial "ort (cable re"lacement) or ethernet"ort (wireless bridges, wireless distribution system).

In this document we mostly restrict ourselves to true wireless 1As, because what

doesn't run natively %#!+I! is not 2fun2 /-)

3.& 'rofessional and (ome ireless LANs

ow that 0ireless 1As are getting towards lower "rice, 0ireless 1A manufacturers

are no longer targeting mobile commercial users only but also the home mar$et. omevendors, such as !roxim, offer two distinct line of "roduct based on the same technology

(and same "rotocol), the ange1an3 for "rofessionals and ym"hony for home users.

As the business version of those 0ireless 1As are more ex"ensive than the home

"roducts, one might wonder what 4ustify the "rice difference a"art from the "ac$aging,the mar$eting and software bundle.

%he radio modems may "resent different %erformances. %he modem is usually the most

ex"ensive "art of the device, and re"lacing analog "arts by less "erformant ones may

reduce the "rice. %he result may be a lower sensitivity, or less filtering of the ad4acent

bands or channels, which may reduce range and "erformance, es"ecially for high numberof nodes or collocated networ$s (which matter most for business environment).

%he host interface may be different. %he business line may offer more o"tions, such as

5thernet, erial and !#I, whereas home version may offer *. %he home line may alsolac$ security (through encry"tion) or "ower management.

*ut in most cases, the hardware between the two lines is exactly the same. In fact, most

of the differences usually reside in the Access 'oints. %his is why 1ucent offer 6

different Access !oints de"ending on usage and targeted at different $ind of users, butonly one ty"e of card for all ty"es of users.

Access !oints for home users are mostly designed to interface with a "hone line (or

I, 1 or cable modem) and "rovide a "roxy or mas&uerading feature, allowing the

user to share its I! access between the nodes of the networ$.

7n the other hand, Access !oints for businesses connect directly to the 1A via 5thernet

or act as wireless re"eaters, with o"timised bridge functionality, higher "erformance,


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offer a wide range of management features (diagnostic, statistics, access control...)

roaming and out of range forwarding (see chapter +,-,.).

o, before investing your money, you have to as$ yourself what networ$ configurationyou are really after and which features you really do need...

3.) *i"ital radios and chan"in" the %rotocol

7ne &uestion "o""ing u" in my mailbox is the ability of doing "rotocol 89' (%MA,0ireless A%M) with device 8:' (a well $nown 0ireless 1A). A variant of this &uestion

is "eo"le trying to im"lement a s"ecific scheme or o"timisation in the ;


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telecommunication o"erators, for the army... sually, to use a fre&uency band, you must

negotiate with these bodies, register your architecture and buy the right to use the

fre&uency.

%hese organisations, aware of the "ros"ects of local radio communications for individual

users, have allocated some s"ecific fre&uency bands to be used in a more flexible way.%he oldest and most commonly used ones are located at B## allocates both the B


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%he D @ unlicensed bands are another very com"licated story.

5%I was the first to o"en the D @ band, and so far, the D.3 @ band is dedicated to

(i%erLan(see chapter 4,-), and the D.6 @ band reserved for (i%erLan --(alias*A, see chapter 4,2). As they have done for 5!/and0E%', only systems that fully

conform to those standards (!hy and MA#) may o"erate in the band.

In the tates, the >## has allocated the band between D.3 and D.; @ (N-- #and) with

some very liberal rules (no s"read "ectrum mandated, no channels allocated). %o limitsystems, they have introduced com"licated "ower rules, ma$ing the use of around 3or a 3 Mb+s signalling rate modulatedby a == chi"s code (li$e the 0avelan), the result is a signal s"read over 33 M@ of

bandwidth.

Any narrowband interferer, because it uses only a small "art of the total bandwidth used

by the system, will a""ear much wea$er to the irect e&uence system (I thin$ it will bemuch clearer if you loo$ at the "icture below). Moreover, the demodulator use the same

code as the transmitter to match the received signal, which decrease further signals not

modulated by the code (this is called the "rocessing gain of the code, == chi"s as used in;


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s"reading). 7ne the other hand, the fact of having one single fixed channel (as o""osed to

>re&uency @o""ing) eases the tas$ of the higher layers (MA#).

*ecause it uses a large channel, a irect e&uence system has only a few channelsavailable in the bandwidth (E for the Wavelan- on different fre&uencies). %hose channels

are totally se"arate (they don't generate interferences on each other). irect e&uencealso offers the "ossibility to use "artially overla""ing channels for systems in ad4acent

areas, increasing slightly the number of channels. *ut this last solution tends to increasethe noise and decrease the "erformance of the system, because all those systems usually

o"erate with the same code (and not one code "er fre&uency).

4.3.2 6re/$ency (o%%in"

6re/$ency (o%%in"uses a set of narrow channels and wal$ through all of them inse&uence. >or exam"le, the 3.6 @ IM band is divided in CB channels of = M@.

!eriodically (every 3< to 6


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s"ectrum.8etransmissionis a very usual tem"oral diversity.FE%(>orward 5rror

#orrection) is another $ind of tem"oral diversity. ery often, 2diversity2 is associated

with antenna diversityonly. Antenna diversity is only one form of diversity (a s"acialdiversity).

Antenna diversitymeans that the radio device has two (or more) antennas. %hetransmission conditions on the channel vary a lot over the time. %he channel tends to fade

in and fade out (see chapter 2,6,), so the device has moment of good rece"tion andmoment of bad rece"tion. *ut, these conditions are also de"endant on the s"acial

"osition. *y having two antennas, even &uite close (a few cm), the condition at each

antenna is very often totally different. 7ne antenna may give a "oor signal and the other agood one, and a few ms later it might be the reverse. o, before receiving each "ac$et, the

receiver chooses the best antenna of the two by com"aring the signal strengths, and so

can avoid most of the fade out "eriods.

4.5 *irectional antennas

Most wireless 1As use omnidirectional antennas, but may offer directional antennas

in o"tion. Instead of receiving in every directions, the directional antenna favourrece"tion in a more or less narrow angle. %he narrower the angle is, the higher the gain is

(and the range), because you get rid of more unwanted emissions and bac$ground noise

in the other directions.

0ith directional antennas, it is &uite common to have a few $ilometres of range in line ofsight with "roducts in the IM band. %he first "roblem is that you must of course "oint

each antenna towards the node you intend to communicate with (de"ending on the angle

this needs to be more or less "recise). %he second "roblem is that very directional

antennas tend to be &uite big.

%his is why directional antennas are only suited for fixed "oint to "oint lin$s ("roducts

li$e ireless !rid"es). >or most networ$s where nodes need to tal$ to different other

nodes in different directions and might need to move, omnidirectional antennas are muchmore "ractical.

ectored antennasare very similar to directional antennas, and heavily used in cellular

"hone base stations. A set of wide angle directional antenna are assembled on a vertical

"ole, each one covering one "ortion of the horion (a sector, for exam"le E antennas =3J is

more sensitive and re&uires a better (see chapter 2,4,2).

4.).3 ;2.11 (7 +11 M## and claimed that as their new system was generating the same ty"e ofsignal as a system, it's im"act on other systems in the band was the same, so it should

be authorised as well. After a bit of negociation, the >## did acce"t this extension of the

rule. ote that some >@ vendors also tried to get D M@ >@ channels in the 3.6 @band but failed to obtain it.
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9ucentcame u" with the sim"lest solution, !!M (!ulse !osition Modulation), which is

included in their 2%urbo2 line of "roducts, offering D and =< Mb+s. !!M sim"ly shift the

code used in the modem, each "osition can encode some more bits. !!M is sim"le,chea", but low "erformance.

1arristried M*7J (M-ary *i-7rthogonal Jeying), offering D.D Mb+s and == Mb+s,which is a more com"lex modulation than !!M, so more ex"ensive and more robust. %he

signal "roduced by the transmitter is also less similar to a signal.

%hey both went bac$ to the ;M will only use

large symbol time. @owever, by increasing the symbol time we reduce the bit-rate. %o

overcome this constraint, 7>M transmit the symbols no longer serially but in "arallel K

%his way, we have very high bit rate with large symbol time.

7>M use a set of subcarrier fre&uencies, the fre&uencies being orthogonal. 5achsubcarrier is modulated individually, the bit rate and signal strength of each subcarrier

can be ada"ted to get maximum "erformance of the system (we "ut more bits on the good

subcarriers and less on the bad ones). %hen, the system s"lits the bits to transmit betweenthe subcarriers, each subcarrier is modulated and then combined to "roduce the

transmitted signal (using a >ast >ourrier %ransform).
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%he main drawbac$ of 7>M is that it re&uire a greater fre&uency accuracy (we traded

timing accuracy to fre&uency accuracy). As the 7>M signal contains many subcarrier

very close to each other in fre&uency, the system must be very accurate to match all ofthem.

%he first use of 7>M was in the @i"er1an II standard (see chapter 4,2), but since;


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retransmissions(4ust retransmit the original "ac$et in case of errors - some form of

"ac$et scheduling and retransmission has been "roven to be nearly o"timal in ayleigh

fading channels). %his is usually im"lemented at the MA# level (see chapter +,.,).

@owever, in a few case >5# might be needed in 0ireless 1As. ome receivers, either

due to "oor im"lementation or s"ecific design (li$e having an 5&ualiser), generaterandom (aussian) errors, and might benefit from >5#.

4.;.4 M$lti%ath and delay s%read

adio waves reflect or diffract on obstacles, and are attenuated differently by differentmaterials. %his is exactly li$e light, which goes through glass, is reflected by mirrors and

sto" by most obstacles, exce"t that much more materials are trans"arent or reflector to

radio than to light.

In a real environment li$e an office or a house, there is a lot of surface reflecting radio

(walls, ceilings, metal), being semi-trans"arent to radio (walls, ceilings, humans) oro"a&ue to radio (metal). %his gives trouble estimating the range of the system (see

chapter 2,4). %his also mean that the signal received at a node may come from differentdirections (de"ending on reflections on the environment) with different strength

(de"ending on attenuations), and the receiver sees only the combinations of all these

reflections. %his "henomenon is called m$lti%ath.

Most of the time, multi"ath is good, because the addition of all the reflections of thesignal increase its strength. %he main effect of multi"ath is that rangeis very difficult to

evaluate (see chapter 2,4,-) and the receiver ex"eriencesfading(see chapter 2,6,).

*ut, the main "roblem of multi"ath is that it creates delay s%read. e"ending on thenumber of reflections and the "ro"agation s"eed in different signals, all these signalsdon't arrive exactly at the same time at the receiver. It's li$e the 2echo2 you may hear in

the mountains, the signal going directly will be faster than one reflecting twice on the

walls.

7f course, as radio "ro"agate at the s"eed of light, those difference are very small (below

the microsecond). *ut, when the bitrate of the system increases, those time differences

becomes significant with regards to the symbol time (see chapter 2,3,.), to the "oint of

creating destructive interferences (the current symbol will be corru"ted by the echo of the"revious symbols).

*it rate lower than = Mb+s are relatively immune to delay s"read "roblems (the symbol

time is = s and higher), but as the bit rate increase above = Mb+s the effect of delay

s"read increases. It is considered that systems faster than D M+s should have sometechni&ue to overcome delay s"read.

/ultipath and 0elay !pread :
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%he main techni&ue to overcome delay s"read is using an >/$aliser. An e&ualiser is a big

digital circuit that try to estimate the different com"onents of the signals. %he e&ualiser

need to be trained ("ac$ets includes a s"ecific well $nown training se&uence) to

determine what are the different "ath, their relative timings and strength. %hen, thee&ualiser se"arate the different com"onents of the signal and recalculate the signal

removing the delay s"read.

%he main disadvantage of 5&ualiser is that they are ex"ensive. ecently, some standardsare starting to use =6*M(see chapter 2,3,2), which is a clever modulation techni&ue

minimising the im"act of delay s"read.

5 The MAC level +link layer,%his section of the document focus on the next layer u", the lin$ layer. %his mostlycom"rise the MAC(Medium Access #ontrol) "rotocol. ifferent MA# "rotocols and

techni&ues are "resented.

5.1 Main channel access mechanisms

%he main 4ob of the MA# "rotocol is to regulate the usage of the medium, and this is

done through a channel access mechanism. A channel access mechanismis a way to

divide the main resource between nodes, the radio channel, by regulating the use of it. It

tells each node when it can transmit and when it is ex"ected to receive data. %he channelaccess mechanism is the core of the/"% protocol. In this section, we describe '0/"#

%!/"andpollingwhich are the E main classes of channel access mechanisms for radio.

5.1.1 T*MA

In this cha"ter, we discuss %MA as a channel access mechanism and not its a""lications

and "rotocols based on it.
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T*MA(%ime ivision Multi"lex Access) is very sim"le. A s"ecific node, the #asestation, has the res"onsibility to coordinate the nodes of the networ$. %he time on the

channel is divided into time slots, which are generally of fixed sie. 5ach node of thenetwor$ is allocated a certain number of slots where it can transmit. lots are usually

organised in a frame, which is re"eated on a regular basis.

%he base station s"ecify in the beacon (a management frame) the organisation of the

frame. 5ach node 4ust needs to follow blindly the instruction of the base station. eryoften, the frame is organised as downlin$ (base station to node) and u"lin$ (node to base

station) slots, and all the communications goes through the base station. A service slot

allows a node to re&uest the allocation of a connection, by sending a connection re&uestmessage in it (see chapter +,.,2). In some standards, u"lin$ and downlin$ frames are one

different fre&uencies, and the service slots might also be a se"arate channel.

'0/" channel access mechanism :

%MA suits very well "hone a""lications, because those a""lication have very

"redictable needs (fixed and identical bit rate). 5ach handset is allocated a downlin$ and

a u"lin$ slot of a fixed sie (the sie of the voice data for the duration of the frame). %hisis no sur"rise why %MA is used into all cellular "hone standards (M in 5uro"e,

%MA and !# in the A) and cordless "hone standards (5#% in 5uro"e). %MA is

also very good to achieve low latency and guarantee of bandwidth (where #MA+#A is&uite bad).

%MA is not well suited for data networ$ing a""lications, because it is very strict and

inflexible. I! is connectionless and generates bursty traffic which is very un"redictable by

nature, while %MA is connection oriented (so it has to suffer the overhead of creatingconnections for single I! "ac$ets). %MA use fixed sie "ac$ets and usually symmetrical

lin$, which doesn't suit I! that well (variable sie "ac$ets).

%MA is very much de"endant of the &uality of the fre&uency band. In a dedicated clean

band, as it is the case for cellular "hone standard, %MA is fine. *ut, because of it's
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inflexibility, and because it doesn't really ta$e care of what's ha""ening on the channel,

%MA can't co"e and ada"t to the bursty interference sources found in the unlicensed

bands (unless a retry mechanism is "ut on to" of it).

5.1.2 CMA


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%he "rotocol starts by listening on the channel (this is called carrier sense), and if it is

found to be idle, it sends the first "ac$et in the transmit &ueue. If it is busy (either another

node transmission or interference), the node waits the end of the current transmission andthen starts the contention(wait a random amount of time). 0hen its contention timer

ex"ires, if the channel is still idle, the node sends the "ac$et. %he node having chosen the

shortest contention delay wins and transmits its "ac$et. %he other nodes 4ust wait for thenext contention (at the end of this "ac$et). *ecause the contention is a random number

and done for every "ac$ets, each node is given an e&ual chance to access the channel (on

average - it is statistic).

As we have mentioned, we can't detect collisions on the radio, and because the radioneeds time to switch from receive to transmit, this contention is usually slotted(a

transmission may start only at the beginning of a slot / D< s in ;@ and 3< s in

;


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In the case of =or ;or exam"le, most ;


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nfortunately, radio transmission has a lot of overhead (li$e large synchronisation field

and headers) which is somewhat incom"atible with the small A%M cells. %he main

benefit of A%M small cells is to allow very efficient switching, but this is not needed overradio. At the end of the day, 0A%M doesn't resemble at all to A%M O A%M uses individual

channel for each node and is asynchronous, whereas 0A%M uses a shared medium and is

totally synchronous.

5.2 MAC techni/$es

0e have described the main "rinci"le of #MA+#A (see chapter +,,.), but most MA#

"rotocols use additional techni&ues to im"rove the "erformance of #MA+#A.

5.2.1 MAC retransmissions

As we have seen in the "revious cha"ter, the main "roblem of the %!/"%" protocolis

that the transmitter can't detect collisions on the medium. %here is also a higher error rate

on the air than on a wire (see chapter 2,6), so a higher chance of "ac$ets being corru"ted.%#! doesn't li$e very much "ac$et losses at the/"% layer(see %#! and "ac$et losses

"roblem - chapter +,2,+). *ecause of that, most MA# "rotocols also im"lement %ositiveacknowled"ementand MAC level retransmissionsto avoid losing "ac$ets on the air.

%he "rinci"le is &uite sim"le / each time a node receives a "ac$et, it sends bac$immediately a short message (an ac$) to the transmitter to indicate that it has successfully

received the "ac$et without errors. If after sending a "ac$et the transmitter doesn't receive

an ac$, it $nows that the "ac$et was lost, so it will retransmit the "ac$et (after contendingagain for the medium, li$e in 5thernet).

Most MA# "rotocols use a sto" and go mechanism, they transmit the next "ac$et of the&ueue only if the current "ac$et has been "ro"erly ac$nowledged (no sliding window

mechanism li$e in %#!). %he rationale is that it ma$es the "rotocol sim"ler, minimiselatency and avoid desen&uencing "ac$ets (something that %#! doesn't li$e as well).

/"% retransmissions in %!/"%" :
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%he ac$s are 2embedded2 in the MA# "rotocol, so they are guaranteed not to collide (the

contention starts after the ac$ - see figure). %hese ac$s are very different from the %#!

ac$s, which wor$ at a different level (and on a different time frame). 7f course, broadcastand multicast "ac$ets are not ac$nowledged, so they are more li$ely to fail...

If all modern 0ireless 1A "rotocols im"lement this essential feature, some old "roductsmay lac$ it. 0ireless 0A "rotocols (li$e satellite lin$s) don't im"lement that either,

because the round tri" delay in their case is so long that by the time they would receivethe ac$ they could have sent another "ac$et. If your 0ireless 1A doesn't im"lement

MA# level retransmissions, all is not lost / students of *er$eley have created a "rotocol

calledsnoop(see at ft"/++daedalus.cs.ber$eley.edu+"ub+snoo"+) which filters the %#! ac$sand retransmits the lost "ac$ets before %#! even notices that they are lost (this is still a

lin$ level retransmission, but done 4ust over the MA#).

5.2.2 6ra"mentation

%he radio medium has a higher error ratethan a wire. 0e have ex"lained in the "reviouscha"ter that it was why most "roducts were including MA# level retransmissions to

avoid losing "ac$ets.

MA# level retransmissions solve this "roblem, but is not really "erformant. If the "ac$et

to transmit is long and contains only one error, the node needs to retransmit it entirely. Ifthe error rate is significantly high, we could come to some situation were the "robability

of error in large "ac$et is dangerously close to = (we can't fit a "ac$et between the bursts

of errors due to fading or interferers), so we can't get "ac$et through.

%his is why some "roducts use fra"mentation. >ragmentation is sending the big "ac$ets

in small "ieces over the medium. 7f course, this adds some overhead, because itdu"licates "ac$et headers in every fragments. 5ach fragment is individually chec$ed and

retransmitted if necessary. %he first advantage is that in case of error, the node needs onlyto retransmit one small fragment, so it is faster. %he second advantage is that if the

medium is very noisy, a small "ac$et has a higher "robability to get through without

errors, so the node increases its chance of success in bad conditions.

5.2.3 7T


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*ut, for a node "laced in between, these simultaneous transmissions have a com"arable

strength and so collide (in its receiver). %his node could be im"ossible to reach because

of these collisions.

%he fundamental "roblem with carrier sense only is that the transmitter tries to estimate if

the channel is free at the receiver with only local information. %he situation might be&uite different between those two locations.

An sim"le and elegant solution to this "roblem ("ro"osed by !hil Jarnin his MA#A"rotocol for A9.3D) is to use 7T


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*ecause the %+#% handsha$ing adds a significant overhead, usually it is not used for

small "ac$ets or lightly loaded networ$s.

5.2.4 7eservation and service slots

7ne of the main "roblem of %MA and !olling "rotocol is for the base station to $nowwhen the nodes want to transmit. In #MA+#A, each node sim"ly waits to win a

contention, so this "roblem doesn't exist. @owever, %MA and !olling usually re&uire a

service slotor reservation slotmechanism.

%he idea is to offer a "eriod of time where nodes can contend (com"ete) and send to thebase station some information about their traffic re&uirements (a reservation re&uest

"ac$et), this "eriod of time coming at regular interval (the remaining of the time, nodes

4ust obey the base station normally). %he base station feeds the reservation re&uests to itsscheduling algorithmand decides the main frame structure (when each node will

transmit). %his "eriod of time for sending reservation re&uests is either called service slot

(if it is use for more "ur"ose li$e cell location and roaming) or reservation slot (if it is useonly to re&uest a transmission or connection).

If the MA# is connection oriented, the rate of new connection is low, so usually a single

service slot is enough (see figure in chapter +,,). If the MA# is "ac$et oriented, the rate

of re&uests is higher, so usually the "rotocol offer many reservation slots together (seechapter +,,-). odes use a sim"le"loha protocolin the slots / they transmit, and if it fail

(collision with other re&uests or medium errors) they bac$off a random number of slots

before retrying.

!rotocols which use many different channels, such as cellular "hone, can even have a

dedicated service channel se"arate from other transmissions, instead of multi"lexingservice re&uests with the data traffic.

5.3 Network to%olo"y

%he to"ology of 0ireless 1A is very different from traditional 1As. %he connectivity

is limited by the range, so we usually don't have com"lete coverage (some node may not

see each other). %his brea$s some assum"tions of higher layers. %o overcome this, eitherthe networ$ is divided in cells managed by an"ccess $oint, or the networ$ use/"%

level forwarding.

5.3.1 Ad8hoc network

Ad-hoc networ$ is the sim"lest form of 0ireless 1A is a networ$ com"osed of a few

nodes without any bridging or forwarding ca"ability. All nodes are e&ual and may 4oin orleave at any time, and have e&ual right to the medium. In fact, it's very much li$e an

5thernet, where you may add or remove node at discretion. %his is the $ind of radio

networ$s de"loyed in homes of small offices.
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through the wired networ$ and via the access "oint where the destination is registered

(that is o$t of ran"e forwardin").

A few systems use as well the access "oint as a networ$ central coordinator of thechannel access mechanism (%MA and "olling mode). %his is a bad idea, because it

decreases the overall reliability and flexibility of the system / every node must be able tocommunicate at any time the access "oint in order to wor$, even if it wants to

communicate with a close neighbour.

"ccess $oints# roaming and radio /"% forwarding :

8oaming @ "ccess $oints 8adio /"% forwarding

5.3.3 7adio MAC forwardin"

%he forwarding mechanism designed around"ccess $oints(see chapter +,-,.) re&uires a

fixed wired infrastructure to lin$ the Access !oint. %his might be satisfactory for mostusages, but is not ade&uate for ad-hoc networ$s.

ome MA# "rotocol (such as @i"er1an - see chapter 4,-) "rovide a MAC level

forwardin", where every node of the networ$ can be used to relay the message on the air

to the destination. %he "rotocol doesn't rely any more on a fixed infrastructure, but on allthe wireless nodes on the "ath.

o, how do we found the o"timal "ath through the nodes to the correct destination ? %his

forwarding mechanism use management message to "ro"agate networ$ changes andto"ology information, and from those messages nodes can com"ute the o"timalforwarding tables. odes must im"lement the forwarding ca"ability and "ro"agate

message based on those routing tables. In fact, each node of the networ$ acts as a ad-hoc

wireless bridge.
http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#aphttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.std.html#Hiperlanhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#aphttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.std.html#Hiperlan


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*roadcast and multicast messages are a bit of a "roblem (they have always been on

bridging technologies) / all nodes 4ust re"eat them and the strategy is to flood the networ$

with them (that's the only way to ma$e sure they reach all "ossible destinations).

ome access pointsalso offer the "ossibility to be configured as ireless 7e%eaters,

which "rovide the same $ind of radio forwarding but in a managed way.

adio MA# forwarding is elegant and interesting, but all the forwarding consume some

more radio bandwidth, which is already limited to start with.

5.4 ome thro$"h%$t considerations

If the "hysical layer "eo"le are mostly tal$ing range and d*, MA# layer "eo"le are (or

should be) concerned about the through"ut of the system.

5.4.1 !it8rate vers$s ma9im$m $ser thro$"h%$t

1i$e for wired "roducts, most radio 1A vendors indicate only the #it8rateof their

"roducts (also called signalling rate). >or exam"le,Ethernetis =< Mb+s, =or exam"le, we

might s"lit big "ac$ets into small inde"endent fragments to decrease the error "robability

(see chapter +,.,.on fragmentation). Ac$s and %+#% add also some overhead.@aving a slotted contention decreases the collisions but ma$es the average contention

delay larger as well.

0hen you add all this, it starts to ma$e a significant difference. If in the case of 5thernetyou may ho"e to reach ;


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5.4.2 M$ltirate system considerations

Most vendors offer multirate systems (see chapter 2,3,), the lower rate allowing a

greater coverage and the higher rate allowing greater through"ut at lower range, and offera mechanism for each node to ada"t the bit-rate de"ending on channel conditions.

*asically, when "ac$ets start to fail, the node reduce the rate.

7f course, "eo"le are li$ely to benchmar$ nodes in relatively close "roximity (two nodes

on the table), when the system will use the highest rate, but the real advantage of 0ireless1As is usually given at higher range (in the garden, moving around), and in this case

the system is li$ely to select the lower rate (and maybe suffer from "ac$et losses and

retransmission due to range), so the "erformance will be less.

@owever, those rate ada"tation schemes are not always the most clever. 0hen there is aninterferer in the band, reducing the rate may increase marginally chances of "ac$ets to get

through, but most of the time having longer transmission time 4ust increase the

"robability of collision. In cases where there is lot's of contention (lot's of nodes with lot'sof traffic), some "roducts do reduce the rate which doesn't hel" to reduce to congestion

(I've seen that "ersonally). In those "articular cases, you may want to fix the rate yourself

to the highest and disable the rate reduction feature.

@aving a multi-rate system also im"act the overhead of the system, es"ecially at highrates. All the basic "art of the "rotocol (headers, management messages, contention) is

designed for the slowest rate, so when going to higher rate their relative sie increase

(their duration remain the same while the "ayload duration decreases).

>or exam"le, when sending the same =D


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%he second exam"le is devices im"lementing only one transmit buffer. %his saves some

cost (memory, com"lexity), but, as the buffer may be either written by the driver or

transmitted over the air but not both at the same time, this creates dead time over the airwhile the driver refills the buffer and reduces the available through"ut. %his was one of

the "erformance gain between the first and the second generation of 5thernet cards in the

old days.

%he "rotocol might also "erforms better when many node are active than when only oneof them transmits. >or exam"le, the contention window in #MA+#A (number of

contention slots) im"act the "erformance O a larger contention window will decreases the

collisions but when there is a few nodes, those will wait on average longer to access thechannel (the common ;or the individual through"ut to be the maximum through"ut, the node must be able to

manage multi"le slots and multi"lex data between these slots.

5.4.4 Contention and con"estion

In the "revious cha"ter we examine why the shared through"ut could be higher than theindividual through"ut. *ut, the reverse can also be true (and is actually more li$ely for

#MA+#A systems).

0hen there is many nodes sending "ac$ets on the networ$, the "robability of having two

nodes choosing the same slot in the contention window increases. 0hen two nodeschoose the same slot (and they are first), their "ac$ets collide and are lost. %his mean that

when the level of contentionincreases, the number of retry increases as well, so the

"erformance of the networ$ dro" u" to the "oint of congestion.

In fact, ;


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A solution to this "roblem is to use %+#% (seechapter +,.,-), because %+#%

ma$es each collision much shorter. In fact, with %+#% enabled, ;or exam"le, with a >re&uency @o""ing system having =.G Mb+s user through"ut, by"utting =D networ$s, each on a different ho""ing "attern, we should have in theory a

36 Mb+s aggregate through"ut. In fact, because the different >re&uency @o""ing "atterns

2collide2 on the same fre&uency (and also suffer from co-channel interference) from time

to time, the actual aggregate through"ut is less, and is in this exam"le only =D Mb+s.

These collisions of the ho%%in" %atterns is why

6re/$ency (o%%in" can?t offer $% to )0 networks on

the )0 channels +#$t only $% to 15 in this case,... &

ome ireless LAN standards
http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#rtshttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#rtshttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#retryhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#rtshttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#retry


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A short gallery of the most famous 0ireless 1A standard (but unfortunately not

necessarily the most wides"read...).

&.1 ->>> ;2.11

%he main "roblem of radio networ$s acce"tance in the mar$et "lace is that there is notone $ni/$e standardli$e 5thernet with a guaranteed com"atibility between all devices,

but many "ro"rietary standards "ushed by each inde"endent vendor and incom"atiblebetween themselves. *ecause cor"orate customers re&uire an established uni&ue

standard, most of the vendors have 4oined the I555 in a effort to create a standard for

radio 1As. %his is ->>> ;2.11(li$eEthernetisEEE 6


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;M "hysical layer is a very close co"y of the one used in

1iper9an (so they might be some sort of com"atibility - see chapter 4,2), using D3subcarriers in a 3< M@ channel, offering G, =3 and 36 Mb+s and o"tional B, =;, EG, 6;

and D6 Mb+s bit-rates. o "roducts are yet on the mar$et.

ery soon after, ;


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o"timal route within the @i"erlan networ$ (the routes are regularly automatically

recalculated). @i"erlan is also totally ad-hoc, re&uiring no configuration and no central

controller.

%he main deficiency of @i"erlan standard is that it doesn't "rovide real isochronous

services (but comes &uite close with time to live and "riority), doesn't fully s"ecify theaccess "oint mechanisms and hasn't really been "roved to wor$ on a large scale in the real

world. 7verhead tends also to be &uite large (really big "ac$et headers).

@i"er1an suffers from the same disease as ;M carriers used to carry data and6 additional are used as references ("ilot carriers - total is D3 carriers, E=3.D $@ s"acing).

@i"er1an II is a ireless ATMsystem (see chapter +,,2), and the MA# "rotocol is a%MA scheme centrally coordinated with reservation slots. 5ach slot has a D6 * "ayload,

and the MA# "rovide A (segmentation and reassembly - fragment large "ac$ets intoD6 * cells, see chapter +,.,.) and AN (Automatic e&uest - MA# retransmissions, see

chapter +,.,). %he scheduler (in the central coordinator) is flexible and ada"tive, with a

call admission control, and the content of the %MA frame change on a frame basis to

accommodate traffic needs. @i"er1an II also defines "ower saving and security features.

@i"er1an II is designed to carry A%M cells, but also I! "ac$ets, >irewire "ac$ets (I555

=EB6) and digital voice (from cellular "hones). %he main advantage of @i"er1an II is that

it can offer better &uality of service (low latency) and differentiated &uality of service

(guarantee of bandwidth), which is what "eo"le de"loying wireless distribution systemwant. 7n the other hand, I'm worried about the "rotocol overhead, es"ecially for I!

traffic.

&.5 =%enAir

7"enAir is the "ro"rietary "rotocol from 'ro9im. As !roxim is one of the largest

0ireless 1A manufacturer (if not the largest, but it de"ends which numbers you are
http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.modem.html#ofdmhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#watmhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#fragshttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#retryhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.modem.html#ofdmhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#watmhttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#fragshttp://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.mac.html#retry


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im"ose tight constraints on the modem (timing and filtering), ma$ing it high cost. %he

A's"ecification, by releasing slightly those constraints, allows for a much chea"er

im"lementation, but still $ee"s a good "erformance.

%he MA# "rotocol is im"lemented in software and digital, so doesn't contribute that

much to the final cost of the "roduct (exce"t in term of develo"ment cost). eleasingsome hardware constraints "revented the use of the ;


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of a ireless !, and in fact loo$ing into the huge s"ecification we can see some

similarities in the design.

*lue%ooth offers the "ossibility to create a set of "oint to "oint wireless serial "i"es(8f%omm) between a master and u" to G slaves, with a "rotocol (!0$) to bind those "i"es

to a s"ecific a""lication or driver. %he *lue%ooth mindset is very vertical, with various"rofiles defining every details from bit level to a""lication level. %#!+I! is only one

"rofile, im"lemented through !!! in a s"ecific "i"e. %here are other "i"es for audio,7bex... 0ith *lue%ooth, nodes need to be ex"licitely connected, but they remember

bindings from one time to another.

%his is miles away from the current wireless 1A a""roach (connectionless broadcastinterface, native I! su""ort, cellular de"loyement, horiontal "lay), so *lue%ooth doesn't

fit %#!+I! and wireless 1A a""lications too well. 7n the other hand, as a wireless *,

it fulfil a role that regular wireless 1As can't, because %#!+I! discovery and binding

"rotocols are more heavyweight.

#urrently, *lue%ooth is moving very slow (my first reading of the s"ec was autumn BC -

then called M#-1in$) due to its com"lexity and the inherent limits due to the "rotocol

design ("eo"le are learning how to wor$around 2features2), but eventually some "roducts

should reach the mar$et and later on software su""ort should come...

In summary, if all you want is to run %#!+I!, you may find it chea"er and more effective

to 7% wait for *lue%ooth and live without the hy"e.

Documents

Anatomy of a Radio LAN