white paper_latency matters.pdf
TRANSCRIPT
7/27/2019 white paper_latency matters.pdf
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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
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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.Fiberopticcablestypicallymeasurearound
70%ofthespeedoflightwhereascoppercablevariesfrom40%to80%dependingontheconstruct.CoaxialcableiscommonlyusedandmanytypeshaveaVFof66%.
Satellitecommunicationlinksuseelectromagneticwavestopropagateinformationthroughtheatmosphereandspace.Theinformationisconvertedfromelectricalsignalstoradiosignalsbythetransmitterandtheantenna.Oncetheseradiosignalsleavetheantenna,theytravelapproximatelyatthespeedoflightforfreespace.
Let’scalculatehowlongitwilltakeanemailtotravelfromNewYorktoLondonassumingthatwearetheonlyuseronaprivatecommunicationschannel.
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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 willtake 214 milliseconds
- Serialization of the same 1500 byte packet on a 100 Mbps LAN willtake 120 microseconds
Serializationcanrepresentasignificantdelayonlinksthatoperatealowertransmissionrates,butformostlinksthisdelayisatinyfractionoftheoveralllatencywhencomparedtotheothercontributors.
Voiceandvideodatastreamsgenerallyusesmallpacketsizes(~20msofdata)tominimizetheimpactofserializationdelay.
Data Protocols and Latency
Datacommunicationsprotocolsatvariouslayersintheprotocolstackusehandshakessynchronizethetransmitterandreceiver,forthetransmitterandreceivertoupdateeachotheronlinkstatusandtocorrectforerrosintransmission.Thesehandshakestaketimetopropagateacrossthelinkandcanaddtothelatencyofthetransmissionofinformationfromsourcetodestination.
What is Network Latency and Why Does It Matter?
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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.Transmissionrateiscommonlymeasuredasthe
numberofbitsmeasuredoveraperiodofonesecond.The“maximumtransmissionrate”describesthefundamentallimitationofanetworkmedium:
IfthemediumisacopperLocalAreaNetwork,maximumtransmissionratesarecommonly10,100,or1000Megabitspersecond.Theseratesareprimarilylimitedbythepropertiesofthecopperwiresandthecapabilitiesofthenetworkinterfacecardarealsoafactor.
Fiber-opticthetransmissionratesrangefromaround50Mbpsupto100Gbps.Unlikecoppernetworks,theprimaryfactorlimitingfiber-optic
transmissionratesistheelectronicswhichoperatesateachendofthefiber.
Wirelesslocalareanetworks(LANs)andsatellitelinksusemodems(modulator/demodulator)toconvertdigitalbitsintoananalogmodulatedwaveformatthetransmitterendofalink,andthenatthereceiveendademodulatorwillthenconverttheanalogsignalbackintodigitalbits.Thelimitingfactorintransmittinginformationoverradio-basedchannelsisthebandwidthofthechannelthatisavailabletoaparticularsignalandthenoisethatispresentthatwillcorruptthesignalwaveform.
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Radio Channel Bandwidth and Noise
Signalstransmittedusingradiowavesoccupyradiospectrum.Radiospectrumisnotanunlimitedresourceandmustbeshared.Topreventradiointerferencebetweenuserstheuseofradiospectrumiscontrolledbynearlyeverygovernmentontheplanet.Theamountofradiospectrumoccupiedbyanygivenradiosignaliscalleditsbandwidth.
Thenatureofradiospectrumuseisbeyondthispaperbutit’simportanttounderstandthatgenerallytheoccupiedradiospectrumofamodemsignalwillincreasewiththedatarate:
- Higher modem data rates cause the modem to occupy moreradio bandwidth
- Lower modem data rates will let the modem occupy lessradio 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 ratethan 10 megabits every second.
- A satellite link using modems operating at a 600 Mbps rate cannotflow any more than 600 megabits every second.
It’sveryimportanttounderstandthatdatabandwidthisamaximumdataflowobtainableoveragiventransportationsegmentoveragivenperiodoftime.
What is Network Latency and Why Does It Matter?
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Latency and TCP/IP
ThefinaldetailrequiredforunderstandingwhylatencyisimportantisanunderstandingofwhatisgoingonattheTransportprotocollayer.The
Transportlayersitsbetweentheapplication-specificApplicationlayerandtheIPlayerthatisusedforpacketrouting.TheTransportlayerperformsanumberoffunctionsincludingsequencingthepacketsintotheirproperorder.
There are two types of Transport layer protocols:
- connectionless- connection-oriented
Connectionlessdataprotocolssimplypushthedatapacketstotheenddestinationwithoutregardforpacketlossorpacketorder.Connectionlessprotocolsareoftenusedforreal-timecommunicationssuchasvoiceor
interactivevideowhereretransmissionoflostpacketsisnotusefulbecausetheywillarrivetoolatetobeplayedout.IntheInternet,themostcommonconnectionlessTransportlayerprotocolistheUserDatagramProtocol(UDP).
Connection-orientedprotocolsaremorecomplex.Theseprotocolsrelyontheestablishmentofa“connection”whichrequireshandshakesandinteractionbetweenthesourceanddestinationcomputers.Connection-orientedTransportprotocolsprovideerrorfreesequenceddeliveryofpackets.Themostcommonconnection-orientedTransportprotocolusedintheInternetistheTransmissionControlProtocol(TCP).IfpacketsarriveoutofordertheTCPlayerputsthembackinorder.Ifpacketsaremissingorcorruptedin
transmission,TCPusesanacknowledgement(destinationbacktosourcefeedback)mechanismtoaskforthemissingorcorruptedpackettoberetransmitted.TheTCPprotocolalsomeasurestheperformanceofthelinkintermsofavailablebandwidth,delayanderrorrate.Usingawindowingtechnique,itadjuststhetransmissionrateofdatapacketstomatchtheabilityofthechannelbetweensourceanddestinationtoacceptthemanddeliverthemcorrectly.AstheTCPprotocolatthesourceanddestinationdeterminethatthechannelisoperatingefficientlywithlowdelay,itwillopenthewindowtoallowmorepacketstobeoutstanding(inflightbetweenthesourceanddestinationandnotyetacknowledged).Thishastheeffectofincreasingthetransmissionrate.Aserrors,congestion,ordelayincreases,
theTCPprotocolwillclosedownthewindowtoslowdownthetransmissionrate.Becauseofthehighdelaysingeosynchronoussatellitelinks,theTCPprotocolactstoreducetheoverallthroughputofthelink.Tocompensateforthis,satellitelinkequipmentvendorshavedevelopedTCPspoofingmechanismsthatfaketheTCPhandshakebyterminatingtheTCPlinklocallytothetransmittingsiteandtrytoconvincethetransmittingsitethatthelinkhaslowerdelaythanitreallyhas.Thisspoofingworksifthelinkisrelativelyuncongestedandhasaverylowerrorrate,whichisnotnecessarilythecaseforsatellitelinks.
What is Network Latency and Why Does It Matter?
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Satellite Link Latencies
Satellitelinkscanintroducelargerlatenciesthanmostterrestrialnetworkingsegmentsduetolongdistancesfromthegroundstationstothesatellite.The
followingtableshowsthelatencycausedbypropagationdelaysfromtwotypesofsatelliteconfigurations,1)theO3bNetworksMEOorbitconstellationatanaltitudeof8063Kilometers,2)ageosynchronoussatelliteat35,786Kilometers.
Altitudeiskeydeterminateinlatencybecausethehigherthesatellitethegreaterthelatency.
Forsatelliteswhichoperateasabent-pipe,thepropagationdelaysaredoubledsincethesignalhastotravelbothuptothesatelliteandbackdowntotheearthbeforeitreachesthenextsegmentofthenetwork.
Thefollowingtablecomparestwotypesofsatellitesusingagroundstationin
Lagos,Nigeria.
What is Network Latency and Why Does It Matter?
Latency Calculations – London Internet Exchange (LINX) to Lagos
GatewaytoSatellitekm
CustomertoSatellitekm
GatewaytoSatellitemsec
CustomertoSatellitemsec
Total SatRTTmsec
Fiberto LINXkm
FibreRTTmsec
Total SatRTTmsec
O3b w/8 SatellitesSpainGWY Lagos
10126 8135 33.8 27.1 121.7 2144 10.7 132.5
GeosyndchronousSatUKTeleport Lagos
38,469 35,847 128.2 119.5 495.4 135 0.7 496.1
Fiber
LondonLagoskm
TotalFiberRTT
8,000 80.0
Satellite Latency Calculations
TheO3bNetworksMEOsatelliteusingtoateleportinSpainwillexperienceroundtriptime(RTT)ofapproximately122msec.Ifweaddintheaverage
InternetlatencyfromtheSpainteleporttotheLondonInternetExchange(LINX)(~10msec.),weendupwithanoveralllatencyfromLagostoaEuropeaninternetsiteof132.5msec.O3b’s132.5msec.roundtriplatencyisclosetofiber’s80msec.RTT
Bycomparison,thesameLagoscustomersiteusingageosynchronoussatellitetoaUKTeleportandthenontoLINXwillhavelatenciesof496msec.
ToactuallyretrieveawebpagefromanInternetserveratLINX,willtakesomeadditionaltimeforthedatarequestpacketsandtheir
acknowledgments:
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What is Network Latency and Why Does It Matter?
This data request time includes:
- Therequestpacketfromtheusertothewebserver- Thewebserveracknowledgingtherequest- Thewebserverpushingtorequesteddatatotheuser
- Thedataarrivingattheuser’scomputer
Geosynchronoussatelliteusersmustwaitalmost750msec.beforetheystartgettingdata,whereasthelowerlatencyO3bsatellitelinkwillreceiveitnearly4xsooner.
Anotherproblemwithhighlatencylinksistheintroductionofalargeamountofinter-packetjitterduetotheTCPprotocolflowcontroloperatinglessefficientlyoverlongerdelaypaths.ThefollowingplotsweremadeusingaTCP/IPpacketcaptureutility.Theseplotsshowthepacketbitrateonthey-axisandtimeofdayonthex-axis.Thedatabeingtransmittedwas
theun-cachedweb-pagere-loadofthecontentfromtheCNNwebpage(http://www.cnn.com).
Theonlyconditionchangedduringwasthedelaybetweenpackets–thetransmissionrateremainedthesame.
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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’seasytoseethattheO3b
Satelliteconstellationwillofferusersanoticeablybetterinternetexperiencewithmoreimmediatefeedbackandquickeraccesstodata.
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Applications
Whatapplicationscannowberunoverlowerorbitsatellitesthatwerepossibleafewyearsago?
Manyapplications,suchasbasicwebsitedownloads,Internetvideostreamingthatoperatedsluggishly(orwereveryslowtostartup)overgeosynchronoussatellitelinksnowhavemuchfasterresponseandworkmuchbetteroverO3b’slowlatencyconnections.Videoconferencingandregularvoicecallsovergeosynchronoussatelliteswereverystiltedandnon-interactive.Conversationswere“half-duplex”andonepartyhadtowaitfortheotherpartytofinishtalkingbeforetheycouldstartup.Youcouldnotbreakintoaconversationandinterrupt.VoicecallsandvideoconferencingaremuchmorenaturalandinteractiveoverO3b’slinks.Interactivegamingplayerswhowereontheendofageosynchronoussatellitelinkwereoftenkilledbeforetheycouldgeta
shotoff.Nowtheycanbefullparticipantsintheaction.Manybusinessapplicationssuchasremotedatabaseaccessandinteractivetransactionprocessingcontainsoftwaretimersthatdonotadjustforlongdelaysovergeosynchronoussatellites.Examplesincludemanybankingapplicationsandairlinereservationandschedulingapplications.Nowtheseapplicationswillworkforusersthatareinremoteunderservedlocations.
Summary
WehavedescribedthestructureofIP-basedpacketswitchednetworks,
thefunctionsofthevariousprotocollayers,andthecausesoflatencyinpacketswitcheddatanetworks,suchastheInternet.Latencyandoverallthroughputisdominatedbytwofactors,thelengthoftheroutethatthepacketshavetotakebetweensenderandreceiverandtheinteractionbetweentheTCPreliabilityandcongestioncontrolprotocolsandthispathlength.O3bNetworkssatelliteconstellationinamuchlowerMEOorbithassignificantlylowerpathlengthandthereforesignificantlylowerlatencythantraditionalgeosynchronoussatellites.O3b’snetworklatencyandthroughputapproximatethatoffiber-basedterrestrialnetworksandpermitusersinremotelocationsthatarenotservedbyfibertorunmanyapplicationsthatonlyworkoverfiber.
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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.
TheHeadercontainsinformationusedforroutingthepackettothe
destination.Thedatacanbeanyinformationwhichneedstobetransportedsuchasasnippetofstreamingmusicoraportionofemailtraffic.TheexactconstructofthedataportionofanIPPacketisdefinedbythedataprotocolthatisbeingcarried.Dataprotocolswillbediscussedlater.
Tounderstandexactlywherelatencyoccurs,it’svaluabletoknowhowthismostbasicunitofnetworkingdataisbuiltandtransported.ForthisweturntotheOSIModel:
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The OSI Model
TheOSImodelwascreatedtodescribetheprocessofturningyourapplicationdataintosomethingthatcanbetransportedontheinternet.
TheupperlayersoftheOSImodeldescribethingsthathappenwithintheapplicationsthatarerunningonthecomputer.Theseapplicationsarewebbrowsers,emailprograms,etc.
Thelowerlayersarewhereinformationtoandfromapplicationsareturnedintodatafortransportonanetwork.Thisiswheredataencapsulationoccursandourbasicnetworkingdataelement–theIPDatagramor“packet”isbuilt.
Thefollowingdiagramshowstheencapsulationprocessinwhat’sknownastheTCP/IPStack.ThepreciseworkingsoftheTCP/IPstackcanbedifferentbetweenvariouscomputeroperatingsystems.Thesedifferencesmayseem
trivialaslongastheprotocolsareimplementedproperlybutwhenseekingtheabsolutehighestlevelsofperformanceit’simportanttoknowthatthenetworkstackimplementationcanbeasignificantcauseofnetworkingperformancevariability.
What is Network Latency and Why Does It Matter?
Application LayerMessageformat,Human-MachineInterfaces
Presentation Layer
Codinginto1sand0s;encryption,compression
Session LayerAuthentication,permissions,sessionrestoration
Transport LayerEnd-to-enderrorcontrol
Network LayerNetworkaddressing;routingorswitching
Data Link LayerErrordetection,flowcontrolonphysicallink
Physical LayerBitstream:physicalmedium,methodofrepresentingbits
T r a n s p o
r t s e r v i c e
U p p e r L a y e r s
7
65
4
3
2
1
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UpperLayers
UpperLayers
Upper Layer Message
UpperLayer
Headers
UpperLayer
Headers
TCP/UDPHeader
Upper Layer(Application) Data
Upper Layer(Application) Data
TCP/UDP Message
UpperLayer
Headers
TCP/UDPHeader
IPHeader
Upper Layer(Application) Data
IP Datagram
UpperLayer
Headers
TCP/UDPHeader
IPHeader
Layer 2Header
Layer 2Header
Upper Layer(Application) Data
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. Duringthis process the application data is wrapped into IP Datagrams which arecommonly called “packets”. Packets are then transmitted by the sendingcomputer in the network
2. Packets are passed along the network (purple line) until they reach thedestination computer
3. Packets are received from the network by the destination computer andare passed up through the stack. During this process the application datais extracted and the passed along to the destination application.
TheadditionalencapsulationatLayer2iscalledframing.ThisisthestagewheretheIPDatagramisturnedintobitswhichareappropriateforaparticulartypeofnetwork.
Layer1isthephysicalnetworkmediumconnection.Thislayerhandlestheconversionofthelayer2bitsintoelectrical,optical,orradiosignalsthatcanbetransported.Thenetworkinterface,oftencalledtheNICorNetworkInterfaceCard,canbefiber-optic,copperwire,orawirelessradiointerface.
What is Network Latency and Why Does It Matter?