VoIP Over Wireless Mesh Networks - Implications and Challenges

7/29/2019 VoIP Over Wireless Mesh Networks - Implications and Challenges

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VoIP over Wireless Mesh Networks:

Implications and Challenges

May 1, 2008

Carlo Alberto Boano

School of Information and Communication Technology

Kungliga Tekniska hgskolan (KTH)Stockholm, SwedenPolitecnico di Torino

Turin, Italy

[email protected]

Md. Sakhawat Hossen

School of Information and Communication Technology

Kungliga Tekniska hgskolan (KTH)Stockholm, Sweden

[email protected]

ABSTRACT

Wireless MeshNetworks (WMNs)have significantly captured the research community attention in the recent

past,emergingasakeytechnologyfornextgenerationwirelessnetworks,showingrapidprogress,andinspiring

numerousapplications.At the same time,VoIP serviceshad increased tremendously theirpopularityand role,

becomingoneofthekillerapplicationstoday,sincetheyprovideanattractiveapproachtodeliveringvoicetraffic

over different types of IP networks. However, providing high quality multimedia services in a flexible and

intelligentmannerandhandlingrealtimeapplicationsoverWirelessMeshNetworks introducemanychallenges

andissuestobeconsideredandsolved.AfteraquickoverviewofWMNsandtheirissues,thispaperfocuseson

understandingthechallengesthatmustbefacedwhendeliveringvoiceandrealtimetrafficoverWirelessMesh

Networks.Subsequentlyfocuswillbemovedontheanalysisofsomeoftheexistingapproachesandhowdothey

trytosolvethesechallenges.

KEYWORDS

VoIP,WMNs,WirelessAdhocnetworks,MANET,SIP.

1. INTRODUCTION

Inthelastfewyears,WirelessMeshNetworks(WMNs)havebeenabletocharmthecommunity,standingoutas

a key technology for nextgeneration wireless networking, thanks to characteristics like speed, easiness, and

inexpensiveness that they bring to wireless access. In addition to being widely accepted in the traditional

application sectors of adhoc networks, WMNs are also undergoing rapid commercialization in many other

application scenarios such asbroadbandhome networking, community networking, building automation, high

speedmetropolitan

area

networks,

and

enterprise

networking

[1]. Because

of

their

advantages

over

other
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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wirelessnetworks,WMNsareundergoingrapidprogressandinspiringnumerousapplications.However,thereare

stillmany technical issuesandchallenges in thedesignofWMNs.Oneof themost important issues ishow to

efficientlysupportrealtimetraffic,and inparticularaVoiceover IP(VoIP)application,which isexpectedtobe

oneoftheneuralgicapplicationsforfuturewirelessnetworks.

In fact, in the recent years, we are facing a quick proliferation of VoIP telephony market, and all its related

services,

made

possible

by

the

increasing

pervasiveness

of

high

speed

internet

connections.

The

rapid

diffusion

of

VoIPisduetothefactthatitoffersbusinessessignificantcostsavings,increasedrevenues,andbettercustomer

services.VoIPprovidesindeedanattractiveapproachtodelivervoicetrafficoverdifferenttypesofIPnetworks,

andithasthusbecomearealityinWMNs.

As both Wireless Mesh Networks and VoIP grow in popularity, it becomes very important understanding the

challengesthattheVoiceoverInternetprotocolserviceposeswhendeployedoveraWMN,andhowbehavesthe

overallnetworkperformancewhenhandlingrealtimeapplications.Inordertoprovideabetterunderstandingof

these issuesandproblems,thispaperpresentsadetailed investigationofcurrentstateoftheartforVoIPover

WMNsandtheadditionalchallengesintroducedbyadhocnetworksandMANET.

Therest

of

the

paper

is

organized

as

follows:

sections

2and

3give

an

overview

of

WMNs

and

VoIP

technologies;

section4analyzes themainchallengesofVoIPoverWMNs,andsection5describestheexistingapproaches in

ordertosolvetheseissues.Thefinalpartofthepaperisdedicatedtoremarks,proposalsandconclusionsmade

bytheauthors.

Thisarticlehasbeenwrittenbybothauthorsatthesametime,whichfocusedtheirknowledgerespectivelyon:

CarloAlbertoBoano:VoIPoverWirelessMeshNetworksbackgroundandMACchallengesinWMNs. Md.SakhawatHossen:routinglayerchallengesinWMNsandadditionalchallengesinadhocNetworks.

2.WIRELESS

AD

-HOC

NETWORKS

AND

WIRELESS

MESH

NETWORKS

2.1WIRELESSADHOCNETWORKS

Awirelessadhocnetworkisadecentralizedwirelessnetwork.Thenetworkisadhocbecauseeachnodeiswilling

to forwarddata forothernodes,and so thedeterminationofwhichnodes forwarddata ismadedynamically

basedon thenetwork connectivity.This is incontrast towirednetworks inwhich routersperform the taskof

routing. It is also in contrast tomanagedwireless networks, in which a special node, known as access point,

managescommunicationamongothernodes.

Thedecentralized

nature

of

wireless

ad

hoc

networks

makes

them

suitable

for

avariety

of

applications

where

centralnodes cannotbe reliedon, andmay improve the scalabilityofwirelessadhocnetworks compared to

wirelessmanagednetworks,thoughtheoreticalandpracticallimitstotheoverallcapacityofsuchnetworkshave

beenidentified[2,3].Minimalconfigurationandquickdeploymentmakeadhocnetworkssuitableforemergency

situationslikenaturaldisastersormilitaryconflicts.

Oneof thekey concepts thatareoftenapplied toadhocnetwork ismobility,which is themanagementof a

mobilehostconnectedtothe Internet. Inthiscase,wetalkaboutaparticularsubsetofadhocnetworkscalled

MANETs,inwhichmobilitymanagementinvolvesthedecisionofif,whenandwheretoperformthehandoverto

anothernetwork[4].


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2.1.1MANETNetworks

MobileAdhocNETworks (MANETs)areagroupofmobilenodes interconnected throughwirelessadhoc links.

Thosenodescanmovefreelyanddynamicallyautoorganizing,selfconfiguringandthuscreatingatemporaryand

arbitrarytopologywithoutanypreexistinginfrastructure,sowithoutanyhierarchy.Theroutersarefreetomove

randomly and organize themselves arbitrarily; thus, the network's wireless topology may change rapidly and

unpredictably. Such a network may operate in a standalone fashion, or may be connected to the Internet.

Roamingin

such

network

architectures

is

very

complex

and

causes

many

new

problems.

AwellknowninstanceofMANETisthesocalledVANETs(Vehicularadhocnetworks)andInVANETs(Intelligent

VANETs),whichareusedforcommunicationamongvehiclesorbetweenvehiclesandroadsideequipment,and

canprovidemultipleautonomicintelligentsolutionstomakeautomotivevehiclestobehaveinintelligentmanner

duringspecialevents.

Themost important issue inMANETs isthattherequirementofsmoothandadaptivedeliveryofrealtimeand

multimediaapplicationsmakesthedesignofmobilitymanagementschemeevenmorechallenging.

2.2

WIRELESS

MESH

NETWORKS

(WMNs)

Mesh networking is a way to route data, voice and instructions between nodes, which allows continuous

connections and reconfiguration around broken links by hopping from node to node until the destination is

reached. If all the nodes are connected, the network is called fully connected. The components of mesh

networksaregenerallynotmobile,andcanconnecttoeachotherviamultiplehops[5].

Meshnetworksareselfhealing:thenetworkcanstilloperateevenwhenanodecrashesoraconnectionbreaks

down. As a result, a very reliable network is formed. This concept is applicable to wired networks, software

interaction,andwirelessnetworks.Whenmeshnetworking concept isapplied to this lastarea,we talkabout

WMNs,

and

it

is

where

this

paper

focuses

on.

2.2.1KindsofWMNs

Accordingto[1],thearchitecturesofWMNscanbeclassifiedintothreetypes:

Infrastructure/BackboneWMNs.Meshroutersformaninfrastructureforclients,preciselyameshofselfconfiguring and selfhealing links among themselves. Thus, a backbone for conventional clients is

provided, andenables integration of WMNswith existingwirelessnetworks, through internetcapable

gateway/bridgefunctionalitiesinmeshrouters.

ClientWMNs.Thistypeofarchitectureprovidespeertopeernetworksamongclients,thusclientnodesconstitutetheactualnetworkandperformroutingandconfigurationfunctionalitiesaswellasproviding

end

user

applications

to

customers.

A

mesh

router

is

hence

not

required

for

these

types

of

networks.

Consequently, requirements of enduser devices increase when compared to infrastructure meshing,

since in client WMNs the endusers must perform additional functions such as routing and self

configuration.

HybridWMNs.Thisarchitectureresultsfromthecombinationofinfrastructureandclientmeshing:meshclientscanaccessthenetworkthroughmeshroutersaswellasdirectlymeshingwithothermeshclients.

It isthearchitecturethatwrapsmostofthebenefits(asshown inthenextsection),andanexample is

showninfigure1.


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Figure1:HybridWMNsarchitecture(adaptedfrom[1]).

2.2.2MaincharacteristicsofWMNs

WirelessMeshNetworksaremultihopwirelessnetworks,butwithawirelessinfrastructure/backboneprovided

by mesh routers, that in case of hybrid architecture is robust and allows redundancy. The redundant paths

betweenbackboneendpointsimprovereliability.Meshrouters,thatcanbeequippedwithmultipleradios,have

minimal mobility and perform dedicated routing and configuration, which significantly decreases the load on

meshclientsandotherendnodes.Mesh routersalso integrateheterogeneousnetworks, includingbothwired

andwireless.Thus,multipletypesofnetworkaccessexistinWMNs.

ThemainadvantagesofWMNsaretheeasinessofdeployment,thereductionofthenumberofAccessPointsto

theInternetthatallowsalowcostforinstallation,andthefactthatcommunicationispossibleeveninsituations

wherecertainsystemsareoverloaded.However,highdelayincaseofroutebreaksandunpredictabilityofroute

qualityareknown issues inWirelessMeshNetworks.Anotherwellknown issue forWMNs is the toughness to

calculatetheNetworkCapacity.Theoreticalnetworkcapacityisinfactstillunknown,especiallywhenthenumber

ofnodes issmall.Thereason isthattheassumptionsaboutthenetworksizeornodedensity intheasymptotic

analysisdonotmatchtheactualscaleofanyWMNs.

Thanksto

their

characteristics,

Wireless

Mesh

Networks

can

be

widely

used

in

todays

world,

in

fields

like

broadbandhomenetworking,communityandneighborhoodnetworking,enterprisenetworking, transportation

systems,buildingautomationandpeertopeercommunications[1].

2.2.3SimilaritiesanddifferencesbetweenWMNsandadhocnetworks

Meshnetworkingistheevolutionoftheadhocnetworking:fromacompletelyisolatedautoconfigurednetwork,

wemove toamorepragmaticparadigm that isbasedon flexibleextension (wireless)ofwired infrastructure:

mesh networks are realizedby a combinationof fixedor semifixednodes andmobile nodes, interconnected

throughwirelesslinkstocreateanautoconfiguredmultihopnetwork.Mobilityofendnodesissupportedeasily

throughthewirelessinfrastructure:althoughmeshroutersareusuallystatic,meshclientsareoftenmobile,and

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thisallowsacostefficientextensionofnetworkcoverage.Adhocnetworkingcanbethusconsideredasasubset

ofWMNs[1]:whileadhocnetworkingtechniquesarerequiredbyWMNs,theadditionalcapabilitiesnecessitate

more sophisticated algorithms and design principles for the realization of WMNs. In particular, the main

differencesbetween(hybrid)WMNsandadhocnetworkaregivenbythefollowingpoints:

Wirelessinfrastructure.WMNsconsistofawirelessbackbonewithmeshrouters.Thewirelessbackboneprovideslargecoverage,connectivity,androbustnessinthewirelessdomain,whereasconnectivityinad

hocnetworks

depends

on

the

individual

contributions

of

end

users,

which

may

not

be

reliable.

Dedicated routing and configuration. In adhoc networks, enduser devices also perform routing andconfiguration functionalities for all other nodes. However, WMNs contain mesh routers for these

functionalities; hence the load on enduser devices is significantly decreased, which provides lower

energyconsumptionandhighendapplicationcapabilitiestopossiblymobileandenergyconstrainedend

users.Moreover,theenduserrequirementsarelimitedwhichdecreasesthecostofdevicesthatcanbe

usedinWMNs.

Multiple radios. Mesh routers can be equipped with multiple radios to perform routing and accessfunctionalities.Thisenablesseparationoftwomaintypesoftrafficinthewirelessdomain.Whilerouting

andconfigurationareperformedbetweenmeshrouters,theaccesstothenetworkbyenduserscanbe

carriedoutonadifferent radio.This significantly improves the capacityof thenetwork.On theother

hand, in adhoc networks these functionalities are performed in the same channel, thus performance

decreases.

Mobility.Sinceadhocnetworksprovide routingusing theenduserdevices, thenetwork topologyandconnectivitydependonthemovementofusers.Thisimposesadditionalchallengesonroutingprotocols

aswellasonnetworkconfigurationanddeploymentwithrespecttoWMNs.

Rateatwhichtopologyisexpectedtochange.Meshnetworksareexpectedtobelongdurationnetworkswith low mobility rates, whereas multihop adhoc network are mainly exploited in short duration

networkswithhighmobility.Thusinadhocnetworksproblemsareharder,sinceeverythinghastoadapt

fasterand

there

is

greater

overhead

due

to

the

changes

introduced

in

the

topology.

Despiteofthedifferences,wirelessadhocnetworksandwirelessmeshnetworkssharethemaincommonissues.

Security isverydifficult toachieve indistributed systemarchitectures,andmanyare the securityattacks that

threatenvariousprotocollayers[6].ButthemostimportantchallengethatWMNsandwirelessadhocnetworks

must face is the scalability issue, which come from communication protocols that suffer the multihop

communication.Routingprotocolsmaynotbeabletofindareliableroutingpath,transportprotocolsmaylose

connections,andMACprotocolsmayexperiencesignificantthroughputreduction.Thereasonforlowscalability

isthattheendtoendreliabilitysharplydropsasthescaleofthenetwork increases[1].This issue iscriticalfor

deploymentofrealtimeInternetapplicationslikeVoIP,thatrequiresshortdelaysandlowpacketloss,andthus

oneof

the

most

challenging

tasks

is

how

to

re

design

WMNs

in

order

to

deal

with

real

time

and

multimedia

requirements.

3. VoIPANDITSREQUIREMENTS

VoiceoverInternetProtocol(VoIP),alsocalledIPTelephony,israpidlybecomingafamiliartermandtechnology

that is invading enterprise, education and government organizations. Exploiting advanced voicecompression

techniques and bandwidth sharing in packetswitched networks, VoIP can dramatically improve bandwidth

efficiency.It

allows

also

the

creation

of

new

services

that

combine

voice

communication

with

other

media

and


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dataapplicationssuchasvideo,whiteboarding,andfilesharing.ButthecostsavingsachievedbyVoIPbyusing

existingdatainfrastructuresalongwitheasydeploymentbenefitsarethemainreasonsdrivingthesteadygrowth

ofVoIP[7].

3.1SOMEDETAILSONVoIPAVoIPsystemrequiresthefollowingoperationstobeperformed:

Voice

digitalization,

which

is

the

conversion

of

voice

from

an

analog

to

a

digital

signal;

Noisecancellation,whichistheprocedureforseparatingthevoicesignalfromenvironmentalnoise; Voicecompression,whichreducestheamountofbandwidthnecessarytotransmitdigitalizedvoice.

Voice compression andnoise cancellationareusually achieved through theuseof codecs,whichdifferentiate

themselves in the type of compression used. After compression, voice is transmitted as IP packets requiring

appropriatesignalingprotocolssuchasH.323orSIP.

H.323 is the ITUT recommendation for multimedia communications over packetswitched networks. This

protocol, initiallydesignedtosupportvideocommunicationsover localareanetworks,hasalsobeenappliedto

VoIP transmission. The H.323 specification includes standards such as theH.255.0protocol,which isused for

registration,calladmissioncontrol,andcallsignaling,andH.245which isused formediumand logicalchannel

control.

TheSessionInitiationProtocol(SIP)istheIETFstandardfortheestablishmentofmultimediasessions.SIPusesa

syntax which is similar to that used by HTTP and supports user mobility. SIP protocol considers two typesof

network entities: a client and a server. SIP is defined in [RFC 2543] as ASCIIbased, applicationlayer control

protocolsthatcanbeusedtoestablish,maintain,andterminatecallsbetweentwoormoreendpoints.Justlike

other VoIP protocols, SIP is designed to provide the functions of signaling and session management within a

packet telephonynetwork.Signalingenablescall information tobecarriedacrossnetworkboundaries.Session

managementprovidestheabilitytocontroltheattributesofanendtoendcall.ComparedtoH.323,SIPisamuch

more streamlined protocol, developed specifically for IP telephony [8]. SIP is simpler and more efficient than

H.323,andittakesadvantageofexistingprotocolstohandlecertainpartsoftheprocess.

3.2VoIPQoSREQUIREMENTSIPnetworksdonotarrangeforamechanismthat isabletoguaranteeanordereddeliveryofthepackets.They

dontevengiveanycertaintyaboutqualityofservicesincetheyarebesteffort.ActualVoIPapplicationsneed

to face latencyproblems (timeof transitandelaborationmustbe reduced)anddata integrity (prevent lossor

hacking of information contained into the packets). The reconstruction of the received packets deals with

damaged packets or disordered packets, and those events makes the audio stream very challenging to be

reconstructed correctly with an acceptable latency. Network administrators can guarantee a bandwidth large

enoughtoreducelatencyandlossrateinprivatenetworks,butitisverychallengingtodothatwhenInternetis

used

as

transmitting

media.

Competing

data

and

VoIP

flows

would

result

in

a

degradation

of

the

voice

quality,

which isalready impactedbyseveralotherparameters suchasdelayjitterandpacket loss.For these reasons,

qualityofserviceplaysafundamentalroleinVoIPservices.InordertoobtainabetterQoS,severaltechnologies

havebeenwidelyappliedtoVoIPcallsinordertoimprovetheirperformance.Themostrelevantare:

Packet loss concealment (PLC). This technique improves robustnessof VoIP calls over the packet lossproblem.Thus,giventhesamepacketlossratio,aVoIPclientwithPLCcanhavehigherquality.

Silencesuppression(SS).Whenaphonecallismade,thereisalargepercentageofsilenceinbetweentalkspurts. Usually only background noise traffic is filled into these silent gaps. In order to reduce such

overhead,silencegapscanbesuppressed.

Frameaggregation.VoIPpacketsareusuallyverysmall. Iftheyaresentoverthenetworkonebyone,then

the

protocol

overhead

of

RTP

and

beyond

layers

will

waste

alarge

percentage

of

bandwidth.

Frame


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aggregation is thus fundamental inorder toassemblemultipleVoIPpackets intoone IPpacket for the

sameuser.Although this solutionhasalreadybeendeployed, it can causea largepacketizationdelay,

whichfurtherdegradestheVoIPcallquality.Lotsofstudieshavebeencarriedoutonthisproblemand

theywillberesumedinsection5.4.

However, although all these techniques improve the quality of voice calls, it is still not enough: quality is

unsatisfactory

if

only

best

effort

delivery

is

considered.

In

the

Internet

backbone,

network

capacity

is

abundant

and thusqualityof serviceofvoice calls canbeensured simply through thewellknowndifferentiated service

(DiffServ)modelbasedQoSschemes.ThechallengingissuesofVoIPmainlylieinthefirstorlastmileoftheIP

networkbecausesuchportionofIPnetworkusuallylacksenoughbandwidthandbecausethefirstorlastmileof

IPnetworksareusuallywireless.Thus,investigatingVoIPoverlast/firstmileinwirelessnetworksisanimportant

researchtopic.

3.2.1VoIPoverwireless

Whenconsidering theproblemof transmittingmultimedia trafficoverwirelessnetworks,numerousadditional

challenges are encountered. Wireless scenarios are in fact characterized by highly timevarying channel

conditionsand

consequently

the

available

bandwidth

seen

at

the

application

level

is

highly

variable

and

thus

not

suitableformultimediatransmissionwithhighQoSrequirements.

Anotherissueisthatvoicetransmissionismoresensitivethandatatransmission:VoIPisarealtimeapplication,

making itparticularly sensitive topacket loss thatcanbecaused inawirelessnetworkbyweak signals, range

limitations,andinterferencefromotherdevicesthatusethesamefrequency.Infact,accordingto[9],inamulti

hopwirelessnetworkoperatingona single channel, theUDP throughputdecreaseswith thenumberofhops

between4and7 times.Thiseffect isproducedbydifferentpacketsof the same flow competing formedium

accessproducingchannelinterferencebyusingunlicensedbands(2.4GHz,5GHz).

Security isabiggerconcernoverwireless,because sending telephonecallsoverapublic IPnetworkpresents

more security risks than using the proprietary closed networks. Wireless adds another layer of security

concerns,with

transmissions

going

over

the

airwaves

instead

of

cables

and

thus

subject

to

easier

interception.

CommonVoIPprotocolssuchasSIPhavetheirownsecurityvulnerabilities.

Finally,olderwirelessLANequipmentisnotreadyforVoIP:itisnotpossibletosimplyrolloutVoIPoverexisting

data WiFi network. For good performance, especially in the enterprise space, there should be the need of

wirelessLANhardwareandsoftwarespecificallydesignedtoworkwithvoicetrafficandaddresstheprioritization

andsecurityissues[10].

3.2.2VoIPoverWirelessMeshNetworks

Aswirelessmeshnetworksgrowinpopularity,withnewpublicandprivatedeploymentsannouncedalmostdaily,

thecommercialneedtoaddvoiceapplicationsrequiresthenetworktoexpanditsoverallperformanceinorderto

handle

multimedia

applications.

Problems

like

scalability,

bandwidth

degradation,

network

latency,

and

application priority contention will easily arise as soon as realtime applications are deployed, and these

phenomenaarefurtherexacerbatedwhencoveringlargegeographicareas.

Ingeneral,theseverityofproblemsvarieswidelybasedontheparticularwirelessmesharchitectureusedinany

deployment:singleradio,dualradio,ormultiradio.Inthenextsection,wedeeplyanalyzethechallengesofVoIP

overWMNs,sinceitisoneofthemostattractiveIPnetworksinthelast/firstmile,withmanypossibleapplication

scenariosliketheonesdescribedin2.2.2.


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4. CHALLENGESOFVoIPOVERWMNs

Asemerged in section3, themostcritical requirement forVoIP services isQoS.ForanetworkdeliveringVoIP

calls,acriticalrequirementisthatVoIPcallsmaycoexistwithothertrafficandgivenacertaintrafficloadofother

traffic,thenumberofcallsneedstobeashighaspossible.Unfortunately,ashighlighted insection2,manyare

theissuesofWMNs,likethedifficultytocomputetheoverallcapacityorthescalabilityissue.Routingprotocols

maynot

be

able

to

find

areliable

routing

path,

transport

protocols

may

lose

connections,

and

MAC

protocols

may

experiencesignificantthroughputreduction.SincecentralizedmultipleaccessschemessuchasTDMAandCDMA

aredifficulttoimplement,accordingalsoto[1],itisneededtorevisitallthelayers:fromthelowestonetothe

applicationlayertoreallyobtainanefficientsolutionforVoIPdeploymentoverWMNs.Thisisnotonlydueto

the scalability issue, but is also driven by the rapid progress of semiconductor, RF technologies, and

communicationtheory,thathaveundergoneasignificantrevolution.Manyapproacheshavebeenproposedto

increase capacity and flexibility of wireless systems, like directional and smart antennas, MIMO systems and

multiradio/multichannelsystems.Alltheseadvancedwirelessradiotechnologiesrequirearevolutionarydesign

inhigherlayerprotocols,especiallyMACandroutingprotocols.

Inordertoanalyzetheexistingapproachesandproposalinsection5,therestofthissectionwillcarefullystudy

thechallengesintroducedbyeachprotocollayer.

4.1PHYSICALLAYERThere are two main open issues in thephysical layer: firstly new wideband transmission schemes other than

OFDMorUWBareneededinordertoachievehighertransmissionrateinlargerareanetworks.Multipleantenna

systemshavebeenresearchedforyears,buttheircomplexityandcostarestilltoohightobewidelyacceptedfor

commercialization.Frequencyagile techniquesare still in theirearlyphase,and tremendous researcheffort is

neededbefore theycanbeaccepted forcommercialuse.Secondly, the interactionwithhigher layersmustbe

improved to best utilize the advanced features provided by the physical layer, thus components should be

designedinawaythathigherlayerscanaccessorcontrolthem.

4.2MACLAYERMAC iscriticaltotheperformanceofWMNs:manyproblemsarerooted inthisprotocol layer.WMNsbasedon

IEEE 802.11 MAC have many wellknown issues such as hidden nodes and exposed nodes. The hidden node

problem occurs when a node is visible from the Wireless Access Point (AP), but not from other nodes

communicatingwiththatAP;theexposednodeproblemoccurswhenanodeispreventedfromsendingpackets

to other nodes due to a neighboring transmitter. These issues cause scalability problems of WMNs, like

throughput thatdrops quickly as the number of hopsor nodes increases, thus they severely limit theoverall

performance.

Todealwith those issues, the IEEE802.11e standardhasbeendesigned to improve theQoSof thebasic IEEE

802.11MAC.However,lookingdeeplyinameshenvironment,wecanrecognizetworelevantissues[1]:

Differentvaluesofarbitraryinterframespace(AIFS)inenhanceddistributedchannelaccess(EDCA)helptoprioritizetransmissionsindifferentflows,butcannotresolvethehiddennodeorexposednodeissue.

TheQoSmechanismofhybridcontrolfunction(HCF)cannotresolvehiddennodeorexposednodeissueseither. Moreover, the HCF requires the existence of centralized control by a QoS access point (QAP),

whichexposestwoissuesinWMNs.OneisthecentralcontrolschemeisnotfavoredbyaWMN,andthe

otheristhatQAPmaynotbeavailableinaWMN.


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Intherecentpast,researchersfocuswasintuningandupgradingtheexistingsinglechannelMACs,forexample

adjustingparametersofCSMA/CA likecontentionwindowsizeorbackoff.However,thesesolutionshavebeen

proven to achieve a low endtoend throughput, because they do not significantly reduce the probability of

contentions amongneighboringnodes [1].Recently, research targethasbeenmovedonproposing innovative

MACProtocols.Randomaccessprotocols suchasCSMA/CAarenotanefficient solutionbecauseof theirpoor

scalability inamultihopnetwork.Thus,revisitingthedesignofMACprotocolsbasedonTDMAorCDMAisone

newgoal,

as

well

as

developing

new

multi

channel

MAC

protocols.

The

main

approaches

are

multi

channel

singletransceiverandmultichannelmultitransceiverMAC (inwhichdifferentnodesmayoperateondifferent

channelsoronseveralsimultaneouschannels),andmultiradioMAC,whereanetworknodehasmultipleradios

inwhichcommunicationsaretotally independent [1],eachwith itsownMACandphysical layers.Whenmulti

channeloperation is considered, theperformanceof aMAC canbe improved, and scalabilityproblem canbe

solved.

4.3ROUTINGIn WMNs, the mesh backbone established among mesh routers is an adhoc network. Currently, most

implementation practices adopt the existing routing protocol for mobile adhoc networks with certain

modifications.Since

mesh

routers

in

WMNs

is

nearly

stationary,

the

complicated

procedures

designed

for

mobile

adhoc routing to capturemobilityarenotnecessary for the routingprotocol in themeshbackbone. Inother

words,aroutingprotocolformobileadhocnetworkwillcontainunnecessaryoverhead,whichreducesnetwork

throughputofWMNs.Thisactuallymakesamobileadhocroutingprotocolinefficient inWMNs.Inparticular,a

mobile adhoc routing protocol is usually slow in response to the infrequent change of network topology in

WMNs.Suchinfrequentchangeisusuallyduetolinkfailureortemporarydropoflinkquality.Thus,alightweight

routingprotocol is theultimategoal forWMNs.For these reasons,despiteof theavailabilityofmany routing

protocolsforadhocnetworks,thedesignofroutingprotocolsforWMNsisstillanactiveresearcharea,andthe

solutionswouldtheoreticallyachievethefollowingfeatures:

MultiplePerformanceMetrics.Minimumhopcountasaperformancemetrictoselecttheroutingpathhas been demonstrated to be ineffective in many situations. New performance metrics need to be

developed,anditwouldbenicetointegratemultipleperformancemetricsintoasingleroutingprotocol.

Scalability.Settingupormaintainingaroutingpathinaverylargewirelessnetworkmaytakealongtime,thusitiscriticaltohaveascalableroutingprotocolinWMNs.

Robustness.WMNsneed tobe robust to link failuresor congestion,and routingprotocols shouldalsoperformloadbalancing.

Efficient Routing with Mesh Infrastructure. Considering the minimal mobility and no constraints onpowerconsumptioninmeshrouters,theroutingprotocolinmeshroutersisexpectedtobemuchsimpler

thanadhocnetworkroutingprotocols.

4.4TRANSPORTLAYERManyRateControlProtocols(RCPs)areproposedforwirednetworks.Anadaptivedetectionratecontrol(Adhoc

TCPFriendlyRateControl)schemeisproposedforadhocnetworkin[11],whereanendtoendmultimetricjoint

detection approach is developed forTCPfriendly rate control schemes.However, there is nodeployed traffic

protocolforWMNs,andclassicalTCPdonotdifferentiatecongestionandnoncongestion lossesandasaresult

the network throughput quickly drops due to the unnecessary congestion avoidance [12]. Recently a WMNs

transport protocol has been proposed in [13], which includes both efficient hopbyhop rate adjustment and

reliabilitymechanisms toachievehighperformance reliabledata transport inWMNs.However, thiscannotbe

consideredasatransportprotocolsuitedonrealtimeapplications,becauseitcontainsanexcessiveoverheadfor

featuresthat

are

neither

necessary

nor

priority

for

multimedia

applications.


10/22

4.5APPLICATIONLAYERChallengesofVoIPoverWMNsinvolvealsotheTCP/IPapplicationlayer.Figure2demonstratesatypicalscenario

when deploying SIPbased VoIP overWMNs. Usually the SIPproxy server acts as an intermediatedevice that

receivesSIPrequestsfromaclientandthenforwardstherequestsonbehalfoftheclienttothenextSIPserverin

thenetwork.Proxyserverscanprovidefunctionssuchasrouting,reliablerequestretransmission,authentication,

authorizationandsecurity.TheconnectionbetweenWMNsisgivenbytheMPLSbasedIPcorenetworkthrough

labeledge

routers

(LERs)

those

operate

at

the

edge

of

MPLS

networks

and

use

routing

information

to

assign

labelstodatagramsandthenforwardsthemintotheMPLSdomain.

Figure2:SIPbasedVoIPinWMNs(adaptedfrom[14]).

WhendeployingSIPoverthisWMNsscenario,wemustfacemanynewchallengingissuesthatarecausedbythe

instabilityofthewirelessenvironmentandbyusermobility:callsetupdelay,accessbandwidthpredictionand

reservationand

call

admission

control.

Typical

scenario

consists

in

WMN

as

access

network

to

the

Internet,

in

MPLStechnologytoguaranteeQoSintheIPcorenetwork,andinVoIPapplicationsthatintendtogooutoftheir

own localWMNsforcounterparts inthe Internet.Therefore,whenSIP isusedtosetupaVoIPsession, itmust

faceaheterogeneousnetworkenvironment,whichincreasesthecomplexityofthesignalingprocessandcausesa

longcallsetupdelay. IntrafficengineeringenabledMPLSnetworks,ConstraintbasedRoutingLabelDistribution

Protocol(CRLDP)orResourceReservationProtocolwithTrafficEngineeringExtensions(RSVPTE)isemployedto

setupalabelswitchedpath(LSP)dynamicallyforaconnectionwithQoSrequirements,thusthetotalsessionset

updelayofaVoIPcallshouldbethesumofSIPsignalingandMPLSsignalingtimes[14].

WhendesigningSIParchitecturesforWMNs,usersinWMNsarefreetomovetoanywhereanytime,andwireless

channelconditions

may

vary

from

time

to

time.

These

two

facts

can

clearly

result

in

varying

access

bandwidth

requirements in WMNs. To accommodate this variation, the best way is to let WMN gateway mesh routers

dynamicallyreserveaccessbandwidthfromtheIPcorenetwork,sincethefixedbandwidthreservationapproach

isnotefficient inthisscenario.Forexample,therecanbetwostraightforwardwaystoreservethefixedaccess

bandwidthforvariablerequirements.Thefirstwayiscalledtheoptimalusersatisfactionscheme,whichreserves

themaximumbandwidththataWMNever requires.Thesecondway iscalled theoptimalcostscheme,which

reserves theminimumbandwidth thataWMNever requires.Nevertheless,bothof thesemethodshave their

shortcomings. The optimal user satisfaction scheme is not economic, although it can always provide enough

accessbandwidthforWMNusers.Theoptimalcostschememaynotensurethatallusersaresatisfied,althoughit

isabletoreducetheexpenseofWMNoperators.Dynamicaccessbandwidthreservationrequirestheprediction

ofoutgoing

traffic

load

in

WMNs.

As

there

is

always

adistinction

between

the

exact

access

bandwidth

10


11/22

11

requirementand thepredictedaccessbandwidth requirement, the calladmission controlmechanismmustbe

implemented.

ACallAdmissionControl (CAC)mechanismmust in factbeemployedwhen thepredictedandreservedaccess

bandwidthisdifferentfromtherealone.CACisusedtoacceptorrejectconnectionrequestsbasedontheQoS

requirements of these connections and the system state information. CAC prevents oversubscription of VoIP

networks

and

is

a

concept

that

applies

only

to

real

time

media

traffic

but

not

to

data

traffic.A

CAC

mechanism

complements the capabilities of QoS tools to protect audio/video traffic from the negative effects of other

audio/videotrafficandtokeepexcessiveaudio/videotrafficawayfromthenetwork.CACcanalsohelpwireless

meshnetworkstoprovidedifferenttypesof traffic loadwithdifferentprioritiesbymanipulatingtheirblocking

probabilities[14].

4.6ADHOCNETWORKADDITIONALCHALLENGESAsalreadymentionedinsection2.2.3,WMNsandWirelessadhocnetworkshavesimilarproperties,butthelast

onesadd insomeextentsadditionalchallengesduetotheadhocnatureand lackof infrastructure.Despiteof

beingaWMNssubset, it ismoredifficult todeploy the legacy InternetApplicationandprotocol to theadhoc

domain.

The

rest

of

the

section

describes

the

main

challenges

that

wireless

ad

hoc

networks

adds

to

the

WMNs

ones.

4.6.1Powerconsumption

Wireless adhocnetworknormally adopts carrier sensemultipleaccesswith collision avoidance (CSMA/CA) to

control medium access, but technical research and real system measurement both indicate that CSMA/CA

consumesmorepowerthantimedivisionmultipleaccess(TDMA)systems.Thereasonbehind isthatanadhoc

nodehastocontenttheradiochannelandhastoalwayskeepawakeiftheyhavepacketstosendortoreceive.

Thepowerconsumptionissuesofawirelessadhocnodecanbecriticallyanalyzedintotwostages,i.e.idlemode

andactivemode[15].

4.6.2

Mobility

management

Mobilitymanagementisanadditionalchallengetomobileadhocnetwork(MANET)asitchangesIPaddressmore

frequently due to using hierarchical addressing scheme, global connectivity to Internet and deployment of

autoconfiguration.WhenanIPaddresschanges,theperformanceofroutingprotocoldegradeshencethequality

ofrealtimecommunicationlikeVoIPalsodegrades.ForinternetconnectedMANETweneedaccesspoint(AP)as

a gateway to the internet and mobility management of this kind of ad hoc network can be separated into

differentnetwork layer issues. Layer twohandovermeansmobilenodesmoving fromoneAP toanotherAP

withoutchangingitsIPaddress.LayerthreehandovermeansthatadhocnodeshavetoacquireanewIPaddress

afterthehandover,sincethetwoAPsareindifferentsubnetworks.Toresumeavoicesession,applicationlayer

handoversuchasSIPmobilitymightbenecessary.

4.6.3Endpointdiscovery

Adhocnetwork canwork in isolatedorconvergedmode. Inboth the cases thenormalprocedureofSIPAOR

(AddressofRecord)bindingresolutionandtheroutingofSIPrequestmessagesbasedonsomecentralizedand

often preconfigured entities cannot be applied due to node mobility. SIP end point discovery in an ad hoc

network is semantically similar to the service or peer discovery process in P2P networks. A P2P network is

generallyconstructedasanoverlaynetworkovertheInternetandtheserviceorpeerdiscoveryprocessinvolves

thediscoveryofaparticularserviceorthecontactinformationofapeerwithouttheuseofanyInternetrouting

infrastructure.HoweverP2Pprotocolscannotbeappliedtoindependentnodeaddressingschemeanddoesnot

supportrandomnodemobilityas isthecasewithmobileadhocnetworks.Moreover,theydonotconsiderthe


12/22

12

underlying routing topology,which is an important criterion forensuring routingefficiency,particularly in the

contextofadhocnetworks[16].

5. EXISTINGAPPROACHES

Withrespect

to

the

challenges

discussed

in

section

4,

we

present

in

this

section

the

most

relevant

approaches

thathavebeenproposedtosolvethemostimportantdescribedissues.

5.1MACLAYEREXISTINGAPPROACHESAs already discussed in section 4.2, MAC is critical for WMNs performances, and lots of solutions had been

proposed,startingfromenhancementsofexistingprotocols.Howeverthemostpromisingsolutionsaregivenby

designofnewMACprotocols:research is focusingonTDMAmultichanneldesign,butalsonew interestingand

innovativeapproacheshavebeenproposed.InthissectionwedescribeaTDMAapproachandanadaptationlayer

betweenMACandroutinglayerthatenhancesthequalityofvoicecalls.

5.1.1

DMT

MAC

(Distributed

Multi

channel

TDMA

MAC)

In [17], a TDMA multichannel MAC protocol is proposed to improve the scalability of WMNs with a vertical

scheme to aggregation schemes as in [9]. It is characterizedby theuseofTDMA, and can supportdedicated

queuing for voice calls based on admission control and time slot allocation instead of prioritized queuing for

different traffictypes.ThesocalledDMTMAC (distributedmultichannelTDMAMAC) isasolutionproposed in

ordertoresolvethescalabilityissuesintheMACprotocolofWMNs,andcanbeextendedfornodeswithmultiple

radios. There are four major components in this approach: the TDMA frame generation, the synchronized

channelswitching,theMACsignalingandthetimeslotandchannelallocation.

TheTDMAframeisalignedwiththebeaconintervalofanIEEE802.11MAC.Ineachframe,acommonslotwitha

commonchannel

is

used

in

all

nodes

to

send

packets

related

to

management

and

signaling.

Given

atime

slot,

a

packet tobe sentonanodemustcheck if its flow to thedestination isallocatedwith sucha time slot. If the

answeristrue,thenthepacketissent;otherwise,ithastobequeuedandwaitforthenextchance.Inorderto

avoid packet loss, interference, and hardware related errors, two conditions must be satisfied by channel

switching:transmissions intheoldchannelmustbeendedatthesametimeandnewtransmissions inthenew

channelmustbestartedatthesametime.

ThesynchronizationfunctionprovidescommontimingforallnodesinWMNs,sothataTDMAframeisdesigned.

The start time of channel switching can be synchronized among all nodes by triggering switching only at the

beginningofeachtimeslot.Toensurethatanewtransmissionbeginsatthesametimeforallnodes,thestart

timeofthenewtransmissionmusthavethesamevaluerelativetothestarttimeofeachtimeslot.

Thetimeslotandchannelallocationalgorithmdeterminesthechannelineachtimeslotforallnodes,anditmust

work inadistributedmanner,sincenocentralizedcontroller isavailable inWMNs.Inthedistributedapproach,

informationabouttopology,channelandtimeslotallocationmustbeexchangedbetweennodesformorethan

twohops,astheinterferencerangeofanodeexpandsfurtherthantwohops.Thus,asignalingprocedureinthe

MAClayerisneededtosendandreceivesuchinformation.Sincetimeslotandchannelallocationisdonelocally

amongnodes,theallocationinformationmustbepropagatedtoothernodesthataremorethantwohopsaway

viaMACsignaling.Inthisway,thelocaltimeslotandchannelallocationwillnotcauseanyconflictwithanother

localallocationthatistwohopsaway.


13/22

5.1.2SoftMACforVoIPsupport

In[18],asimpleQoSschemeisproposed inorderto improvethequalityofvoicecallsoverWLANscalledLayer

2.5SoftMAC.It liesbetween802.11MAC layerand IP layer,and itenhancesthe limitedcoordination in802.11

MACvia softwaremechanisms that supportVoIP services regulatingnetwork load andpacket transmissionof

bothrealtimeandbestefforttrafficamongneighboringnodesinadistributedmanner.Theobjectiveistokeep

thechannelbusyaswellastimeandcollisionratebelowappropriatelevels,ensuringacceptableVoIPquality.

Eachnodewillexchangeitsrealtimeandbestefforttrafficinformationexplicitlytoitsneighborsbybroadcasting

andmeasuringthecurrentlinkstatussuchascapacityandpacketlossratio.Inthisway,itispossibletoestimate

theconsumedandavailablebandwidthforadmissioncontrol,andtherateofbestefforttrafficwillbecontrolled

byaratecontrolmechanismateachnode.Basedonthedecisionsofcontrolplanemodules,priorityqueuingand

trafficshapingaretoregulatetherateofpackettransmission.Animportantthingthatmustberemarkedisthat

unlike inwirednetworkswhere theutilizationofagiven link isdeterminedby thebandwidthconsumptionof

existingflowstraversingthelink,inamultihopwirelessnetworkdeterminingthelinkutilizationisnolongeras

trivial. The fraction of air time represents the utilization of a physical channel that maps the bandwidth

requirementattheapplication layertotheactualairtimerequiredatawireless link,takingheaderoverheads,

timevarying

link

capacity

and

packet

collision/loss

ratio

into

account.

Threearethekeymodulesinthisapproach:

AdmissionControl forVoIP traffic,whichperformsadmissioncontrol to regulate theVoIP traffic load,eachnodeneedstoestimatetheresidualfractionofairtimeofitsincidentwirelesslinks.

RateControl forBE traffic, which regulates the rate of besteffort packets to reduce their impact onexistingVoIP flows.More specifically, a portionof the residual air time left by the realtime traffic is

allocatedtobestefforttrafficinadistributedfashion.Theactualairtimecostateachlinkwillvarywith

the currently measured link capacity and packet loss ratio, so the traffic shaper at SoftMAC needs to

adjust the besteffort transmission rate accordingly. In the priority queueing module, nonpreemptive

priorityis

always

provided

to

VoIP

traffic

at

each

node,

so

best

effort

packets

can

only

be

passed

to

the

MAC layer when there is no VoIP packet waiting to be served. To achieve this goal, data buffering is

movedfromthe802.11MAClayertolayer2.5SoftMACinordertoregulatepackettransmission.

PriorityQueueingforServiceDifferentiation,whichisusedtoprovideVoIPtrafficwithhighpriorityandbestefforttrafficwithlowpriority.Thesignalingtrafficforresourcereservation,linkstatusmeasurement

andother informationupdateaswellastheroutingtraffic isgiven thehighestpriority.Henceathree

queuepriorityschedulingisusedateachnodeforpacketschedulingandbuffermanagement.

Figure3:

SoftMAC

architecture

and

components

for

VoIP

(adapted

from

[18]).

13


14/22

14

5.2ROUTINGLAYEREXISTINGAPPROACHESThe ideal lightweight routingprotocol forWMNs reasonsexplained in section4.3hasnotbeendeployedyet:

despite of the availabilityof many routing protocols for adhoc networks, the design of routing protocols for

WMNsisstillanactiveresearcharea.

Therearepredominantlytwotypesofroutingprotocolsolutionsformobileadhocnetworks,whichareprimarily

used

for

WMNs

with

required

modifications:

the

so

called

reactive

and

proactive

approaches.

Reactive

protocols

establishroutesondemandfloodingthenetwork.Theyhaveaminimalbandwidthconsumptionandlowsetup

time.ExamplesareAODV (AdhocOndemandDistanceVector) [RFC3561]andDSR (DynamicSourceRouting

protocol).Proactiveprotocols check continually the state of the network, and the routing table is constantly

updated.Thebandwidthconsumption isconstantandtheroutingtable isalwaysavailable.Someexamplesare

OLSR (Optimized Link State Routing protocol) [RFC 3626] and HSR (Hierarchical State Routing protocol). It is

impossibletosayaprioriwhichthebestsolutionis,sincethedecisionisatradeoffthatmustbeconsideredfor

eachparticularscenario.Severalresearchstudiessuggestreactiveprotocolsovertheproactiveones,particularly

forhighlydynamicnetworks.Proactiveroutingprotocols insuchnetworkssufferfromhighoverheadsandslow

convergence.However,reactivestrategycanalsosufferfromunacceptabledelayinroutediscoveryprocessdue

toprohibitive

flooding

traffic

to

get

rid

of

broadcast

storm

problem.

Although therearenotexistingapproachessolely forWMNs,wenowdescribethecrosslayerdesignbetween

layer2and3proposedtoworktogetherwiththeDMTMAC in[17],andthe InterdomainRoutingProtocolfor

MultihomedWirelessMeshNetworksdescribedin[19]thatenablesabetterhandoff.

5.2.1Crosslayerdesign

In[17],acrosslayerdesignbetweenMACandroutingprotocolsisalsocarriedouttoachievefastrediscoveryof

new routing path and also fast roaming. Two schemes are discussed: crosslayer detection of link failure to

increasethespeedofrouterediscovery,andnetworktopologyconsistencyinsurance.

RouteRe

Discover:

In

aWMN,

although

mobility

is

not

aconcern

to

mesh

routers,

link

failure

can

still

causes

the

changeofnetworkconnectivity.Whenalinkbetweentwonodesdoesnothavesatisfyingqualityoranodefails,

all traffic flows thathavea routingpathvia this linkmust selectanew routingpath.Formostadhoc routing

protocolsexcept for geographic routing, rerouting for a traffic flow involves the processof settingupanew

routingpathfromoneendnodetotheotherendnode.This isareallyslowprocessduetotheslownessoflink

failuredetection,andduetotheslownessofnewroutingpathsetup.

Inordertoexpeditetheprocessofrouterediscovery,MACandroutingcrosslayerinteractionisneeded.Inthe

MAClayer,severaltypesofmanagementframesareavailable.Basedontheseframes,linkfailurecanbeeasily

detectedwithinashorttimeperiod.Thus,linkfailuredetectionismuchfasterintheMAClayerthanthatinthe

networklayer.

The

detection

of

link

failure

by

the

MAC

protocol

can

be

sent

to

arouting

protocol

via

the

cross

layer communication.To increase the speed of setting up a new routingpath, it is preferred to send routing

messageswithguarantee.BasedontheTDMAframestructure,aminitimeslotcanbeallocatedtotherouting

protocolforsendingroutingrelatedmessages (thusasortofdedicatedtransmissionforroutingmessages). In

thisway,signalingofaroutingprotocolcanbefaster.

TopologyConsistency:InaWMN,bothroutingandMACprotocolsneedtocollectinformationaboutthenetwork

topology.Usuallyaroutingprotocolderivesthenetworktopologybasedonpacketsreceivedattheroutinglayer,

whileaMACprotocolfindsoutthenetworktopologybasedonMAClayerpackets.Suchadifferencemaycause

topology inconsistency,and incasesuchdifferencescomeout,packetswillnotbe routed inacorrectway.To

avoidthis

problem,

MAC

and

routing

must

rely

on

the

same

packets

and

the

same

criteria

to

derive

network


15/22

topology. ThusMAC and routing layers use the same packet type toderive network topology information. In

addition,thispacketmustbearoutinglayerpacketratherthanMAClayer.Otherwise,itwillbefilteredoutinthe

MAC layer andbecomes invisible to a routingprotocol.Moreover,when a routinglayer packet is received at

eitheraMACoraroutingprotocol,thesamelinkqualitymetricmustbeusedtodeterminethelinkconnectivity.

5.2.2AninterdomainroutingprotocolformultihomedWMNs

Ahybridroutingprotocolformultihomedwirelessmeshnetworksispresentedin[19],whichintegrateswireless

andwired connectivity and optimizes the use of the wireless medium by shortcutting wireless hops through

wired connections. The novelty of this approach is the use of overlay multicast to autodiscover Internet

gateways,tocoordinatedecisionsbetweenaccesspointsduringmobileclienthandoffsandloweroverhead.This

approach uses multicast groups to coordinate decisions and seamlessly transfer connections between several

Internetgatewaysasmobileclientsmovebetweenaccesspoints. Internetgatewaysjointothemulticastgroup

called Internet Gateway Multicast Group (IGMG) on which they periodically advertise their wired interface IP

addressandthismodelusesanycasttoforwarddatapacketsfromaclienttotheclosestInternetgateway.The

multicastroutingishandledbytheunderlyingoverlayinfrastructure,andmulticasttreesarecalculatedinaway

similartothatofMOSPF.Whencalculatingbestroutesthisapproachallowsdifferentroutingmetricsforwired

andwireless

links

and

considers

both

wireless

and

hybrid

route.

Moreover

this

approach

supports

inter

domain

handoffforbothTCPandUDPconnections.

5.3ANENHANCEDSIPPROXYSERVERFORWIRELESSVoIPINWMNsTheCOPS(CommonOpenPolicyService)ProtocolisapartoftheInternetprotocolsuite,andspecifiesasimple

client/servermodelforsupportingpolicycontroloverQoSsignalingprotocols.Policiesarestoredonserversorso

calledPolicyDecisionPoints (PDPs)and areenforcedon the clients,namedPolicyEnforcementsPoints (PEP).

WheneverPEPneedstomakeadecision,itsendsallrelevantinformationtothePDP.

In [14], anenhancedSIPproxy server forVoIP inWMNs isproposed toovercome the technical challenges in

wireless VoIP deployment explained in section 4.5, and its framework is shown in figure 4. In particular, the

enhanced SIP proxy server utilizes COPS messages to negotiate with the MPLS label edge routers about the

overallaccessbandwidthrequirementonbehalfofallSIPterminalsinaWMN.

Figure4:TheframeworkoftheenhancedSIPproxyserver(adaptedfrom[14]).

ThelabeledgeroutersexchangetrafficengineeringsignalingwithotherroutersinsidetheMPLScorenetworkto

set up the corresponding LSPs; in this way, the LSPs required by SIP telephony are set up in the MPLS core

network before SIP calls are made. As a result, the SIP call set up delay in the MPLS network is decreased

significantly.Toevaluate thevalueof theoutgoingbandwidth,predictionalgorithmareused,and if theWMN

doesnothaveenoughoutgoingbandwidth toaccommodateallSIP calls, theenhanced SIPproxy servermust

15


16/22

16

utilizeacalladmissioncontrolmechanismtodeclinesomeofthecallrequests.Criticalforthisapproachisthusa

suitablebandwidthpredictionalgorithm.

5.4VoIPTRAFFICAGGREGATIONSCHEMESFORWIRELESSMESHNETWORKSAsalreadyexplainedinsection3.2.2,oneofthemainproblemsofmeshnetworkisdealingwithscalability,and

scalabilityisfullyaffectedalsobytheoverheadproblemexplainedin3.2.Generaltrafficaggregationschemescan

beused

to

improve

the

performances

and

limit

scalability

problems.

A

good

approach

in

this

way

is

in

[20],

where

theproposedpacketaggregationtechniqueisadaptive,andincreasesthescalabilityofIEEE802.11basedWMNs.

It isperformedontopoftheMAC layer,sothattheycouldreducetheoverheadduetobothprotocolheaders

and the contentionmechanism regulating the IEEE802.11 standard.Butanewkindofaggregation scheme is

goingtobeavailablewhentheIEEE802.11nwillbedeployed.Accordingto[21]itwillimproveaggregationsince

interframespaceswillbereducedtozero(ZIFS),andbothMACandPHYheaderswillbeaggregatedatthesame

time.(AMPDUandAPPDU).

Butaggregationonvoice,isusuallymadeineagertype,whichmeansthatisperformedwithafixedaggregation

delaybudget like in [8,22],hence irrespectiveof thecongestionsituation in thenetwork. In [23],voice frame

aggregationfor

wireless

mesh

networks

is

made

instead

with

lazy

frame

aggregation

(LFA)

that

can

outperform

eagermethods.ThecoreideaoftheLFAinthemultihopwirelessenvironmentisthatateachwirelessrouter,the

voiceframesfromthesamecallthatconcurrentlyresideintheLLCqueueatthetimeofaggregationarepacked

intothesameMACframe,withoutwaitingforanymorevoiceframestoarrive.Bysimplypackingasmanyvoice

framestogetherasinducedbythegivenloadconditiononthe802.11WLAN,theLFAalgorithmalwaysfindsthe

optimaldegreeofaggregation,anddoesnot incuranyunwarranteddelaycost.This is insharpcontrast to the

existingframeaggregationmethodsthatoperatewithafixedframeaggregationdelaybudget.

5.5EXISTINGAPPROACHESTOSOLVETHEADHOCNETWORKADDITIONALCHALLENGESAsmentioned insection4.6,Wirelessadhocnetwork introducesadditionalchallenges likepowerconsumption

andmobility

constraints.

However

the

main

issue

is

the

end

point

discovery.

In

the

rest

of

the

section

we

describe

threedifferentapproachesthathavebeenproposedtosolvetheseissues.

5.5.1Gatewayasproxyserver

ThescalabilityissueofVoIPservicesininternetconnectedMANETsisaddressed in[24]:anapproachwherethe

SIPproxy iscolocatedat theMANETgateways.Theauthorscompared thisnewapproachwith the traditional

approachwheretheSIPproxyislocatedintheaccessnetwork.Thenewapproachsignificantlyreducescallsetup

latency for successful calls. The proposed method uses the gateway as proxy/registrar server with limited

capabilityinsteadofusingthepeertopeerSIPorservicediscoveryframeworkwheretheunderlyingSIPprotocol

hasbeenmodifiedhence true interoperability isnotguaranteed. In thisproposedmodel if twoMANETnodes

wantto

communicate

with

one

another

they

must

go

through

the

gateway

even

if

they

are

just

one

hop

away

fromeachother.Thenoveltyofthisapproachistheguaranteedinteroperability.

InsteadofusingpreconfiguredSIPoutboundproxyserverIPaddressineveryMANETnodeorDHCPtodiscover

it, this model propose the use of gateway discovery mechanisms to inform SIP user agent clients inside the

MANETabouttheexistenceofgatewayswithSIPproxy/registrarfunctionalities.InordertoinformMANETnodes

aboutgatewayswiththesefunctionalities,thismodelproposethemodificationofGW_INFOmessage(asshown

infigure5)throughtheinsertionofthebitPinthereservedfieldwhichindicatesgatewaycapabilitytoactasa

SIPproxy/registrar.Thisdoesnotincreaseoverheadbutmobilenodeshavenowadditionalfreedomtochoosea

propergatewaybasedonitsSIPproxycapabilities.


17/22

Figure5:GW_INFOmessageformatinSIPproxycolocatedatGW(adaptedfrom[24]).

Theselectedgatewaydiscoverymechanismhasastrongimpactontheoverallperformanceduetothenumberof

messagesexchangedversuslatencyandbyincreasingthenumberofgatewayscallsetupdelaycanbedecreased

for single hop call as well as multihop call. However, using the proxy colocated at the gateway could have

drawbacks,becauseitisthendifficulttooffer3GPP/IMScompliantservices.

5.5.2LooselycoupledandtightlycoupledapproachestoenableSIPbasedsessions

In[16]twoapproachestoenableSIPbasedsession inwirelessadhocnetworkarepresented,namelya loosely

coupledapproach

(LCA),

where

the

SIP

endpoint

discovery

is

decoupled

from

the

routing

procedure

and

atightly

coupledapproach (TCA),which integrates theendpointdiscoverywitha fullydistributedclusterbased routing

protocolthatbuildsavirtualtopologyforefficientrouting.

Figure6:LooselycoupledandtightlycouplingapproachesforSIPendpointdiscovery(adaptedfrom[16]).

Figure6showsthefunctionaldiagramsoftheseapproaches.TheLCAworksontopoftheadhocroutingprotocol

and uses the similar technique that AODV uses for route discovery for a given destination IP address. This

approachdefinestwomessagesnamelySIPREQandSIPREPtolocateanendpointcorrespondingtotargetAOR.If

anynodewantstodiscoveranyothernodethentherequestingnodebroadcastsanUDPbasedSIPREQmessage.

Toavoid

the

broadcast

storm

it

uses

an

expanding

search

technique

and

to

circumvent

congestion

the

originating

nodeusesexponentialbackoffalgorithmfortheretransmissionofSIPREQmessage.UponreceivingtheSIPREP

messageonlythetargetnodeorthenodewhichhasamappingoftheTargetSIPURIunicastsSIPREPmessageto

theoriginatingnode.AftertheendpointdiscoverythesubsequentSIPmessageroutingandmediapacketrouting

ishandedovertoAODV.

TCAisanintegratedapproachwheretheendpointdiscoveryiscoupledwithadistributedClusterBasedRouting

Protocol(CBRP)whichcreatesavirtualtopologywiththeclusterheadsformingabackbonenetworkthatisused

toroutebothSIPmessageanddatapackets.Thisisatrulydistributedapproachwheretheclusterheadshostthe

proxies and SIP registrars. The cluster heads are connected with each other either directly or via specially

designated gateway nodes. To deal with the power consumption issue minimal number of cluster heads and

17


18/22

gatewaynodesformaMinimalDominatingSet(MDS)becausetheyarethemostcomputationallyintensivenodes

and hence saving the total energy consumption of the whole network. The authors proposed appropriate

approach for cluster head selection, cluster formation and gateway selection which ensures virtual topology

whereeachclusterheadcanreachtoits2hopand3hopneighboringclusterheadthroughthegatewaynodes.

Moreover all the member nodes are 1hop away from a cluster head which ensures the connectivity of any

membernodewithanyothermembernodeviathegateway.

AclustermemberidentifiesitsclusterheadfromtheHELLOmessagesentbytheclusterheadandregistrarswith

the corresponding SIP registrar by sending SIP registrar message meanwhile the cluster head deals with the

locationservicesassociatedwiththeregistrarandkeepsmapofalltheSIPURIandnodeaddressofthecluster

members.InthisapproachtheclusterheadalsoactasaSIPproxyandforwardingnodewhichisresponsiblefor

routingroutediscoverymessage.

When a clustermembernodewants toestablish a sessionwithanother clustermembernode, it sends a SIP

INVITEmessage to thecorrespondingproxyof the requestingnode.Theproxy thensendsthismessage to the

neighboringclusterheadsorproxiesinordertodiscovertheroutetothetargetnode.Ifanyoftheneighboring

proxies

has

the

target

AOR

registered

with

itself,

it

sends

the

INVITE

message

to

the

target

node,

or

it

forwards

themessageto itsneighboringclusterheadsafterrecordingtheproxyaddress intheRecordRoutefieldofthe

SIPmessage.ThetargetnodeonreceivingtheINVITEmessagesendsbackaSIPOKmessageviathereverseroute

specifiedbythelistoftraversingproxiesinRecordRouteheaderfield.Whentherequestingnodegetsbackthe

SIPOKmessage then itknows about the route to the target,which isused subsequently forbothSIP session

establishmentandmediapacketdelivery.Theintermediateclusterheadskeepacopyoftherouteinlocalcache

to reduce the overhead for subsequent route discovery. Table 1 represents the summary of performance

comparisonofthetwoapproachesderivedfromsimulationresultsgivenbytheauthors.

Table1:ComparisonbetweenLooselyCoupledApproach(LCA)andTightlyCoupledApproach(TCA).

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5.5.3VoIPsystemimplementationusingUPnP

In [25],ChangetalhaspresentedanAdHocVoIPsystem implementationusingUPnPwith freeconfiguration.

TheyhavealsoproposedaSIPassistanttosolvetheproblemofAdHocnetworkwithnoparticularSIPserver.In

theproposedmodelwhenanodeenters inanAdHocnetwork itwillretrieveallthedevice information like IP

address and corresponding names using the UPnP protocol. The UPnP deals with the addressing, discovery,

description,control,andeventrelatedissues.

Allthe

nodes

in

this

system

support

UPnP

protocol

and

there

is

no

particular

server

for

signal

redirecting

or

routinginsteadthemodelisprovidedwithaSIPassistantshowninfigure7,whichcollectsinformationfromUPnP

andconvertsthemtoawellknownformatforSIPuseragent.

WhenanodeentersinthenetworkitadvertisesitselfinthenetworksoallothernodesthatsupportUPnPcanbe

awareof its existence.On theother hand when an access pointenters into the network it multicasts search

request andusingUPnPprotocol and allothernodes respond to the accesspointwith thebasic information.

Detail services likeaccessiblevariables,actionscanbeobtainedby theeventandcontrolmechanismofUPnP

protocol.Moredetailed information likeSIPcontactURL,correspondingname,availability,capabilityandsoon

canbeobtainedfromdescriptionstep.

Figure7:SystemArchitecturefor5.5.3approach(adaptedfrom[25]).

6.CONSIDERATIONSANDPROPOSALS

Insection4,wehighlightedtheneedofrevisitingalltheTCP/IPlayersinordertoovercometheWMNschallenges

and limitationswhen transmitting realtime traffic, andwedescribed in section5 themainexistingproposed

solutions.

Concerning theMAC layer,tuningparameterssuchascontentionwindowandbackoffhasbeenprovennot to

reduce significantly the probability of contentions [1], thus they must not be considered as an outstanding

solution. Atthesamelevel,theadaptationlayerdescribed insection5.1.2betweenMACandrouting layer isa

verysmart

approach,

but

we

think

it

cannot

be

considered

as

adefinitive

solution

either,

because

even

if

experimentalresultsshowsthatitdrasticallyreducestheendtoendonewaydelayofVoIPpackets[18],itdoes

notsolve thescalability issue:notsomanyVoIPcallscanbe supportedat thesame time.TDMAMultichannel

MAC,hasbeenexperimentallyproven[17]toeffectivelyimproveboththescalabilityofnetworkthroughputand

thedelayjitter,thustheymustbeconsideredasoneofthecurrentmostefficientsolutions.

Wethinkthatthewinningconceptaboutapproachin[17]isnotthemultichannelinnovation,butitsinsteadthe

crosslayerdesign:theyhaveinfactbasicallymergedMACandroutingprotocolsandcombinedthemsotoclosely

lettheminteract.WethinkthatalsosinglechannelMACsolutionwouldrepresentarealityifimplementedcross

layer. InWMNs,thedynamicityofthetopology impactsonmultiple layers,and inorderto improvethedesign

efficiency,we

believe

that

cross

layer

design

is

indispensable.

But

we

would

propose

adifferent

cross

layer

from

19


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20

theoneadoptedin[17].Insteadofmergingseveralprotocolsintoonecomponent,wekeeplayersseparate,and

weimprovetheperformanceofaprotocollayerbyconsideringasortofparameterspassingbetweenprotocol

layers.Forexamplethe linkqualitycanbepassedbetweenphysicalandrouting layer,sothattherouting layer

canreact immediatelyto linkfailuresandreroutepackets;orthepacket lossratecanbepassedbetweenMAC

and transport layer to let the transport layerdistinguish thecongestion. Implementing thena routingprotocol

that would exploit all those parameters as multiple performance metrics, we can think on using proactive

protocolsrather

than

reactive,

given

the

assumption

that

mesh

network

has

generally

not

that

much

mobility.

Combiningsucharchitecturewithtechniqueslikelazyframeaggregation[23]andtheenhancedsipproxyserver

in[14]wecanimprovethequalityofthevoicecallandminimizingthedelayjitter,thusovercomingalmostallthe

challengesdescribedinsection4.Ourfutureworkwillbeconcentratedonprovidinganimplementationscheme

ofsuchaproposal,andverifyingexperimentallytheperformance.

CONCLUSION

Wireless Mesh Network is an emerging technology for the next generation wireless networks and VoIP is

nowadaysoneofthemostpopularapplicationsfordeliveringmultimediatrafficduetocostefficiency.However,

whendeployingVoIPandotherlegacyrealtimeapplicationsoverWMNsmanychallengesarise,andthesituation

gets worse for wireless adhoc networks due to the lack of infrastructure. This paper analyzes the main

challenging issues of VoIP over WMNs deployment, examining also the particular case of wireless adhoc

networks,andtriestocarefullyevaluatetheexistingapproaches inordertounderstandhowtheymitigatethe

challenges.However,acompletesolutiondoesnotcurrentlyexist,andresearchers,developerandmanufacturers

needtoworktogethertoexplorethisareainordertomitigatethechallenges.

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Documents

VoIP Over Wireless Mesh Networks - Implications and Challenges