What is Network Latency and Why Does It Matter? · PDF fileFour key causes of latency are: propagation delay, serialization, data protocols, routing and switching, and queuing and

www.o3bnetworks.com 1

Contents

ThispaperispresentedbyO3bNetworkstoprovideclarityandunderstandingofacommonlymisunderstoodfacetofdatacommunicationsknownaslatency.ItisourgoalthatthereadertakesawayaclearunderstandingoflatencyandgainsinsightintothesignificantadvantagethatO3bNetworksbringstothesatellitecommunicationsmarketforthedevelopingworld.

Executive Summary 2

What Causes Latency? 2

Transmission Rate and Bandwidth 4

Latency and TCP/IP 6

Satellite Link Latencies 7

Applications 10

Summary 10

Appendix 11

What is latency? 11

Networking 101 11

What is Network Latencyand Why Does It Matter? by O3b Networks


Executive Summary

Internetdataispackagedandtransportedinsmallpiecesofdata.Theflowofthesesmallpiecesofdatadirectlyaffectsauser’sinternetexperience.Whendatapacketsarriveinasmoothandtimelymannertheuserseesacontinuousflowofdata;ifdatapacketsarrivewithlargeandvariabledelaysbetweenpacketstheuser’sexperienceisdegraded.

Latencyandoverallthroughputisdominatedbytwofactors,thelengthoftheroutethatthepacketshavetotakebetweensenderandreceiverandtheinteractionbetweentheTCPreliabilityandcongestioncontrolprotocols.

ThispaperwilladdressthesourcesofdelaystointernetdataanddescribewhyO3bNetworkssatelliteconstellationhaslowerpathlengthandlatencythantraditionalgeosynchronoussatellitesandinsomecasesfiberbasednetworks.

What Causes Latency?

Regardlessofthespeedoftheprocessorortheefficiencyofthesoftware,ittakesafiniteamountoftimetomanipulateandpresentdata.Whethertheapplicationisawebpageshowingthelatestnewsoralivecamerashotshowingatrafficjam,therearemanywaysinwhichanapplicationcanbeaffectedbylatency.Fourkeycausesoflatencyare:propagationdelay,serialization,dataprotocols,routingandswitching,andqueuingandbuffing.

Propagation Delay

Propagationdelayistheprimarysourceoflatency.Itisafunctionofhowlongittakesinformationtotravelatthespeedoflightinthecommunicationsmediafromsourcetodestination.Infreespace,thespeedoflightisapproximately3x105km/sec.Thespeedoflightislowerinothermediasuchascopperwireorfiberopticcable.Theamountofslowingcausedbythistypeoftransmissioniscalledthevelocityfactor(VF).

Mostpeoplearesurprisedtolearnthatcopperwireandfiber-opticcableshavesimilarvelocityfactors.Fiberopticcablestypicallymeasurearound70%ofthespeedoflightwhereascoppercablevariesfrom40%to80%dependingontheconstruct.CoaxialcableiscommonlyusedandmanytypeshaveaVFof66%.

Satellitecommunicationlinksuseelectromagneticwavestopropagateinformationthroughtheatmosphereandspace.Theinformationisconvertedfromelectricalsignalstoradiosignalsbythetransmitterandtheantenna.Oncetheseradiosignalsleavetheantenna,theytravelapproximatelyatthespeedoflightforfreespace.

Let’scalculatehowlongitwilltakeanemailtotravelfromNewYorktoLondonassumingthatwearetheonlyuseronaprivatecommunicationschannel.

What is Network Latency and Why Does It Matter?


Ignoringtheactualroutestakenbyunderseacablesduetotheocean’sfloor,let’sassumethepathfromNewYorktoLondonisthegreatcircledistanceof5458km.

Propagation delay = distance/speed:

The email sent using a copper link: 5458 / 197863.022 = 23.58 msThe email sent using a fiber-optic link: 5458 / 209854.720 = 26.01 msThe email sent using a radio link: 5458 / 299792.458 = 18.21 ms

Thesearethelatenciescausedonlybypropagationdelaysinthetransmissionmedium.Ifyouweretheonlyonesendingonesingledatabitandyouhadunlimitedbandwidthavailable,thespeedofthepacketwouldstillbedelayedbythepropagationdelay.

Thisdelayhappenswithoutregardfortheamountofdatabeingtransmitted,thetransmissionrate,theprotocolbeingusedoranylinkimpairment.

Serialization Delay

Serializationistheconversionofbytes(8bits)ofdatastoredinacomputer’smemoryintoaserialbitstreamtobetransmittedoverthecommunicationsmedia.Serializationtakesafiniteamountoftimeandiscalculatedasfollows:

Serializationdelay=packetsizeinbits/transmissionrateinbitspersecond

For example:

- Serialization of a 1500 byte packet used on a 56K modem link will take 214 milliseconds

- Serialization of the same 1500 byte packet on a 100 Mbps LAN will take 120 microseconds

Serializationcanrepresentasignificantdelayonlinksthatoperatealowertransmissionrates,butformostlinksthisdelayisatinyfractionoftheoveralllatencywhencomparedtotheothercontributors.

Voiceandvideodatastreamsgenerallyusesmallpacketsizes(~20msofdata)tominimizetheimpactofserializationdelay.

Data Protocols and Latency

Datacommunicationsprotocolsatvariouslayersintheprotocolstackusehandshakessynchronizethetransmitterandreceiver,forthetransmitterandreceivertoupdateeachotheronlinkstatusandtocorrectforerrosintransmission.Thesehandshakestaketimetopropagateacrossthelinkandcanaddtothelatencyofthetransmissionofinformationfromsourcetodestination.



Routing and Switching Latencies

InIPnetworkssuchastheInternet,IPpacketsareforwardedfromsourcetodestinationthroughaseriesofIProutersorswitchesthatcontinuouslyupdatetheirdecisionaboutwhichnextrouteristhebestonetogetthepackettoitsdestination.Arouterorcircuitoutageorcongestiononalinkalongthepathcanchangetheroutingpathwhichinturncanaffectthelatency.

HighperformanceIProutersandswitchesaddapproximately200microsecondsoflatencytothelinkduetopacketprocessing.IfweassumethattheaverageIPbackbonerouterspacingis800km,the200microsecondsofrouting/switchingdelayisequivalenttotheamountoflatencyinducedby40kmoffiber;routing/switchinglatencycontributestoonly5%oftheendtoenddelayfortheaverageinternetlink.

Queuing and Buffer Management

Anotherissuewhichoccurswithinthetransportlayersiscalled“queuinglatency”.ThisreferstotheamountoftimeanIPpacketspendssittinginaqueueawaitingtransmissionduetoover-utilizationoftheoutgoinglinkaftertherouting/switchingdelayhasbeenaccountedfor.Thiscanadduptoanadditional20msoflatency.

Transmission Rate and Bandwidth

TransmissionRateisatermusedtodescribethenumberofbitswhichcanbeextractedfromthemedium.Transmissionrateiscommonlymeasuredasthenumberofbitsmeasuredoveraperiodofonesecond.

The“maximumtransmissionrate”describesthefundamentallimitationofanetworkmedium:

IfthemediumisacopperLocalAreaNetwork,maximumtransmissionratesarecommonly10,100,or1000Megabitspersecond.Theseratesareprimarilylimitedbythepropertiesofthecopperwiresandthecapabilitiesofthenetworkinterfacecardarealsoafactor.

Fiber-opticthetransmissionratesrangefromaround50Mbpsupto100Gbps.Unlikecoppernetworks,theprimaryfactorlimitingfiber-optictransmissionratesistheelectronicswhichoperatesateachendofthefiber.

Wirelesslocalareanetworks(LANs)andsatellitelinksusemodems(modulator/demodulator)toconvertdigitalbitsintoananalogmodulatedwaveformatthetransmitterendofalink,andthenatthereceiveendademodulatorwillthenconverttheanalogsignalbackintodigitalbits.Thelimitingfactorintransmittinginformationoverradio-basedchannelsisthebandwidthofthechannelthatisavailabletoaparticularsignalandthenoisethatispresentthatwillcorruptthesignalwaveform.



Radio Channel Bandwidth and Noise

Signalstransmittedusingradiowavesoccupyradiospectrum.Radiospectrumisnotanunlimitedresourceandmustbeshared.Topreventradiointerferencebetweenuserstheuseofradiospectrumiscontrolledbynearlyeverygovernmentontheplanet.Theamountofradiospectrumoccupiedbyanygivenradiosignaliscalleditsbandwidth.

Thenatureofradiospectrumuseisbeyondthispaperbutit’simportanttounderstandthatgenerallytheoccupiedradiospectrumofamodemsignalwillincreasewiththedatarate:

- Higher modem data rates cause the modem to occupy more radio bandwidth

- Lower modem data rates will let the modem occupy less radio bandwidth

Sinceradiospectrumisalimitedresource,theoccupiedradiobandwidthisanimportantlimitingfactorinwirelessandsatellitelinks.

Noiseintheradiochannelwillperturbtheanalogsignalwaveformandcancausethedemodulatoratthereceivertochangeadigitaloneintoazeroorviceversus.Theeffectofnoisecanbeovercomebyincreasingthepowerlevelofthetransmittedsignal,orbyaddingafewextraerrorcorrectingbitstothedatathatisbeingtransmitted.Theseerrorcorrectingbitshelpthereceivercorrectbiterrors.However,theerrorcorrectionbitsincreasethebandwidththatisrequired.

Data Bandwidth

Indatatransmission,thedatabandwidthissynonymoustothetransmissionratebeingused.Bandwidthisimportantbecauseitdefinesthemaximumcapacityofadatalink.

- A 10 Mbps copper LAN cannot sustain traffic flowing at a higher rate than 10 megabits every second.

- A satellite link using modems operating at a 600 Mbps rate cannot flow any more than 600 megabits every second.

It’sveryimportanttounderstandthatdatabandwidthisamaximumdataflowobtainableoveragiventransportationsegmentoveragivenperiodoftime.



Latency and TCP/IP

ThefinaldetailrequiredforunderstandingwhylatencyisimportantisanunderstandingofwhatisgoingonattheTransportprotocollayer.TheTransportlayersitsbetweentheapplication-specificApplicationlayerandtheIPlayerthatisusedforpacketrouting.TheTransportlayerperformsanumberoffunctionsincludingsequencingthepacketsintotheirproperorder.

There are two types of Transport layer protocols:

- connectionless - connection-oriented

Connectionlessdataprotocolssimplypushthedatapacketstotheenddestinationwithoutregardforpacketlossorpacketorder.Connectionlessprotocolsareoftenusedforreal-timecommunicationssuchasvoiceorinteractivevideowhereretransmissionoflostpacketsisnotusefulbecausetheywillarrivetoolatetobeplayedout.IntheInternet,themostcommonconnectionlessTransportlayerprotocolistheUserDatagramProtocol(UDP).

Connection-orientedprotocolsaremorecomplex.Theseprotocolsrelyontheestablishmentofa“connection”whichrequireshandshakesandinteractionbetweenthesourceanddestinationcomputers.Connection-orientedTransportprotocolsprovideerrorfreesequenceddeliveryofpackets.Themostcommonconnection-orientedTransportprotocolusedintheInternetistheTransmissionControlProtocol(TCP).IfpacketsarriveoutofordertheTCPlayerputsthembackinorder.Ifpacketsaremissingorcorruptedintransmission,TCPusesanacknowledgement(destinationbacktosourcefeedback)mechanismtoaskforthemissingorcorruptedpackettoberetransmitted.TheTCPprotocolalsomeasurestheperformanceofthelinkintermsofavailablebandwidth,delayanderrorrate.Usingawindowingtechnique,itadjuststhetransmissionrateofdatapacketstomatchtheabilityofthechannelbetweensourceanddestinationtoacceptthemanddeliverthemcorrectly.AstheTCPprotocolatthesourceanddestinationdeterminethatthechannelisoperatingefficientlywithlowdelay,itwillopenthewindowtoallowmorepacketstobeoutstanding(inflightbetweenthesourceanddestinationandnotyetacknowledged).Thishastheeffectofincreasingthetransmissionrate.Aserrors,congestion,ordelayincreases,theTCPprotocolwillclosedownthewindowtoslowdownthetransmissionrate.Becauseofthehighdelaysingeosynchronoussatellitelinks,theTCPprotocolactstoreducetheoverallthroughputofthelink.Tocompensateforthis,satellitelinkequipmentvendorshavedevelopedTCPspoofingmechanismsthatfaketheTCPhandshakebyterminatingtheTCPlinklocallytothetransmittingsiteandtrytoconvincethetransmittingsitethatthelinkhaslowerdelaythanitreallyhas.Thisspoofingworksifthelinkisrelativelyuncongestedandhasaverylowerrorrate,whichisnotnecessarilythecaseforsatellitelinks.



Satellite Link Latencies

Satellitelinkscanintroducelargerlatenciesthanmostterrestrialnetworkingsegmentsduetolongdistancesfromthegroundstationstothesatellite.Thefollowingtableshowsthelatencycausedbypropagationdelaysfromtwotypesofsatelliteconfigurations,1)theO3bNetworksMEOorbitconstellationatanaltitudeof8063Kilometers,2)ageosynchronoussatelliteat35,786Kilometers.

Altitudeiskeydeterminateinlatencybecausethehigherthesatellitethegreaterthelatency.

Forsatelliteswhichoperateasabent-pipe,thepropagationdelaysaredoubledsincethesignalhastotravelbothuptothesatelliteandbackdowntotheearthbeforeitreachesthenextsegmentofthenetwork.

ThefollowingtablecomparestwotypesofsatellitesusingagroundstationinLagos,Nigeria.


Latency Calculations – London Internet Exchange (LINX) to Lagos

Gateway to Satellite km

Customer to Satellite km

Gateway to Satellite msec

Customer to Satellite msec

Total Sat RTT msec

Fiber to LINX km

Fibre RTT msec

Total Sat RTT msec

O3b w/8 SatellitesSpain GWY Lagos

10126 8135 33.8 27.1 121.7 2144 10.7 132.5

GeosyndchronousSatUK Teleport Lagos

38,469 35,847 128.2 119.5 495.4 135 0.7 496.1

Fiber

London Lagos km

Total Fiber RTT

8,000 80.0

Satellite Latency Calculations

TheO3bNetworksMEOsatelliteusingtoateleportinSpainwillexperienceroundtriptime(RTT)ofapproximately122msec.IfweaddintheaverageInternetlatencyfromtheSpainteleporttotheLondonInternetExchange(LINX)(~10msec.),weendupwithanoveralllatencyfromLagostoaEuropeaninternetsiteof132.5msec.O3b’s132.5msec.roundtriplatencyisclosetofiber’s80msec.RTT

Bycomparison,thesameLagoscustomersiteusingageosynchronoussatellitetoaUKTeleportandthenontoLINXwillhavelatenciesof496msec.

ToactuallyretrieveawebpagefromanInternetserveratLINX,willtakesomeadditionaltimeforthedatarequestpacketsandtheiracknowledgments:



This data request time includes:

- Therequestpacketfromtheusertothewebserver- Thewebserveracknowledgingtherequest- Thewebserverpushingtorequesteddatatotheuser- Thedataarrivingattheuser’scomputer

Geosynchronoussatelliteusersmustwaitalmost750msec.beforetheystartgettingdata,whereasthelowerlatencyO3bsatellitelinkwillreceiveitnearly4xsooner.

Anotherproblemwithhighlatencylinksistheintroductionofalargeamountofinter-packetjitterduetotheTCPprotocolflowcontroloperatinglessefficientlyoverlongerdelaypaths.ThefollowingplotsweremadeusingaTCP/IPpacketcaptureutility.Theseplotsshowthepacketbitrateonthey-axisandtimeofdayonthex-axis.Thedatabeingtransmittedwastheun-cachedweb-pagere-loadofthecontentfromtheCNNwebpage(http://www.cnn.com).

Theonlyconditionchangedduringwasthedelaybetweenpackets–thetransmissionrateremainedthesame.


TheaddednetworklatencyanditaffectontheflowofTCPdataspreadthewebpageloadovertime.

• The 50 ms latency link took 3 seconds• The 150 ms latency link took 5 seconds• The 300 ms latency link took 11 seconds• The 600 ms latency link tool 17 seconds

Thespreadingofnetworkdataovertimereduceswhat’scalledthe“EffectiveBandwidth”ofalink.Packetsarestillbeingtransportedatthesamebitratebutduetolatencyitistakingmuchmoretimeforalloftheweb-pagepacketstoarrive.

It’sthis“spreadingovertime”behaviorofhighlatencynetworkswhichbecomesnoticeabletotheuserandcreatestheimpressionthatalinkisnotoperatingatahighspeed.

O3brecentlyconductedanotherdemonstrationofreal-worldeffectsoflatencyusingthetimetoloadawebpage.Thisisaverycommonactivityandclearlyshowsusersthatlatencydirectlyaffectsthewayauserobtainsdatafromtheinternet.

ThefollowingplotsshowtheeffectsoflatencyonthetimetoloadtheWallStreetJournalwebpage(http://www.wsj.com):

Web Page Load Time: wsj.com to Florida

Itisclearthatwebpageloadtimesdramaticallyincreasewhenlatencyincreases.Inthiscasedoublingthelatencynearlydoubledthewebpageloadtimes

Whenlookingatthebasiclatencynumbers,it’seasytoseethattheO3bSatelliteconstellationwillofferusersanoticeablybetterinternetexperiencewithmoreimmediatefeedbackandquickeraccesstodata.



Applications

Whatapplicationscannowberunoverlowerorbitsatellitesthatwerepossibleafewyearsago?

Manyapplications,suchasbasicwebsitedownloads,Internetvideostreamingthatoperatedsluggishly(orwereveryslowtostartup)overgeosynchronoussatellitelinksnowhavemuchfasterresponseandworkmuchbetteroverO3b’slowlatencyconnections.Videoconferencingandregularvoicecallsovergeosynchronoussatelliteswereverystiltedandnon-interactive.Conversationswere“half-duplex”andonepartyhadtowaitfortheotherpartytofinishtalkingbeforetheycouldstartup.Youcouldnotbreakintoaconversationandinterrupt.VoicecallsandvideoconferencingaremuchmorenaturalandinteractiveoverO3b’slinks.Interactivegamingplayerswhowereontheendofageosynchronoussatellitelinkwereoftenkilledbeforetheycouldgetashotoff.Nowtheycanbefullparticipantsintheaction.

Manybusinessapplicationssuchasremotedatabaseaccessandinteractivetransactionprocessingcontainsoftwaretimersthatdonotadjustforlongdelaysovergeosynchronoussatellites.Examplesincludemanybankingapplicationsandairlinereservationandschedulingapplications.Nowtheseapplicationswillworkforusersthatareinremoteunderservedlocations.

Summary

WehavedescribedthestructureofIP-basedpacketswitchednetworks,thefunctionsofthevariousprotocollayers,andthecausesoflatencyinpacketswitcheddatanetworks,suchastheInternet.Latencyandoverallthroughputisdominatedbytwofactors,thelengthoftheroutethatthepacketshavetotakebetweensenderandreceiverandtheinteractionbetweentheTCPreliabilityandcongestioncontrolprotocolsandthispathlength.O3bNetworkssatelliteconstellationinamuchlowerMEOorbithassignificantlylowerpathlengthandthereforesignificantlylowerlatencythantraditionalgeosynchronoussatellites.O3b’snetworklatencyandthroughputapproximatethatoffiber-basedterrestrialnetworksandpermitusersinremotelocationsthatarenotservedbyfibertorunmanyapplicationsthatonlyworkoverfiber.



Appendix:

What is latency?

Definition of Latency from Wikipedia:

Latencyisatimedelaybetweenthemomentsomethingisinitiated,andthemomentoneofitseffectsbeginsorbecomesdetectable.Thewordderivesfromthefactthatduringtheperiodoflatencytheeffectsofanactionarelatent,meaning“potential”or“notyetobserved”.

Mostpeopleunderstandthatittakestimeforwebpagestoloadandforemailstogetfromyouroutboxtothedestinationinboxandyes,thisisaformoflatency.Butinordertounderstandwhythishappensweneedtothinkaboutlatencyatalowerlevel:

Latencyisatimedelayimpartedbyeachelementinvolvedinthetransmissionofdata.

Theremainderofthispaperwilldiscussthespecificmechanismsinvolvedandhowlatencylimitstheperformanceofadatacommunicationslink.

Networking 101

It’simportantforthereadertounderstandtheverybasicelementsofnetworkingtoproperlygraspthelatencyissue.

Earlynetworkingengineersanticipatedtheneedbeabletohandlethousandstomillionsofusersononecohesivenetwork,andthustheTCP/IPnetworkingmodelwasdeveloped.

ThekeydesignfeatureoftheTCP/IPnetworkingmodelistheconceptofencapsulationwhichistheideaoftakingdataandwrappingitinacommoncontainerforshipping.ThecontainerthatwasdevelopediscalledtheIPDatagram,alsoknownasan“IPPacket”.

TheIPPacketisaverysimplething:aheader,followedbydata.

TheHeadercontainsinformationusedforroutingthepackettothedestination.Thedatacanbeanyinformationwhichneedstobetransportedsuchasasnippetofstreamingmusicoraportionofemailtraffic.TheexactconstructofthedataportionofanIPPacketisdefinedbythedataprotocolthatisbeingcarried.Dataprotocolswillbediscussedlater.

Tounderstandexactlywherelatencyoccurs,it’svaluabletoknowhowthismostbasicunitofnetworkingdataisbuiltandtransported.ForthisweturntotheOSIModel:



The OSI Model

TheOSImodelwascreatedtodescribetheprocessofturningyourapplicationdataintosomethingthatcanbetransportedontheinternet.

TheupperlayersoftheOSImodeldescribethingsthathappenwithintheapplicationsthatarerunningonthecomputer.Theseapplicationsarewebbrowsers,emailprograms,etc.

Thelowerlayersarewhereinformationtoandfromapplicationsareturnedintodatafortransportonanetwork.Thisiswheredataencapsulationoccursandourbasicnetworkingdataelement–theIPDatagramor“packet”isbuilt.

Thefollowingdiagramshowstheencapsulationprocessinwhat’sknownastheTCP/IPStack.ThepreciseworkingsoftheTCP/IPstackcanbedifferentbetweenvariouscomputeroperatingsystems.Thesedifferencesmayseemtrivialaslongastheprotocolsareimplementedproperlybutwhenseekingtheabsolutehighestlevelsofperformanceit’simportanttoknowthatthenetworkstackimplementationcanbeasignificantcauseofnetworkingperformancevariability.


Application LayerMessageformat,Human-MachineInterfaces

Presentation LayerCodinginto1sand0s;encryption,compression

Session LayerAuthentication,permissions,sessionrestoration

Transport LayerEnd-to-enderrorcontrol

Network LayerNetworkaddressing;routingorswitching

Data Link LayerErrordetection,flowcontrolonphysicallink

Physical LayerBitstream:physicalmedium,methodofrepresentingbits

Tran

spor

t se

rvic

eU

pper

Lay

ers 7

6

5

4

3

2

1


Upper Layers

Upper Layers

Upper Layer Message

Upper Layer

Headers

Upper Layer

Headers

TCP /UDPHeader

Upper Layer(Application) Data


TCP/UDP Message

Upper Layer

Headers

TCP /UDPHeader

IPHeader


IP Datagram

Upper Layer

Headers

TCP/UDPHeader

IPHeader

Layer 2Header

Layer 2Header


Layer 2 Frame

1 0 0 1 0 1 1 0 1 1 0 0 1 0 1 1

TCP /UDP

IP

Layer 2

Layer 1

TCP /UDP

IP

Layer 2

Layer 1

The TCP/IP Stack

Thetransportofnetworkdataisathreestepprocess:

1. Data from a source application is passed down through the stack. During this process the application data is wrapped into IP Datagrams which are commonly called “packets”. Packets are then transmitted by the sending computer in the network

2. Packets are passed along the network (purple line) until they reach the destination computer

3. Packets are received from the network by the destination computer and are passed up through the stack. During this process the application data is extracted and the passed along to the destination application.

TheadditionalencapsulationatLayer2iscalledframing.ThisisthestagewheretheIPDatagramisturnedintobitswhichareappropriateforaparticulartypeofnetwork.

Layer1isthephysicalnetworkmediumconnection.Thislayerhandlestheconversionofthelayer2bitsintoelectrical,optical,orradiosignalsthatcanbetransported.Thenetworkinterface,oftencalledtheNICorNetworkInterfaceCard,canbefiber-optic,copperwire,orawirelessradiointerface.


Documents

What is Network Latency and Why Does It Matter? · PDF fileFour key causes of latency are: propagation delay, serialization, data protocols, routing and switching, and queuing and