Seriesof PubliationsC, No. C-2014-3
A New TCP-friendly Link-Layer Protool for Wireless
WAN and Satellite Networks
Davide Astuti
Helsinki,September2014
Lientiate Thesis C-2014-3
Universityof Helsinki
Department ofComputer Siene
P.O.Box68 (Gustaf Hällströminkatu 2b)
Davide Astuti
Department ofComputer Siene,UniversityofHelsinki
Lientiate Thesis C-2014-3
September2014
121 pages
Abstrat. Theprovisionofwirelessaesstovoieanddataserviesexperienedanimpressivegrowthof
importane duringthe last20years. MostoftheInternetappliationsemploytheTCP/IPprotool
suitetodelivertheirservies. TCPhasbeendevelopedtooperateinaterrestrialnetworkenvironment.
However,with therapid risein wireless ommuniationin reentyears,it hasbeomeimportant to
adapt TCP to heterogeneousenvironments that inlude bothwireline networks and Wireless
Wide-Area Networks (W-WANs), suh as satellite and terrestrial wireless networks, in order to optimize
performane.
Thespeilinkharateristisofwirelessandsatellitelinks,inpartiularhighlateniesandfrequent
framelossesdueto bit-orruption,aet performaneof transportprotools. TCPsuers beauseof
low bandwidth, long onnetionset-up times, high lateny and errorrate, ineient use of air link
apaity. MostofnextgenerationsatellitesystemsutilizesBandwidth-on-Demand(BoD)mehanisms
to eiently share radio resoures among a large number of users. BoD mehanisms often auses
additionallink delay,whih hasadetrimental eetonTCPperformane.
In this thesis, we propose anew TCP/IP-friendly link level protool, namely Satellite Link Aware
Communiation Protool (SLACP) whih inorporates a novel error-reoverymehanism aimed at
drastiallyredue theresidualPaketError Rate(PER) asseenbyhigherlayerswithoutproduing
signiantextra delay. Theprotoolhasbeenvalidated with experimentalevaluations onduted in
aDigital Video Broadasting-ReturnChannel System (DVB-RCS) satellite environment, where the
BoDtehniqueDemandAssignmentMultiple Aess(DAMA)is employed to regulatetheaess to
the satellite hannel. Cross-layerinterations between link layer and transport layer aswell asthe
impat of BoD mehanismson TCP dynamis are investigated. Furthermore, experiments using a
seletedsetofstate-of-the-artTCPenhanementsinonjuntionwithourTCP/IP-friendlylink-level
protoolhavebeenarriedout. Theproposedmehanismsandimprovementsofthelinkandtransport
layerwould berelevantformanywirelessWANsystems.
Aknowledgements
Thisthesishasbeenwritten thankstothe helpand supportofanumberofpeople. First,Iwishto
expressmydeepestgratitudetoProfessorKimmo Raatikainenforgivingmehisinvaluable support
and the opportunity to join the Department of Computer Siene at University of Helsinki as
ErasmusstudentrstandasPhDstudentlateron. Hisguidaneandfeedbakplayedaruialpart
forthewritingofthisthesis. Unfortunately,Iouldnotsharetheompletion ofthisworkwithhim.
Iannotforgetallthediretions,suggestions,onstrutivefeedbakIreeivedfromLeturerMarkku
Kojo during ountless hours spent to disuss protool design tehniques and the experimental
methodology whih has been applied to the work in this thesis. Writing this thesis would not
havebeen possiblewithout him.
Iwishtothankallthe membersofmyprojetteamforreatingaheerfulworkingenvironmentand
providingvaluablesupportandfeedbakduringallthephasesoftheexperiments. Speialthanksto
Aki Nyrhinenfor our exellent and produtive ollaboration duringthe implementation ofSLACP
protool. I will always remember the hallenging atmosphere in the lab and the joy after results
wereahieved.
MythankstoJ.Laan forhisontributionto thedesign oftheFEC mehanismintheSLACPand
J. Fimes for implementing the Reed-Solomon enoder and deoder for the SLACP. Alatel Spae
provided theNEPsatellite emulator platformusedintheexperiments. Thisworkhasbeen arried
out asa partof the TRANSATprojet ofEuropean Spae Ageny. TheDepartment of Computer
Siene hasprovidedthesupportand equipment Ihave neededtoprodue and ompletemythesis
and thefundingofmy studies.
SpeialthankstoSimoneLeggioandGiovanniCosta,mybestfriendswithwhomIgrowuptogether
duringmyuniversitystudiesandworkingyears. Wesharesomanyexperienesandjoyfulmoments
that Iannot hoose even one to write here without thinking to another one whih woulddeserve
to be mentioned aswell.
Myolletivethanksto all the peopleIhave forgotto mention, together withmyapologies.
Finally, Iwishto thankmyparents andmy brotherfor their invaluable supportduring allmylife,
and to mylove Johanna for staying on my side and sharing withme all the diulties and joys I
Contents
1 Introdution 1
1.1 Motivation. . . 2
1.2 ExistingSolutions . . . 2
1.3 Overview ofthe approah . . . 3
1.4 Strutureof the Thesis . . . 4
2 Wireless and Satellite Systems 7 2.1 Overview . . . 7
2.2 GlobalSystem for Mobile Communiation . . . 8
2.3 General PaketRadioServie . . . 9
2.3.1 GPRSProtool Stak . . . 11
2.3.2 Enhaned Data ratesfor GSMEvolution . . . 12
2.4 UniversalMobile Teleommuniations System . . . 12
2.5 Long Term Evolution . . . 14
2.6 SatelliteCommuniation Systems . . . 15
2.6.1 SatelliteConstellations . . . 16
2.6.2 SatelliteLink Charateristis . . . 18
2.6.3 Medium AessControl Protools. . . 19
2.6.4 SatelliteBandwidth-on-Demand . . . 21
3 Link Layer Protools 25 3.1 Overview ofFuntionality inDataLink LayerProtools . . . 25
3.2 Forward ErrorCorretion . . . 26
3.2.1 BlokCodes. . . 28
3.2.2 Convolutional odes . . . 30
3.2.3 Interleaving . . . 32
3.2.4 Criteriafor ode seletion . . . 33
3.3 Automati RepeatRequest . . . 34
3.3.1 ARQPersistene . . . 36
3.3.2 Prosand Cons ofARQ. . . 37
4 Transport Protools 41
4.1 ReliableData Transport . . . 41
4.2 TCP overWirelessand Satellitelinks. . . 43
4.2.1 ProblematiLink Charateristis . . . 43
4.2.2 TCP-basedEnhanements . . . 46
4.2.3 TCP PEP-based Enhanements . . . 50
4.3 UnreliableData Transport . . . 52
4.4 Delay-Sensitive AppliationsoverWireless andSatellite Links . . . 53
5 A New LinkLayer Protool 57 5.1 Overview . . . 57
5.2 LogialChannels andQoS Support . . . 58
5.3 ErrorReovery . . . 60
5.3.1 Aknowledgements . . . 61
5.3.2 Shemeof theproposedFEC mehanism . . . 64
5.3.3 Choosing the appropriate errorreovery sheme . . . 68
5.4 Support for ExpliitLossNotiation . . . 69
5.5 Frames. . . 70
5.6 Cross-layer Flow Control . . . 70
6 Experimental Evaluation 73 6.1 SystemArhiteture . . . 73
6.2 EmulationPlatform . . . 75
6.3 Test Arrangements . . . 76
6.3.1 Satellitelinkerror model. . . 78
6.3.2 TCP improvements . . . 78
6.3.3 Workload . . . 79
6.3.4 Performane Metris . . . 79
6.4 Experiments withRegular TCP . . . 80
6.4.1 Oneow. . . 80
6.4.2 Four ows . . . 85
6.4.3 Summary ofbasi problems withRegularTCP . . . 87
6.5.1 Oneow. . . 88
6.5.2 Four ows . . . 89
6.5.3 Summary ofSLACPadvanes . . . 90
6.6 Experiments with SatelliteTCP andEnhaned TCP . . . 90
6.6.1 SatelliteTCP . . . 90
6.6.2 Enhaned TCP . . . 92
6.6.3 Attaking the SlowStart overshoot . . . 95
6.7 Impatof Demand Assignment MultipleAess(DAMA)alloation delay . . . 97
6.7.1 Spurious TCP RTOs . . . 97
6.7.2 Reduing aessdelay . . . 99
6.7.3 Delayed ACKs . . . 100
6.7.4 ShortTransfers . . . 101
6.8 Further Analysisof SLACP behavior . . . 102
6.8.1 Protool Behavior . . . 102
Typial reovery ase. . . 102
Reovering thelost tail ofa burstof pakets . . . 103
Reovering lost retransmissionsusingFEC redundany . . . 104
6.8.2 Reovery Modes . . . 105
6.9 Summary . . . 107
7 Conlusions 109 7.1 Summary ofthe Thesis . . . 109
Introdution
Theprovisionofwirelessaesstovoieanddataserviesexperienedanimpressivegrowthof
impor-taneduringthelast20years. Avarietyofteleommuniationnetworktehnologieshasbeen
devel-opedforprovidingubiquitousaessforusers. Wide-areaellularnetworks,suhasGeneralPaket
Radio Servie (GPRS) [33℄, Universal Mobile Teleommuniations System (UMTS) [40,112,119℄
and Long Term Evolution (LTE) [2℄ provide onnetivity in a geographially large area. Wireless
loal areanetworks (WLAN) oera highspeed serviein limitedareas. Communiation satellites
anbeintegratedwiththeterrestrialnetworkinfrastruturetosupportaesstotheInternet,
oer-ingglobal overage, broadastand multiastapabilities, Bandwidth-on-Demand (BoD)exibility,
salabilityand reliability.
Transmission Control Protool (TCP) [107,125℄ is the main transport protool in the Internet.
Most of the Internet appliations employ TCP to deliver their servies. TCP has been developed
to operate inaterrestrial network environment. However, withtherapid riseinwireless
ommuni-ation inreent years,ithasbeomeimportant to adapt TCP toheterogeneous environmentsthat
inlude both wirelinenetworksand WirelessWide-AreaNetworks(W-WANs),suhassatelliteand
terrestrial wireless networks, inorderto optimize performane.
The spei link harateristis of wireless and satellite links,in partiular highlatenies and
fre-quent frame losses due to bit-orruption, aet performane of transport protools. TCP suers
beauseoflowbandwidth, longonnetionset-uptimes, highlatenyand errorrate, ineient use
of air link apaity [63℄. Most of the next generation satellite systems utilizes BoD mehanisms
to alloate satellite link apaity for eient sharing of radio resoures among a large number of
users. BoD mehanisms often auses additional link delay, whih hasa detrimental eet onTCP
performane[18,81,123℄.
AnextensivesetofTCP enhanementshasbeenproposedbytheresearhommunity duringthese
years [18,24,37,81,97℄ to improve the performane of TCP over wireless links. Solutions an be
broadlylassied assplit-onnetion, link-layer approahesand end-to-endapproahes[24℄.
In this thesis, we propose a new TCP/IP-friendly linklevel protool, namelySatellite Link Aware
Communiation Protool (SLACP) [18,81℄, whih inorporates a novel error reovery mehanism
aimed at drastiallyreduethe residual PaketError Rate(PER)asseenby higherlayers without
onduted inaDigital Video Broadasting-ReturnChannelSystem (DVB-RCS)[45℄ satellite
envi-ronment, where the BoDtehnique Demand Assignment Multiple Aess (DAMA)is employed to
regulatetheaesstothesatellitehannel. Cross-layerinterationsbetweenlinklayerandtransport
layer aswell as the impat of BoD mehanisms on TCP dynamis areinvestigated. Furthermore,
experiments using a seleted set of state-of-the-art TCP enhanements in onjuntion with our
TCP/IP-friendly link-levelprotool have been arriedout.
1.1 Motivation
TCP performane over W-WAN networks is often poor due to the harateristis of wireless and
satellite links suh as high lateny, high Bandwidth-Delay Produt (BDP), high error rate, long
RoundTripTime(RTT),bandwidthasymmetryandvariableRound-TripTime(RTT)[10,12℄. All
theseharateristishavesevereimpatonTCP performaneinlinkswithhuge propagationdelay,
suh as Geostationary Orbit (GEO) satellites, espeially in ase BoD mehanisms are in use for
alloating hannel resoures [18,81,123℄.
AmajorproblemwithTCPonsistsofitsinabilitytodistinguishbetween paketlossesdue tolink
imperfetions and paketlosses due to ongestion. Inboth ases TCP will redue its transmission
rate, while this is neessary only in the latter ase. Moreover, in Wireless-Wide Area Networks
(W-WAN) links, paket losses often our in bursts resulting in the loss of several segments in a
singleTCPwindow. IntermsofresultingtransportperformanetheTCPlossreoverytendstobe
inadequate withsuhlosspatterns.
Link-level error reovery maybe used to redue the residual error rate asseen by TCP.However,
ross-layerinterations may impatonTCP performane. Theadditional delayintrodued bylink
levelretransmissions mayause signiantly long delayspikes and adversely interferewith
end-to-enderrorandongestionontrolattransportlevel,resulting inunneessarypaketretransmissions,
redutionof thesending rateand waste oflinkapaity.
Typiallywirelesslinksalsopossessotherharateristis,suhasBoDalloationmehanisms,
band-width asymmetry, that mayadversely aet TCP performane. BoD mehanisms introdue
addi-tionallinkdelay; suh adelay maybesigniantly variableover timedepending onthe amount of
tra oered to thenetwork. Thismayresultina detrimental impatonTCP reoveryalgorithm
performane and ause unneessary retransmissions [18,81℄. To our knowledge, little published
researh hasbeendeveloped oninterations between BoD mehanisms and transport protools.
1.2 Existing Solutions
Theshemes proposedtoimprove the performane ofTCP over wireless linksan be broadly
las-siedassplit-onnetion, link-layerandend-to-endapproahes[24℄. Thesplit-onnetionapproah
replaes an end-to-end TCP onnetion with two or more separate onnetions. One of the
on-netions is aross the problemati wireless link allowing TCP modiations for more eient loss
impliations as it breaks the end-to-end semantis of the onnetion [29℄. It also annot oexist
withtheend-to-end useofInternetProtoolseurity(IPse) [79℄.
Inthelink-layerapproah,link-levelerror reoveryisusedloallyonanerror-prone linktoimprove
thereliabilityofthe link. Loalknowledgeofthelinkan be usedto optimize thereovery
meha-nism. Manylink-layer reovery mehanisms arebasedon Automati RepeatreQuests(ARQ)used
in a highly persistent mode of reovering lost frames. This may ause unwanted interation with
TCP retransmissions[24,50℄. In partiular,highly persistent linkARQeasily leads to delayspikes
that an result in suboptimal TCP performane by ausing spurious TCP timeouts, unneessary
retransmissionsand amultipliative dereaseintheongestion windowsize.
The end-to-end approah preserves the end-to-end semantis of TCP. Some of the proposals in
this ategory inlude TCP enhanements that follow the Internet ongestion ontrol priniples
[14,53℄ suggested by Internet Engineering Task Fore (IETF) suh as large initial window [11℄,
TCP SeletiveAknowledgment Option(TCPSACK)[93℄,WindowSaling [77℄,and TCPControl
BlokInterdependene(TCP-CBI)[130℄. Alarge numberof researhproposalsnotfollowing IETF
reommendations hasbeen proposed, suhasTCP Peah [5℄and TCP Westwood[92℄.
1.3 Overview of the approah
Inthisthesiswefousonthelink-layerapproahanddisussthedesigndetailsofaTCP/IP-friendly
link-layerprotoolwithnoveldesign aimed at improving TCP and otherIP-tra performane on
network pathsinvolving W-WAN links.
SatelliteLink Aware Communiation Protool (SLACP) [18,81℄is a logiallink-layer protool
uti-lizing a new hybrid error reovery mehanism. Hybrid ARQ tehniques ombine Forward Error
Corretion (FEC) and ARQ [85℄. SLACP ombines ARQ with FEC in a novel way in order to
redue the residual paket-error rate (PER) and thereby allowing more eient TCP operation.
SLACPuses the interleaving tehnique [85,87℄,but at theframe levelrather than bitlevel.
Theadditional delaydueto ARQisminimized bylimitingtheretransmissionattempts to one,but
the residual PER is still kept at a very low level. This is possible by retransmitting lost frames
together withFEC enoded redundany frames, inreasing theprobability that a lost frame is
re-overedwithoneretransmissionattemptonly. Thisfeatureispartiularlybeneialforlonglateny
links aseah additional retransmission ontributes to inreasing theend-to-end RTT signiantly.
The SLACP design involvesa number of additional measures inkeeping the delay to a minimum,
inludingminimalqueuing inthelinkhead andtheexpediteddelivery ofretransmitted framesand
aknowledgments (repeatrequests).
In order to provide support for delay sensitive tra, SLACP supports several Quality-of-Servie
(QoS) lassesbyimplementingseverallogialhannelsover asinglephysial linkand allowing eah
hannel to be ongured with QoS parameters and error ontrol strategy that best suit to the
IP tra direted over the hannel. Frames transmitted over eah logial hannel are delivered
independently of the frames sent over the other hannels. This avoids the head-of-line bloking
problemourringduring ARQ-based reovery.
SLACP employs ow ontrol between the IP layer and the link layer together with limiting the
oftenowdiretlyto the MediumAessControl(MAC)buer toperformBoD (sinetheamount
of link apaity requested depends on the quantity of data enqueued inthe MAC buer) without
owontrol betweenthe layers andtheexess paketsaredroppediftheMACbuerbeomesfull.
The IP queue will always be empty; thus, proper IP router queue sizes to ontrol total amount of
buering annotbe used and theuse of IPative queue management mehanisms is not eetive.
With SLACP, arriving pakets are forwarded to the MAC layer only if spae is available in its
buer;otherwise,paketsarekeptintheIPbuer. Inaddition,anyunneessarylink-levelbuering
isavoided tominimize the overalldelay byallowing only oneunsent paket bueredinSLACP.
We have implemented the SLACPprotool onLinux. We perform anextensiveset ofperformane
experiments in an emulated satellite DVB-S/DVB-RCS environment with real TCP/IP staks in
theend hosts and employing our implementation ofSLACP over thesatellite segment. A satellite
emulation platform is used to emulate several levels of error rate and a DAMA BoD alloation
sheme onthe satellite return link.
We useaseleted setof state-of-the-artTCP enhanementsinonjuntion withSLACPto further
improve TCP performane over W-WANs. There has been little earlier work in evaluating TCP
performanewhena propersetofTCP enhanements areombined. Studyingtheombined eet
oftheTCP enhanementsforsuhanenvironment allows better understandingofhowwella
state-of-the-artTCPanperform. OurgoalistoshowthatTCPenhanementsalonemaynotbeenough
to provide aeptableperformane(espeiallyinase ofhighPER)and thatusingaseleted setof
TCP enhanements inonjuntion withSLACP protoolis theway to improve TCP performane
inerror-prone W-WAN links.
1.4 Struture of the Thesis
The rest of the thesis is organizedas follow. In Chapter2 we desribe the main harateristis of
mobile wireless teleommuniation and satellite systems. In this thesis, we are interested in the
impat of wireless systems harateristis on the behavior of transport protools. Several mobile
wireless teleommuniation systems and satellite ommuniation systems are desribed to show
whatharaterististheyhaveinommon, whatarethe maindierenesand provide theneessary
bakground to understand theresearh problem.
InChapter3weprovidethebakgroundonlinklayerprotoolsanderrorontrolshemes. Twobasi
ategoriesoferrorontrolmethodstoimprovetheapparent qualityofaommuniationhannelare
desribed: FEC and ARQ. In Chapter 4 reliable and real-time transport protools are desribed
togetherwiththeproblems experienedinwireless environment. Several proposedmitigations and
relatedwork areintrodued.
Themainontributions ofthisthesisarepresentedinChapters5and6. InChapter5weintrodue
a new TCP/IP-friendly link level protool, alled Satellite Link Aware Communiation Protool
(SLACP). Design priniples,main objetives, protool and interfaingdetails aredesribed.
InChapter6wepresent experimentalevaluations ondutedto validatetheSLACP protoolinan
emulated DVB-RCS satellite environment. An overview of the DVB-RCS system arhiteture is
provided, mainly fousing on the elements of thesystem arhiteture relevant to thesope of this
Wireless and Satellite Systems
This hapter desribesthe main harateristis of mobile wireless teleommuniation and satellite
systems. Inthisthesis,weareinterestedintheimpatofwirelesssystemharateristisontransport
protools behavior. Therefore, several mobile wireless teleommuniation and satellite systems
are desribed to show what harateristis they have in ommon, what are the main dierenes
and provide the neessary bakground to understand the researh problem. Althouth a number
of teleommuniation systems are desribed, the pragmati fous of the thesis is in the satellite
systems;themobileterrestial systems arenot usedinevaluatingtheproposalsof thethesis.
2.1 Overview
The provision of wireless aess to voie and data servies experiened an impressive growth of
importane during the last 15 years. Several teleommuniation network tehnologies have been
developedforprovidingubiquitousaessfor users. Wean distinguishwirelessmobile
teleommu-niationsystems andsatellite ommuniation systems.
Despite of their tehnologial dierenes, implementation and appliation elds, mobile
teleom-muniation systemsand satellite systemshave manythings in ommon onerningtheir high-level
struture and their harateristis and featuresprovided to the user. A high-level view of an user
aessing PaketDataNetwork(PDN) servies(e.g. Internet,Intranet) using wireless tehnologies
is depited in Figure 2.1. The user is endowed with a Mobile Devie (MD) onneted to an
a-ess network infrastruture, whih further gives aess to the PDN through a Last-Hop or aess
Router. Therefore,theaessnetworkinfrastrutureanbeseenasaninterfaebetweenthemobile
user andthe PDN. Suh a role an be fullledboth bya wireless mobilesystemand bya satellite
ommuniationsystem 1
.
Wireless and satellite systems raise a multitude of performane issues sine they provide the user
with radio links that typially experiene higher bit error rate, higher lateny and larger delay
variationthan wirelinelinksdo. Charateristis ofwireless andsatellitelinks areoftenomparable
1
TheLast-Hopongurationisonlyoneofthepossiblewaysofemployingasatellitelink. Hereweareinterested
Figure2.1: High-levelview ofa mobileuser aessinga PDN.
andhavesimilardetrimentalimpatonappliationsperformanealthoughtheoriginofthese
prob-lemsisverydierent. Anoverviewofwireless andsatellitesystemsdesribing their harateristis,
ommon omponents andmain dierenesis given below.
2.2 Global System for Mobile Communiation
The Global System for Mobile Communiation (GSM) [109℄ is the pan-European digital ellular
standard published by the European Teleommuniations Standards Institute (ETSI). GSM is a
digital ellular system belonging to the seond-generation of ellular systems that sueeded the
rst generation of analogue mobile phone tehnologies inluding Advaned Mobile Phone System
(AMPS), Total Aess CommuniationSystem (TACS) and Nordi MobileTelephony (NMT).
GSMtehnologyisanextremelysuessfulwirelesstehnologyasitbeametheworld'sleading and
fastest growing mobile standard. It is estimated that at the end of January 2004 there was more
than 1billion GSMsubsribersarossmore than 200 ountriesof theworld [129℄.
GSM was originally designed for providing voie servies. Data servies were limited to a
iruit-swithed9.6Kbpsonnetion,furtherextendedto14.4Kbps[42℄. TheHighSpeedCiruitSwithed
DATA (HSCSD) [43,44,127℄ is an extension to GSM allowing paket data servies at a potential
bitrate of115 Kbps [121℄.
The GSMnetwork infrastruture is shownin Figure2.2. GSM funtionalities aredivided between
three parts: the Mobile Station (MS), the Base Station Subsystem (BSS) and the Network
Sub-system (NSS). The MS is omposed of the terminal and a smart ard alled Subsriber Identity
Module (SIM), whih provides personal mobility to the user. The BSS is omposed of the Base
Transeiver Station (BTS) and the Base Station Controller (BSC). The Base Transeiver Station
inludes the radio transeivers overing a ell and handles theradio ommuniation with the MS.
The BSC manages radio resoures (radio-hannels setup, frequeny hopping, handover) for one or
more BTSs. The Mobile Swithing Center (MSC) isthe entral omponent of theNSS and allows
onneting the GSM network to the xed networks suh as PSTN or ISDN. MSC also manages
mobilesubsriberfuntionalitiessuhasregistration,authentiation, handovers,loationupdating,
andallroutingtoaroamingsubsriber. Inordertoprovidetheseservies,otherfuntionalentities
arepresent in theNSS:the HomeLoation Register (HLR), the Visitor Loation Register (VLR),
Figure2.2: GSM systemarhiteture.
The main limitation of GSM for providing data servies is that it is based on iruit-swithed
transmission. Setting uptheinitial onnetiontakesseveralseonds. Inaddition,aommuniation
hannelisalloatedfora singleuserforthe entire allperiod. Hene, inaseof burstydatatra,
this results in a waste of network resoures and is an expensive serviefor theuser. Onthe other
hand, onethe onnetion hasbeen established, the user benets by thealloated radio resoures
for all theall duration possiblywithoutdelayor bitrate variations.
Inaseaonnetionhandoours,thelinkmaybeunavailableforseveralseonds(linkdisruption).
This has an adverse impat on performane of transport protools as it may ause paket losses
(seeChapter4 formore disussion). The delaylengthaused byanhandomayvaryand depends
on whether the onnetionhando is internal (hando between ells overed by thesame BSS) or
external (handobetween ells underoverageof two dierent BSSs).
2.3 General Paket Radio Servie
General Paket Radio Servie (GPRS) [33℄ is an extension of the GSMsystem. It oersa
paket-swithedbearerservietogetherwiththeexistingiruit-swithedserviesavailableinGSM.GPRS
has been designed to extend dataservies provided by GSM so that users an take advantage of
shorternetwork aesstimes andhigher transmission rates than inGSM. GPRSsupports existing
paket-swithed networks,suhasIPnetworks.
GPRS Support Node (SGSN)and the GatewayGPRS SupportNode (GGSN). The SGSNis
on-netedto oneor more BSC.Itisresponsibleforpaketdelivery towardsthemobilestations within
its geographial servie area. It handles paketrouting, mobility management (attah/detah and
loation management), authentiation and harging funtions. It stores user loation information
inits loationregisteranduserprolesofallGPRSusersregisteredinthatSGSN.TheGGSNats
as a gateway between the GPRS bakbone network and the external paket data networks. It is
responsibleforIPaddressassignment,itstorestheurrentSGSNaddressoftheuserandhisprole.
It alsoprovidesharging funtionality.
On thephysial layer of both GSMand GPRS, Frequeny Division Multiple Aess(FDMA) and
Time Division Multiple Aess (TDMA) are used. TDMA tehnique is applied in multiple
fre-quenies bands(TDMA/FDMA).The frequeny band reserved for GSM/GPRS (890-915 MHz for
uplinkand935-960MHzfor downlink)isdividedin124arrierhannelsof200 kHzwidth. Eahof
thesefrequeny hannels holdseight TDMAhannelsor time slots. Eight timeslotsformaTDMA
frame. The reurreneof onepartiular time slotdenes aphysial hannel.
GPRS diers from GSM inthe way hannels are alloated. In GPRS, a mobile user an alloate
fromonetoeighthannelsperTDMAframe(multi-slotoperationmode). Ahannelisonlyreserved
when paketsare transmitted andit is releasedshortly after thetransmission ifthere areno more
paketsready to be transmitted. Consequently,amobile stationmayreservea dierent numberof
hannelsbasedon itsbandwidthneeds. Instead,inGSMahannelisalloatedto aspeimobile
station for the entire all duration. In order to more eiently support asymmetri data tra
where the amount of data indownlink is dierent than inuplink (e.g. Web transfers), uplink and
downlink hannels arealloatedseparately.
In GPRS a hannel provides a bit rate ranging between 9.05 Kbps and 21.4 Kbps based on the
amount ofhannelodingusedtoprotet dataagainsterrors. Sineausermayalloateuptoeight
hannels,amaximumtheoretialbitrateof171.2Kbpsan beahieved. Inpratie,theahievable
datarates rangefrom 40to 60 Kbps.
GPRS is more suitable for bursty data tra (e.g. Internet appliations) than GSM as its user
billing systemfor dataserviesisbasedon theamountof data volumeeetively transferred. This
is possible as hannels are alloated only when data pakets are sent or reeived and they an be
used by dierent users (statistial multiplexing). This also results in an eient management of
radio resoures. The main drawbak is that the network aess is not deterministi; a user may
not beable to alloatea hannelto transfer newpakets inase all hannels are alreadyalloated
by other users, resulting in an additional delay before paket transmission is ahieved. Suh an
additional omponent of the link lateny may be harmful to performane transport protools (see
Chapter4 for moredisussion).
AsinGSM,usermobilityaetsthelinkavailabilityandmayausepaketlosses,delaysandpaket
reordering. Moreover, onea handoourrednetwork resoures available inthenewell maynot
bethesame asinthe previousone. Therefore, maintaining theprevioustransmissionbit ratemay
Server
BSC
SGSN
GGSN
Application
L2
L1
LLC
SNDCP
RLC
L1bis
MAC
SNDCP
GTP
BSSGP
L2
L1
L1bis
UDP /
TCP
LLC
IP
L2
IP
L1
TCP
UDP /
GTP
L2
L1
Service
Network
Service
Network
BSSGP
LLC: Logical Link Control
GSM RF
GSM RF
MAC: Medium Access Control
RLC: Radio Link Control
BSSGP: BSS Gateway Protocol
GSM RF: GSM Radio Frequency
GTP: GPRS Tunnel Protocol
L1, L2: OSI protocol layers 1 and 2
IP: Internet Protocol (version 4 or 6)
MS (Client)
IP
IP
IP
IP
MAC
RLC
LLC relay
SNDCP: Subnetwork Dependent Convergence Protocol
Application
Figure2.3: GPRSprotoolstak.
2.3.1 GPRS Protool Stak
Figure 2.3 depitsthe GPRS protool stakand the elements involved in a data transfer between
theMSandaServeronnetedtothe GGSNthroughanexternalpaket datanetwork. TheGPRS
Tunneling Protool (GTP) [41℄ is employed between the two GPRS Support Nodes (SGSN and
GGSN) to route data pakets and related signal information into the GPRS bakbone network
(tunneling). UnderGTP, eitherUDPor TCP transport protoolmay be usedfor ommuniation.
UDP is preferred sine the GPRS bakbone provides high reliability and further error ontrol at
transportlayeris not needed. The network tehnology adoptedbelow IP level an be various and
dependson the operator.
The Subnetwork Dependent Convergene Protool (SNDCP) [46℄ is employed between the MS
and the SGSN.It maps thenetwork-level protool to theunderlying logial link ontrol. SNDCP
also providesother funtionssuh asompression, segmentation andmultiplexing ofnetwork layer
messagesto asingle virtual onnetionfromtheSGSN to theMS.
Two sub-layers an be distinguished in the data link layer: the Logial Link Control (LLC) [47℄
and theRadio Link Control/Medium AessControl (RLC/MAC) [49℄ sub-layers. TheLLC layer
onnets the MS with the SGSN and provides a highly reliable link onnetion between the two
peers. The protool usedis an adapted version of the Link AessProedure-D (LAPD) protool
employed in GSM [38℄. Its funtionalities inlude ow ontrol, in-order delivery, error detetion
and orretion, iphering. Variable frame lengthsare supported. AsFigure 2.3 shows, theLLC is
split between the BSC and SGSN. The BSC funtionality is alled LLC relay. The Base Station
performsfuntionsof routingand QualityofServie (QoS) information for theBSS.
The Radio Link Control (RLC) ahieves a reliable linkbetween theMS and theBSS. It performs
segmentation andreassemblyfuntionsofLLCframesintoRLCdatabloksanderrorontrol suh
asAutomatiRepeatreQuest(ARQ)andForwardErrorCorretion(FEC)(seeChapter3formore
details about error orretion). RLC protool uses seletive rejet ARQ protool. The maximum
numberof retransmissionsfor aorrupted frame(ARQ persistene) is ongurable from 0to 3. A
reliable RLC mode ensures that frames are delivered in order, while the ARQ in RLC ombined
withFEC ahieves an high hannel reliability. Four dierent odingshemes (CS-1 to CS-4) with
varying levels of FEC [32,94℄ may be utilized. Most urrently deployed GPRS networks support
only CS-1 andCS-2. The MAClayerregulates theaessamong severalMSs to theradio hannel
by usingalgorithms for ontention resolution, sheduling and prioritization.
The GPRS data link layer provides high reliability but it suers from high and widely varying
Round Trip Times (RTT). Measurements using pingoverexisting GSMnetworks showed thatthe
typialvalueof RTTis around700 ms[61,82℄though themeasurements rangeisbetween 500 and
1100msastheRTTdependsonthenetworkonditionsintheellservingtheMS.Suhhighvalues
and variationrange mayadversely interat withtransport layerprotools (seeChapter 4 for more
details).
2.3.2 Enhaned Data rates for GSM Evolution
EnhanedDataratesforGSMEvolution(EDGE)[99℄orEnhanedGPRS(EGPRS)isadigital
mo-bilephonetehnologythatprovideshigherdatatransmissionratesandimproved datatransmission
reliability ompared to GPRS. EDGE/EGPRS is an enhanement of GSM and GPRS networks,
and ithasbeen designed to failitate the upgradeof existingGPRSnetworks.
The upgrade ompared to a GPRS network involves the base stations (transeiver units) sine a
newmodulationshemeisinuse. Mobileterminalsneedtosupportthenewmodulation andoding
shemes allowing higheruser datarates. No hangesarerequired intheGPRSore network.
EDGEusesthehigher-order PhaseShiftKeying/8(8PSK)modulation,inadditiontotheGaussian
Minimum-Shift Keying (GMSK). Like in GPRS, a rate adaptation algorithm is used to adapt the
modulationand odingshemeto thequalityoftheradio hannel. A newreovery mehanismhas
been introdued to inrease the robustness of data transmission, alled Inremental Redundany
(IR), whih inorporates both FEC and ARQtehniques (see Chapter 3 for more details on error
reovery mehanisms). EDGE supports data rates up to 236.8 Kbps for 4 time-slots (theoretial
maximumis473.6 Kbpsfor 8 time-slots)inpaketmode.
2.4 Universal Mobile Teleommuniations System
TheUniversal MobileTeleommuniations System (UMTS) [40,112,119℄ belongs to thethird
gen-eration of mobile systems and is being developed in the Third Generation Partnership Projet
(3GPP) [128℄. UMTS provides both voie and paket data servies with data rates apabilities
Figure2.4: UTRANarhiteture.
Themaximumdatabitrate availableinUMTS is2Mbps for slowmobile terminalsinpioellular
environments. The data rateof 384 Kbps an be provided inmiroellular environments, whereas
terminalroamingountrysideinmaroellular environmentsanexploitdata ratesupto144Kbps.
One of thekey requirement of UMTS is preservingthe urrent GSM/GPRSinfrastruture
invest-ments of operators during the upgrade to UMTS. The UMTS network struture onsists of three
parts(seeFigure2.4): TheCore Network(CN),a newRadioAessNetwork(RAN)alledUMTS
terrestrial RadioAessNetwork (UTRAN) andtheUser Equipment (UE).
The arhiteture of CN is basedon GPRS network, although the equipment must be upgraded to
supportUMTSfuntionalities. ThemainfuntionofCNonsistsofprovidingswithing,routingand
transitfor usertra aswellasnetwork management apabilities. The CoreNetworkisdivided in
iruitswithedandpaketswitheddomains. PaketswithedelementsareServingGPRSSupport
Node(SGSN) andGateway GPRSSupport Node(GGSN), inheritedfrom GPRSarhiteture.
UTRANisomposedbyoneormoreRadioNetworkSubsystems(RNS)(seeFigure2.4). EahRNS
onsistsoftwoparts,namelyRadioNetworkController (RNC)andNodeB,whihnearlyfulllthe
same funtionalitiesasBSC and BTSof GSMrespetively.
The RNC is onneted to a set of Node B elements. RNCs ommuniates with eah other by
the Iur interfae, whih is not present between GSM BSSs, to autonomously manage funtions
relatedtotheRadioResoureManagement(RRM). Thisresults inareduedburdenfromtheCN.
RNConentrates the most ofUTRAN intelligeneand performs servingontrol funtionssuhas
admission, MediumAessControl (MAC),Radio LinkControl (RLC),and handover.
The Node B handles theradio transmission and reeption within ells. Eah Node Bhandles one
or more ells. Node B supports funtions for handling the physial radio hannel, inluding FEC,
modulation. It measures the urrent Frame Error Rate (FER) and reports to RNC for handover.
NodeBtakesalso partinhandling powerontrol management of theUE.
UMTS diersfrom GSM/GPRS mostlyinthe new priniplesfor airinterfae transmission, whih
is based on CDMA aess sheme instead of TDMA and is implemented within the UTRAN. It
supportstwo modesof operation: FrequenyDivision Duplex(FDD),whihuses W-CDMAaess
tehnique,and TimeDivision Duplex(TDD),whihusesTD-CDMAaesstehnique. TD-CDMA
is an hybrid of TDMA and CDMA shemes, whih allows more than one data burst being
trans-mittedwithin atime-slot,eah withadierent CDMAspreading ode. Combining slotsandodes
properlyallows users tomakeuseofthe instantaneous datarate requiredbytheappliation. TDD
mode allows adjusting the ratio of uplink/downlink tra sothat asymmetrial tra (e.g. Web
tra) isbetter supported. It is likely thatFDD isused to provide maro-miro ellular overage
insimilarwayasinGSMand TDDfor pioellular environments[112,114℄.
The UEsupports the dual mode UMTS/GSMsothatit an exploit highdata rates up to 2 Mbps
insidetheUMTS servieareaanduse theGSM network outsidetheUMTSserviearea.
UMTSlink layerprovides a highreliabilityas inGPRSwitha lower RTT. Typial RTT values in
UMTSare around300 ms[61℄. Interations between linklayer andtransportlayer mayalso our
insuh anenvironment. (SetionChapter 4for a moredetailed disussion).
2.5 Long Term Evolution
Long Term Evolution (LTE) [2℄ is a standard developed by 3GPP for wireless ommuniation of
high-speed data for mobile phones and data terminals. LTE may also be referred as Evolved
UMTS Terrestrial Radio Aess (E-UTRA) and Evolved UMTSTerrestrial RadioAessNetwork
(E-UTRAN). Based on the GSM/EDGE and UMTS/HSPA 2
network tehnologies, LTE provides
higherapaityandlowerlatenyusingnewmodulationtehniquesandimprovedspetraleieny.
LTE provides and inreased peak data rate (up to 100Mbps for downlink with 20MHz, 50Mbps
for uplink with 20MHz), improved spetral eieny (5bps/Hz for downlink and 2.5bps/Hz for
uplink), improvedell edgeperformane(intermsofbitrate) andreduedlateny[1℄. Higherdata
rates are enabled by the use of Orthogonal Frequeny Division Multiple Aess (OFDMA), high
order modulation up to 64 Quadrature amplitude modulation (QAM), large bandwidth (up to 20
MHz)andMultiple-InputMultiple-Output(MIMO)transmissioninthedownlink (upto4x4). The
highesttheoretialdatarateis170Mbps inuplinkand withMIMOtherateindownlink an reah
300 Mbps.
Unlike GPRS and UMTS, LTE ispurely IP-based, both at transport leveland user level. All
ser-vies, inluding voie, are arried using IP (although voie servies an also be arried using the
legay iruit-swithed network). A more at network arhiteture isemployed in LTE ompared
withprevious3GPPmobilenetworks. TheE-UTRANNodeB(eNodeB)intheaessnetwork
om-muniatesdiretlywiththeEvolvedPaketCore(EPC) orEvolvedPaketSystem(EPS)usingthe
2
High Speed Paketdata Aess (HSPA) [3℄ is an upgrade to UMTS networks to inrease data performane.
Improvements inlude High Speed Downlink Paket data Aess (HSDPA) and High Speed Uplink Paket data
Aess(HSUPA).TheombinationofHSDPAandHSUPAisreferred toasHSPA.HSPAevolution(also knownas
S1interfae. TheEPCisomposedoftheseveralfuntionalentities,namelyMobilityManagement
Entity(MME),HomeSubsriberServer(HSS),ServingGateway(S-GW),PDNGateway(P-GW),
Poliy and Charging Rules Funtion (PCRF). The ore network EPC an be interonneted with
othernon-3GPP aesstehnologies, like WiMAXand WiFi.
2.6 Satellite Communiation Systems
A satellite ommuniation systemonsistsof a spae segment and a ground segment. The ground
segmentinludeGatewayStations(GSs),aNetworkControlCenter(NCC),andanOperation
Con-trolCenter(OCC).TheGSistheaesspointtothesatellitehannelandinteronnetstheexternal
network with the satellite network. The NCC and OCCs funtionalities inlude network resoure
management, satellite operation, and orbiting ontrol. The spae segment inlude ommuniation
satellites.
Aommuniationsatelliteprovidesamirowavelinkonnetionbetweentwogeographiallyremote
stations. Insimple terms, itan beseen asawireless repeater reeivingsignals froma stationand
forwarding ittowards one or more reeiver terminals. A satellite is equipped withone or multiple
transpondersomposedofatranseiverandanantenna. Dependingontheiraltitudeorbit,satellites
anevenoverhugeareasoverthesurfaeofthe earth. Thefootprint isdened astheareaovered
by asatellite's transmissionbeam.
Satellites an simply reeive and retransmit signals (bent-pipe satellites) without any additional
operationorinorporateon-boardproessingandswithingfuntionalitiestoprovidebetterservies.
Theadvantagesofon-boardproessingandswithinginludelowererrorrates(enodingtehniques
an be applied), separation of uplink and downlink, higher system eieny, delay and apaity
improvements and possibilityto re-routepakets [83℄.
Themain advantagesof satellite systemsare:
◦
Ubiquitousoverage: Satellitesan overa wide surfaearea over earth andreah every user inthat area regardlessits loation. The size of thefootprint an be thewhole earth asseenfromspaedependingonthesatellitealtitudeand,therefore,entireontinentsanbeovered.
◦
Connetivity: Satellites provide multi-point onnetivity and natural broadast apability, whihmake them suitablefor lowost multiast appliations(e.g. video broadasting).◦
Capaityexibility: Theapaityavailable tousers an beeasily ongured aordingtothe appliation requirements. Both symmetriandasymmetri ongurations areahievable.◦
Transmission osts independent of distane: Due to the wide area overed by a satellite, reeivinga signal fromanywhere insidethearea of overage doesnot vary theost,whih isindependent of distane.
◦
Deployment: Deployment of a satellite network is onvenient espeially for large areas as network overage for a potential big number of users an be ahieved immediately after thesystem has been installed. Therefore, satellite systems are a suitable solution for disaster
Band Earth-to-Spae Frequeny Spae-to-Earth Frequeny
C 5.925-6.425 GHz 3.7-4.2GHz
Ku 12.75-13.25, 13.75-14.5 GHz 10.7-12.75GHz
Ka 27.5-30.0 GHz 17.7-20.2GHz
Q/v 7.2-50.2 GHz 39.5-42.5GHz
Table 2.1: Satellite frequenybands
Satellite systems operate in various frequeny bands. Carrier frequeniesfor Earth-to-Spae
(Up-link) and Spae-to-Earth diretion (Downlink) aredierent. Table 2.1 provides themost ommon
frequeny bands. Current satellite systems operate inC and Ku bands, but the trend is in using
Kabands for future systems. The higher the frequeny band,the smallerantennas areneeded for
ommuniation. Typial sizesof antennas for systems operating inC band have a diameter of 2-3
meters. Inthe aseofKuband,antennas assmall as45 mindiameteran beutilized [66℄. Using
Kabands allows ahieving very high apaitysystems, but environmental impairments to eient
signaltransmissionsuhasfadingand rain attenuation have a moresigniant impat.
The Earth-to-Spae link is a point-to-point and highly diretional link, requiringa high gain dish
antennaatthegroundterminal. TheSpae-to-Earthlinkanovervarioussizeoffootprint. Inase
of smaller footprints, heaper and smaller ground terminals an be utilized as transmission power
isonentrated ina smallregion. Somesatellitessystemsallows dynamiallyrediretingtheir spot
beamsand hangingtheir overage area[126℄.
In the most general network topology, satellite links may be loated at any point inthe network.
Inthisase, the satellitelinkisjustanother ommuniation hannel onnetedwiththerestofthe
network by meansof two satellitegateways. Thistopology isknown asMiddle-Hoponguration.
In this thesis, we are partiularly interested to satellite links providing network aessdiretly to
endusers andthereforeating asaLast-Hoplink. Endusers anaessto satelliteserviethrough
a GS or Satellite Terminal (ST) or by using a dediated User Terminal (UT). Figure 2.5 depits
theLast-Hopongurationofasatellite system,where oneor moreend hostsonneted toaLoal
AreaNetwork(LAN)areonnetedtothe Internet ServieProvider(ISP)viasatellite. AlltheSTs
share the same ommuniation hannel with eah other. Consequently, oordination for resolving
hannelontentionissues isneeded. Dediated protools maybeusedoverthelasthop toimprove
theeieny of the system. Further details areinSetion 2.6.3.
2.6.1 Satellite Constellations
Dierent satellite systems ongurations are based on the altitude of their orbit. Satellite(s) are
lassied into GeoStationary Orbit(GSO) and Non-GeoStationary Orbit(NGSO) satellites.
GSO orGEO satellitesareplaed atan altitudeof 35,786km abovetheequator. Theorbit period
is 24 hours, therefore their revolution is synhronized with the earth's rotation. This gives the
advantage of ontinuous visibility from the earth and makes GEO satellitesthe preferred satellite
ongurationfordistributingtelevision, telephoneanddataommuniationsthroughout theworld.
Duetoitsorbitonguration,aGEOsatelliteanoveraverylargearea. OnlythreeGEOsatellites
LAN
ST
LAN
ST
SATELLITE ACCESS
NETWORK
HUB
BACKBONE
NETWORK
BACKBONE
NETWORK
ST
ISP
Figure 2.5: Last-Hopsatellite onguration.
the satellite position above the equator). An important drawbak of GEO satellites is their high
propagation delay whih ranges between 250-280 ms. This makes suh a satellite onguration
lesssuitable for real-time and interative appliations. Examples of existing GEO systemsinlude
Intelsat [69℄,Inmarsat[68℄, andPanAmSat [101℄.
TheNGSOsatellitesanbelassiedinMediumEarthOrbit(MEO)andLowEarthOrbit(LEO).
As opposite to GEOs, NGSO satellites are in motion with respet to an observer on the surfae
of the earth. Therefore, the earth station needs to antiipate the diretion and the time where
and when the next satellite will appear above the horizon and orient its antennaaordingly.
Af-terwards, the station needs to ontinually trak down the satellites as it moves and re-orient the
antenna. Therefore,oordinating the ight pathsandommuniations handovers requiresomplex
and sophistiatedontrol and swithing systems.
MEOsatellitesorbitarebetween10000and15000km. Theorbitperiodisaround6hours. Theearth
surfaean beoveredby10-12satellitesin2-3planes. MEOsatellitesprovideapropagationdelay
between 110 and 130 ms. An example of existing MEOsystem is the Navstar Global Positioning
System (GPS) [98℄, whih provides navigational data to mobile units anywhere in the world to
measuretheurrentpositionontheearthsurfae. TheGPSsystemisomposedofaonstellationof
24satellites,3satellitesforeahoneofsixorbitsoveringthesurfaeinequalsetions. Thereeiver
positionintermsoflatitude,longitudeandaltitudeanbeauratelydeterminedbyreeivingsignals
transmitted fromat leastfour ofthese satellites.
Thealtitude ofLEO satellitesorbitranges between 700and 2000 km. Theorbit periodis between
100to120minutes. Inordertooveraertainpointofthesurfaeontinuously,LEOonguration
onsistsof 6-8 planes with6 satellites perplane. The ground stationmust perform databuering
or aquire thenext visible satellite before theold one disappears to handle the handover between
two satelliteswithout serviedisruption. Duetotheir loworbit,LEOsatellitesan overasmaller
area than MEO or GEO ongurations. However, the typial propagation delay is around 20-25
ms. Two existingLEOsatellitesystems areIridium[70℄and GlobalStar[60℄. TheIridiumSatellite
Orbit type LEO MEO GEO
Altitude (km) 700-2000 10000-15000 36000
Satellites For Global Coverage
≥
32
10-15 3-4Delay 5-20 ms 100-130 ms 250-280ms
Capaity per Satellite 0.128-10Gbps 0.128-10 Gbps 1-50 Gbps
Handover Frequent Infrequent Never
Level of Complexity High Medium Low
Broadast TV No No Yes
Satellite Lifetime 3-7 years 5-10 years 10-15years
Table 2.2: Comparison of several LEO, MEO,GEO systemharateristis.
(inluding oeans, airwaysand Polar regions). Itemploysa onstellation of66LEO satellites. The
ommerial servie has been launhed in Marh 2001. The Globalstar satellite system provides
teleommuniation servies suh as data network onnetivity, position loation, Short Messaging
Servie(SMS) andall forwarding to over100ountries. Theonstellation is omposedof 48LEO
satellites. Commerial serviestartedinlate 1999.
LEOandMEOsatelliteongurationsprovidealowerdelaythanGEO'sandalowerweightforeah
satellite,but aglobalsatellite onstellation isrequiredto overthelarger areas. Duetotheir lower
propagation delay, LEO and MEO ongurations are more suitable for real-time and interative
appliations. Moreover,they requireasmallerantennaand transmissionpowerthanGEOsasthey
arelosertotheearth. Onthe otherhand,GEO systemsanprovide higherdataapaityandare
oftenmore onvenient for distributing high-volume tra. Furthermore, management of theGEO
systems is less omplex as handling ommuniation handover between satellites is not required.
Table 2.2shows aomparison of LEO, MEO,GEO systemharateristis.
2.6.2 Satellite Link Charateristis
Satellitelinkshave severalharateristis thathave asigniant impaton transportprotools and
dierfrommostterrestrialhannels[64℄. Anoverviewoftheseharateristisisreportedasfollows.
Impat ontransportprotools is disussedin Setion4.2.
Error Rate. Satellite links are subjeted to several environments impairments suh as
interfer-ene,fading, multipath, rainattenuation andshadowing. Thismakessatellite hannels moreprone
to transmission errors than wired links, even though improvements in modulation and adaptive
oding tehniques help to ope with this issues and may ahieve a Bit Error Rate (BER) of the
orderof
10
−
10
inthemost reentsatellite networks. However, ordinarysatellitenetworksaremore
error-prone than terrestrial networks and theBER an be
10
−
7
on average and
10
−
4
in theworst
ase[64℄. Theerrorpatternisalsodierentthaninterrestriallinks. Errorstypiallyourinbursts,
so that time periods where the hannel is in good onditions alternate with periods where bursts
onvarious fators, suhasatmospherionditions and physial layerimplementation (modulation,
oding, interleaving tehniques) usedinthesatellitesystem.
Long PropagationDelay. Asatellitelinkhaveaninherentdelayduetothenitespeedoflight
and the altitude of the orbit utilized in the satellite system (LEO, MEO or GEO). The one-way
delay between two ground stations via a GEO satellite may range between 250-280 ms. If the
satellite linkis both the forward and the return hannel then the RTT would be at least 500 ms
plusthedelaydueto proessing, queuing delayinintermediaterouters alongthepath towardsthe
destination, propagation delay of other links in the path. A long propagation delay onstitutes a
big issue for transport protools as the sender annot rely on a quik feedbak from the reeiver
resulting in a longer time for the sender to determine whether a paket sent has been orretly
reeived or not. Additionally, a long delay together with a high link apaity results in a larger
Bandwidth-Delay Produt (BDP) than the typial one of terrestrial networks. BDP is dened as
the amount of data a protool should have "in ight" (data that has been transmitted, but not
yet aknowledged) at any one time to fully utilize the available hannel apaity [12℄. This is an
issuewithtransportprotools using smallongestion/reeive windowasthey wouldnot beable to
fully exploit the available link apaity. In LEO satellite onstellations, the propagation delay is
signiantly smaller but varies over time due to motion of satellites and hand-over proess. This
mayaet performaneof transportprotools sensitive to RTT variations.
Bandwidth Asymmetry. Link apaityinthedownlink diretionan bedierent fromthatin
uplink diretion. This an be aused by transmission power and antenna size onstraints in the
user equipment. Systems designed for broadast information distribution may be unidiretional
and reeive feedbak from the user via a non-satellite return path, for example a modem link.
Bandwidth asymmetry mayadversely aet performaneof transport protools in asethe return
hannelonstitutes abottlenekfor the ommuniation hannel [23℄.
2.6.3 Medium Aess Control Protools
In satellite systems withmultiple independent stations, radio resoures areshared among the
sta-tions or users. Hene, a mehanism is needed to ontrol aess to theresoure and maximize the
throughput on asatellite linksharedbya numberofhosts.
The satellite layer arhiteture inludes theAessLayer whose role is to provide an eient
pro-tool, alled Medium Aess Control (MAC) protoolto oordinate theaess to thesatellite link
among users. Eah multi-aess link has its own MAC mehanism. Other funtionalities of the
Aess Layer inlude: Terminal registration and authentiation (Log-on), Connetion Admission
Control (CAC), terminal synhronization and power ontrol, resoure management, poliing and
paketsheduling. MACprotools forsatellite ommuniations shouldbedesigned to provide high
hannel throughput, eient use of radio resoures, low transmission delay, reongurability and
protool salability.
The satellite environment poses major onstraints that do not allow the employment of a large
on short link latenies. On the other hand, the long propagation delay present in satellite links
deteriorates theperformane ofmost ofMACprotools designed forterrestrial links,makingthem
not suitable and requiring spei design and optimization. Several aess tehniques in satellite
systemsareoutlined below. Further details an be foundin[6,105,113℄, for example.
Random Aess. In Random Aess, eah terminal an transmit on thelink anytime, without
anyguarantee of suessfultransmission inadvane. Thisis thesimplest tehnique to implement.
Itissuitableforlighttra loadotherwiseasigniantamount ofapaityiswastedduetopaket
ollisionsamongstations. Eahollisionsausesat leastoneadditionalroundtripdelay. Therefore,
ollisions are more detrimental to performane in ase of satellite systems with long propagation
delay as in MEO and GEO systems. An example of protool using Random Aess tehnique is
thePureAlohaprotool, whihanahieveamaximumeieny of18%hannelthroughput. One
variant, namedSlotted Aloha, mayahieve amaximumthroughput of 37%[124℄.
Fixed Assignment Multiple Aess (FAMA). In xed assignment protools, link apaity
is assigned to a terminal statiallyand independently of its ativity and bandwidth requirements.
Using this tehnique, hannel assignment is stritly ontrolled and the aess to the hannel is
ompletely regulated; paket ollisions annot our. However, it is not possible to adapt and
hangeapaityassignmentstoastationinaseitsapaityrequirementshanges,withaonsequent
waste of network resoures. This is the main disadvantage of xed assignment tehniques. Ifdata
transmissioninthenetworkisburstyand/orsporadi,periodsofsilenemaybeasigniantportion
of periods oftransmissions.
Twowaystoimplement FAMAareFrequenyDivisionMultipleAess(FDMA)andTimeDivision
Multiple Aess (TDMA). In FDMA, a spei frequeny band is assigned to eah terminal and
an beused anytime without need of oordination among thestations. Figure 2.6a showshowthe
hannel bandwidth is sub-divided into
N
frequeny sub-hannels whih do not overlap with eah other. Frequeny hannels are separated by interfrequeny protetion bands to limit interferenebetween ontiguous hannels. Thistehnique issimple,but itdoesnot provideexibilityinaseof
new stations are added into the system. An advantage inFDMA is that smaller antennas an be
used.
InTDMA, atime interval
T
f
,alledframe duration,is dividedinN
subintervals,eah of durationT
f
N
(see Figure 2.6b). Eah station an transmit ina predened time slot. The station assembles datainburstsand transmitstheminsequeneduringits assignedtimeslot. TDMAhasbeenusedmore often than FDMA for data and digital voie transmissions, whih an be assembled indata
burstsandreassembledatthereeiverside. AnadvantageofTDMAisthatonly onearrierisused
at anytime,andhene intermodulationproduts resulting fromnonlinearampliation of multiple
arriers are not present [113℄. Disadvantages of TDMA are the higher antenna size and theneed
of areful time slot synhronization in eah terminal of the satellite network, whih inreases the
omplexity ofearth stations.
Code Division Multiple Aess (CDMA). This tehnique usesaspread-spetrumapproah,
Figure 2.6: Frequeny Division MultipleAess
Transmissions of signals oming from multiple terminals overlap both in frequeny and time (see
Figure2.6). Despreadingandreoveringtheoriginalsignalisperformedatreeiverside. InCDMA,
eah stationuses the same arrier frequeny and all theavailable bandwidth, but akey ode
C
i
is assigned to eah stationi
to modulate the arrier (ode-modulation) in a partiular way so that the reeiver station an reover the transmitted signal from thei
th
station by using the
C
i
key ode. During reovery with a partiular keyC
i
, signals oming from other stations are pereived very muh like random noise. CDMAtehniques provide timing exibilities,inherent resistane tointerferene, systemapaity andommuniation privay. However, theimplementation of CDMA
systems an introdue signiant omplexity. Important appliations of CDMA tehniques arein
themilitary setorasthe tehniques areinherently resistant to interferene and jamming.
Hybrid multiple aess sheme. Traditional shemes an beombinedto inreasetheoverall
exibilityandeieny of thesystemandto opewithdierent tra harateristis(sporadi or
steady)inorder to solvetheproblemof eient assignment oftheapaity. An exampleof hybrid
aessshemeis Multi-Frequeny TDMA (MF-TDMA)[4℄. MF-TDMA isa hybrid of FDMA and
TDMA tehniques, inwhih a ombination of timeslot and spei frequeny band is assigned to
eah station(bi-dimensional time/frequeny plan). MF-TDMA redues satellite antennasizes and
transmissionpower,and inreases satellite networkbandwidth [6℄.
2.6.4 Satellite Bandwidth-on-Demand
In asedata tra isburstyand unpreditable, xed alloation tehniques of thesatellite linkare
ineientandausewasteofresoures. Inordertoinreasetheoveralleienyandthroughputof
thesatellite system,MACprotools thatalloateapaityon demandto user requestsareneeded.
Bandwidth-On-Demand (BoD) mehanisms area entral omponent insatellite resoure
manage-ment. They areused by most reent satellite systems to eiently share radio resoures among a
numberofusers[123℄. BoDisinludedintheMAClayerandhandlestheaesstothelinkbetween
and a partiular HUB or gateway terminal, whih typially onnets the satellite network to the
Internet.
Inthisthesis,experimentswithapartiular broadbandsatellitesystemhavebeenarriedout. This
system omplies the Digital Video Broadast Return Channel System (DVB-RCS) standard [45℄.
BoDmehanisms inluded inthisstandard aredesribedbelow.
DemandAssignedMultipleAess(DAMA)isaBoDtehniqueusedinDVB-RCSsatellitesystems.
DAMA allows a number of users to share satellite link resoures on a demand basis. Developing
eientBoDmehanismsforsatellitelinksisadiulthallengeduetothehighpropagationdelay.
Two parameters must be taken into aount when designinga BoDmehanism:
Responsiveness: TimeperiodbetweenarequestisissuedbyaSTtothetimewhenthebandwidth
isassigned.
Lateny : Time interval between two onseutive opportunities for the terminal to demand for
bandwidth.
The total transmissiondelay on thesatellitelink isomposedof thepakettransmission time,the
propagation delay, the responsiveness and thelateny. Hene, the design shallaim at limiting the
two parameters in order to derease the total transmission delay. On the other hand, a too small
value of responsiveness and lateny may lead to a lower data throughput as a higher amount of
bandwidthwouldbe usedfor transmissionof bandwidthrequestsand assignmentsreplies.
DVB-RCS employs a ombination of TDMA and DAMA (paket DAMA). A single hannel is
dividedinto non-overlapping time intervals alledframes. Framesaresubdivided into a numberof
timeslotsof xedlength. Timeslots areusedfor datatra althougha ertainnumberof slotsis
reservedfor signaling (apaityrequests, synhronization,et). Adierent number ofslotsmaybe
assignedaordingto the raterequirements ofaertain onnetion. Thisfeature makes thepaket
DAMA tehnique exible andadaptive to various data tra requirements.
TheDVB-RCSstandard denes fourlasses of apaityassignments[45℄:
Constant Rate Assignment. In the Constant RateAssignment (CRA) lassthe alloated
a-paity is guaranteed and the terminal an utilize its alloated apaity while it is logged on to
the network. CRA requires no dynami signaling from the ST. The tra is not subjet to any
sheduling delay.
RateBasedDynamiCapaity. IntheRateBasedDynamiCapaity(RBDC)lassthe
band-widthalloationisbasedon instantaneous raterequestssent bytheST.The STan request
band-widthuptoa prexedeilingvaluethattheSTisguaranteedto get. Therequestremains eetive
until it is updated bythe ST or until it is timed out. During thetime that a bandwidth request
remains eetive, the ST tra is not subjetto any additional apaity alloation delay. In
on-trast to CRA,RBDCstrategy allows for statistial multiplexing among many terminals, resulting
ina moreeient useof the satellite bandwidth.
Figure2.7showsanexampleofrequest-assignproleforRBDC.Thetimelinesdepitrequestsand
RBDC request
MSL delay
4
2
6
No Rqst
4
0
4
2
6
6
4
0
4
2
6
6
4
0
RBDC Assignment
Figure2.7: Example of RBDCRequest- Assignprole
MSL delay
VBDC request
VBDC Assignment
Estimated Hub
VBDC Queue
(after assign)
4
2
6
No Rqst
4
0
0
5
4
0
5
2
Total = 16
Total = 16
4
1
3
3
2
0
Figure2.8: Exampleof VBDCRequest- Assignment prole
(MSL), whih is the minimum time between the time the request is issued and the beginning of
the frame where the orresponding assignment is reeived. The peuliarity of RBDC isthat if no
apaity request is sent, then the urrent apaity is maintained (until a maximum time limit)as
showed for forthrequest in the gure. A requestfor zeroRBDC apaityis required to terminate
theassignment.
Volume Based Dynami Capaity. In the Volume Based Dynami Capaity (VBDC) lass
thebandwidth is assignedin response to a request by theST. The bandwidth requestis based on
theamount of data waitingintheMACbuer. Asheduler (inthehubterminal) assignsapaity
from a queue of requests among all STs it serves, within the onstraint of the remaining apaity
after CRAand RBDCassignments. Asbandwidth is shared by a numberof terminals there is no
guaranteeontheapaityavailabilityforaspeibandwidthrequest. Hene,theassignedapaity
to a ST an be less than the requested one in ase of link ongestion (when the total of all STs
apaitydemandexeedsthetotalapaity)ormaximumallowedVBDCapaityforthatST(suh
a limit dependson the servieagreement withthe network provider). VBDCstrategy provides an
eientuseofthenetworkresouresbutenqueuedpaketsmayexperieneasigniantdelaybefore
beingtransmitted.
with the RBDC example to show the dierenes on the assignment prole. Sine link ongestion
may be present, the assignment prole typially does not follow the request prole and the HUB
may deny alloating the requested apaity by an ST. Eventually, the total amount of apaity
assignedshould math thetotal amount requested.
Free Capaity Assignment. In the Free Capaity Assignment (FCA) lass apaity that has
not been urrently alloated is assigned freelyto STs inround robinfashion. No apaity request
signalling is neessary, but no apaity guarantee is provided. CRA,RBDC and VBDC apaity
requestsaregrantedrstbeforeassigningFCAapaity. FCAintroduesjitterasitassignsapaity
sporadially to a terminal. Therefore,it is not feasible to jitter-sensitive appliations suh as
real-timeservies.
Asdesribed above,eah lassofapaityassignment presentsadvantages anddisadvantages. The
hoieofwhihlassamongCRA,RBDCandVBDCshallbeusedforaertainappliationdepends
onthetra proleofthe appliation,itsQoSrequirementsandtheost ofprovision ofeahlass.
CRA apaity isthe most expensive as apaity isstatially alloated to an user, whereas VBDC
is the heapest. Eah apaity assignment tehnique has a strong impat on transport protools
performane.
TherehasbeenrelativelylittleresearhontheinterationsbetweenBoDmehanismsandtransport
performane. One of our objetives is to analyze possible interations between dynami resoure
management and transportprotools. An analysisonhowtheRound-Trip-Time (RTT)variations,
whenusingeahofthe apaityassignmentstehniquesdesribed above,impata TCPonnetion
Link Layer Protools
This hapter disusses link layer protools and error ontrol shemes for networks. Two basi
ategoriesoferrorontrolmethodstoimprovetheapparent qualityofaommuniationhannelare
desribed: Forward Error Corretion(FEC) andAutomati RepeatRequest(ARQ).
3.1 Overview of Funtionality in Data Link Layer Protools
A datalinklayerprotool provides adened servieinterfae to the network layer. It handles the
ommuniation between neighbor nodes by performing spei funtions suh as frame
synhro-nization, error ontrol, ow ontrol, addressing, aess ontrol, linkmanagement. Frame
synhro-nizationisrelated todetermining howthestream ofbits providedbytheunderlying physial layer
aregrouped into framesand where thebeginning and theend ofeah frame isloated. Error
on-trolrefersto mehanismstodetet andorrettransmissionerrorsourringontheommuniation
hannel. Thefuntionofowontrolistoregulatetheowofframestransmittedonthelinksothat
a slow reeiver isnot swamped bya faster sender. Addressing funtionshandle theidentity ofthe
stations involved inthe transmissionon aommuniation hannel sharedbymore than two nodes.
Aessontrolregulatestheaesstothelinkamongtheonnetednodes. Linkmanagementopes
withtheinitiation, maintenane ant termination ofthedataexhange.
Data link layer funtionalities are implemented with spei protools. Figure 3.1 depits the
struture of a generi data link layer for a link between two nodes sharing the link also with
other stations. The link layer runs on top of the physial layer. Two sub-layers an be logially
distinguished inMediumAessControl (MAC) and LogialLinkControl (LLC).
The purpose of Medium Aess Control (MAC) is to regulate aess to the shared link. MAC
protools implement methodsto regulate thetransmissionofdataand avoidollisions. Inorder to
identify uniquely the destination node of thetransmission, theMACappends thephysial address
(MAC address)of the destination to the frame 1
. Every node on the network must have aunique
MAC address to ensure proper data transmission and reeption. MAC ahieves the unreliable
1
Physical Layer
Data Link Layer
Network Layer
Physical Layer
Data Link Layer
Network Layer
LLC
MAC
No.
Data
CRC
No.
Data
CRC
Addr
Data
link
A
B
Figure3.1: Strutureof ageneridatalinklayeronnetingtwo nodes
transmissionofframesandtheframesynhronization. ItspeiestheaddressshemeandtheMAC
protool for performing aessontroland linkmanagement funtionalities.
TheLogialLinkControl(LLC)handlesfuntionsoferrordetetionandorretion. Thisisahieved
byaddingtwoadditionaleldstotheframe: asequenenumberanderror-detetionbits(e.g. CRC).
The frame sequene number is used by the reeiver to determine where one or more frames have
not been reeived. Error-detetion bits areusedtodetermine whetherthereeived frameisorret
or orrupted. If the linklayerperformsretransmissions, the sequene number isused todetermine
whih paket needs to be retransmitted. This method is known as Automati Repeat Request
(ARQ). Redundanybitsmay alsobe appended tothe frameinorderto tryto reoveraorrupted
frame without the need of retransmissions. Thistehnique is known asForward Error Corretion
(FEC). In addition to error ontrol funtionalities, LLC implements ow ontrol between sender
and reeiver to avoidthat the reeiverbuer is swamped bya too fastsender ausing some ofthe
reeivedframesbeingdropped. Flowontrolisimplementedwithaspeiprotoolwhihprovides
feedbak to the sender about its buer status, so that the sender an regulate the rate at whih
transmit data.
LLC maysupport three typesof onnetion servies for handling thetransmission: 1)
unaknowl-edged onnetionless servie where no error ontrol is performed; 2) onnetion-oriented servie
where a onnetion between nodes must be established before bloks of data an be transferred
until theonnetionislosed;3) aknowledgedonnetionlessserviewhere dataframesanbe
a-knowledged. Aninterestedreadermayndmoreinformationaboutlinklayerprotoolsin[124,126℄.
3.2 Forward Error Corretion
AForwardErrorCorretion(FEC)shemeonsistsofaddingredundantdatatotheoriginalmessage
to be transferred (oding phase). At the reeiver side, the redundant part of the message is used