voip over wireless mesh networks - implications and challenges
TRANSCRIPT
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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
Md. Sakhawat Hossen
School of Information and Communication Technology
Kungliga Tekniska hgskolan (KTH)Stockholm, Sweden
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.
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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
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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
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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.
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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]).
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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
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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
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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.
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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
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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
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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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