Extending IEEE AVB with Time-triggered Scheduling: A Simulation Study of the Coexistence of Synchronous and Asynchronous Traffic

Transcription

1 Exending IEEE 8. wih Time-riggered Scheduling: A Simulaion Sudy of he Coexisence of Synchronous and Asynchronous Traffic Philipp Meyer, Till Seinbach, Franz Korf, and Thomas C. Schmid philipp.meyer@haw-hamburg.de, seinbach@ieee.org, korf@informaik.haw-hamburg.de,.schmid@ieee.org Deparmen of Compuer Science, Hamburg Universiy of Applied Sciences, Germany Absrac In-car neworks based on Eherne are expeced o be he firs choice for fuure applicaions in he domain of infoand enerainmen. However, he full benefi of a echnologically inegraed in-car nework will only become rewarding wih an Eherne-based backbone, unifying several auomoive domains in a single infrasrucure. Today, here is remarkable ineres in he IEEE 8. Audio/Video Bridging () proocol suie, ha provides end-o-end performance guaranees in Eherne neworks. Bu for he sric iming requiremens of auomoive conrol-raffic, hese guaranees are oo weak. An exension of Eherne wih synchronous ime-riggered raffic can overcome hese limiaions. In his paper, we invesigae he coexisence of synchronous and asynchronous raffic by experimenally adding ime-riggered messages o he credi-based shaper of in a sraighforward way. Based on simulaions and analyical evaluaions, we quanify he impac of such inegraion conceps for a reasonable design range. Our resuls demonsrae he feasibiliy of a shaping sraegy wih concurren and ime-riggered message, bu show a significan impac of he schedule design on he asynchronous sreams. Based on our findings, we provide recommendaions for configuraions ha can improve end-o-end nework performance for in-car applicaions by over %. I. INTRODUCTION Real-ime Eherne is he mos promising soluion o increase bandwidh and reduce complexiy in nex generaion in-car neworking infrasrucures. Today up o elecronic conrol unis of differen funcional domains are inerconneced over heerogeneous and someimes proprieary communicaion echnologies (such as CAN, FlexRay, LIN, MOST or LVDS) resuling in a nework srucure ha is hard o design, manage and mainain. Eherne is an esablished, widely deployed and open sandard echnology wih a variable physical layer and a large base of developmen ools and experise. years afer is invenion Eherne will now ener a new applicaion domain, as BMW sars is series producion of he X model ha conains Eherne based video cameras. In he firs sage of developmen, Eherne will drive in-car mulimedia, bu also camera and laser-scanner based driver assisance sysems. These applicaions ypically require iming guaranees in he order of microseconds. This is a ypical domain for he Eherne Audio/Video Bridging proocol suie []. Eherne is a se of sandards, providing qualiy of service mechanisms for low laency communicaion. In he second sage of developmen, he differen neworking domains (such as chassis, drive-rain, comfor or enerainmen) are expeced o be inerconneced using Eherne based in-car backbone archiecures. In his concep, he backbone replaces he cenral gaeway usually used oday, allowing for a more efficien inerconnecion of domain overlapping funcions. A subse of hese funcions e.g. for auonomous driving have rigid real-ime requiremens, such as an endo-end laency of less han µs [], or jier in he order of microseconds. Eherne was no designed for hese regid requiremens of conrol raffic and hus i canno be used in hese scenarios. There are several proposals o improve for saisfying hese requiremens of exremely low laency and jier. Recenly he sandardisaion of scheduled raffic was sared in Projec Auhorizaion Reques (PAR) 8.Qbv [3]. Scheduled raffic adds synchronous ime-riggered messages o he previously only even-based shaping of Eherne. Timeriggered raffic uses a coordinaed ime-division muliple access (TDMA) muliplexing sraegy o preven muliple ougoing messages from raversing a line card a he same ime. Thereby, ime-riggered raffic can mee he laency required wih predicable minimal jier. In his work, we show simulaion resuls from a concepual implemenaion of Eherne wih addiional imeriggered raffic. We analyse how boh, he implemenaion of synchronous, ime-aware shaping, as well as he designspace of ime-riggered schedules in Eherne, have impac on imporan real-ime relaed nework merics such as laency, jier or bandwidh uilisaion. Our simulaion allows o analyse he design space and exend he analyical wors-case calculaions wih resuls from realisic nework examples o assess he achievable performance. Due o our modular approach, he simulaion model can be adaped o fuure advances possibly defined in IEEE 8.Qbv. To he bes of our knowledge, we conribue he firs open-source simulaion framework for he analysis of upcomming shaping algorihms for Eherne ha combines even-based and ime-riggered communicaion. This paper is organised as follows: In Secion II, we inroduce he conceps of Eherne and ime-riggered Eherne and presen previous and relaed work. Secion III presens he inegraion concep and he simulaion model. In Secion IV, differen shaping and scheduling designs are presened, evaluaed and discussed. Finally, Secion V concludes our work and gives an oulook on our fuure research.

2 Class-A Chassis Class-B Bes- Effor 3 Prioriy Based Selecion Bes-Effor Egress Credi Based Shaper (CBS) Prio Egress Prio Prio 3... Prio cycle Diagnosis Cam RC BE BE BE E-Swich RC BE BE BE cycle RC BE E-Swich Time-Triggered Message Rae-Consrained Message Bes-Effor Message Fig.. IEEE 8.Qav Sender/Forwarding: Transmission selecion scheme Fig.. Prioriising and ime-riggered media access in ime-riggered Eherne II. BACKGROUND & RELATED WORK This secion inroduces IEEE 8. and ime-riggered Eherne and presens previous and relaed work. A. IEEE 8. (and Time Sensiive Neworking) The IEEE 8. Audio/Video Bridging () [] suie, ha is specified by he IEEE Time Sensiive Neworking Group (formerly Audio Video Bridging () Task Group), consiss of proocols for low laency sreaming over 8 neworks. defines he IEEE 8.AS ime synchronisaion proocol for he synchronisaion of disribued endsysems in Eherne. I provides a synchronisaion error of less han µs over a maximum of seven hops using imesamping [], []. IEEE 8.Qav defines queuing and forwarding rules for ime sensiive applicaions in Eherne. For laency requiremens up o a maximum of ms over seven hops, sream reservaion (SR) class-a was defined. SR class-b guaranees laency requiremens of up o ms. Traffic belonging o none of hese SR-classes is reaed as bes-effor raffic. Bes-effor messages cover all legacy Eherne frames. Transmission selecion and raffic shaping in IEEE 8.Qav is organised by prioriies and a credi based shaping (CBS) algorihm (see Figure ). The ransmission of a frame of a sream in a SR class is only allowed when he amoun of available credis is greaer or equal. An upper and lower bound of he credi based shaper limis he sreams bandwidh and bursiness. Messages of nodes ha are unaware of he IEEE 8.Qav proocol are mapped o he prioriies of beseffor raffic o ensure he real-ime capabiliies guaraneed by he sream reservaion classes. Eherne defines a signalling proocol for he dynamic online regisraion of new real-ime sreams. IEEE 8.Qa Sream Reservaion Proocol provides a hree sep signalling process (i.e., sream adverisemen, regisraion and un-regisraion) o reserve resources along he pah beween source and sink. A mos 7% of he oal bandwidh can be reserved. The remaining resources are used for bes effor raffic. B. Time-riggered Eherne (AS68) Several real-ime exensions for Sandard Eherne use he concep of synchronous ime-riggered messages o provide fully deerminisic ransmission wih low laency and jier. Popular proocols are PROFINET [6] or Eherne [7]. In he following, we discuss he ime-riggered conceps based on he Eherne proocol. Due o he naure of imeriggered communicaion he resuls are applicable o oher ime-riggered proocols as well and hus should anicipae he expeced behaviour in PAR 8.Qbv. The Eherne (AS68) specificaion [7] was sandardised in by he Sociey of Auomoive Engineers (SAE) [8]. I is a compaible exension of IEEE swiched Eherne and uses opologies formed of full-duplex links. Three differen raffic classes can be used in a Eherne sysem: For ime-riggered () communicaion, preconfigured schedules assign dedicaed ransmission slos o each paricipan. This coordinaed ime-division-mulipleaccess (TDMA) muliplexing sraegy allows for deerminisic ransmission wih predicable delays. I prevens congesion on ougoing line cards and enables isochronous communicaion wih low laency and jier. To allow for he TDMA concep, a failsafe synchronisaion proocol, wih an error below µs, implemens a global ime among all paricipans. In addiion o synchronised ime-riggered messages, wo even-riggered message classes are defined: Rae-consrained (RC) raffic is inended for he ransmission of messages wih moderae iming requiremens. I limis bandwidh and prioriises according o he sraegy of he ARINC-66 (AFDX) proocol [9]. RC raffic is comparable wih SR Classes A and B. Bes-effor (BE) raffic conforms o sandard Eherne messages ha are ransmied wih he lowes prioriy. The laer allows he inegraion of hoss ha are unaware of he imeriggered proocol and remain unsynchronised. These nodes communicae using bes-effor messages. Figure shows he media access policy for messages of differen raffic classes. C. Relaed & Previous Work Various work has been dedicaed o Eherne-based communicaion in cars. Lo Bello [] provides an overview over differen approaches o Eherne-based auomoive communicaion. Her work argues for deploying IEEE 8. and Eherne in differen applicaion domains. Previous performance assessmens of ime-riggered Eherne and Eherne revealed srenghs and weaknesses in differen applicaion domains for each of he proocols []. As a conclusion an approach wih and ime-riggered messages on a shared infrasrucure as conribued in his work was proposed. There is ongoing research o improve he end-o-end nework performance of Eherne. As uses a prioriy based scheme, he main challenge is o reduce he ime when

3 Time-riggered Virual Links (VL) 3 Timeriggered Scheduler () Class-A Class-B Bes- Effor 3 Prioriy Based Selecion Bes-Effor Egress Time Aware CBS Clock Schedule Por Time-riggered + + Bes-Effor Egress VL VL Prio VL Prio VL Prio... Prio VL 3 3 Daaflow Conrolflow Fig. 3. Transmission Selecion Algorihm for and ime-riggered raffic. CBS uses clock and schedule, enabling communicaion wih no inerference a low prioriy frame delays a high prioriy frame. Imiaz, Jasperneie and Weber [] analyse he impac of non-real-ime cross raffic wih varying maximum frame size on he end-oend laency. Their resuls can also be applied o he concep of ime-riggered raffic in presened in his paper. The IEEE 8. Time Sensiive Neworking Group focuses on wo approaches o reduce laency of ime-sensiive sreams. PAR 8.Qbu [3] inroduces frame pre-empion for concurren sreams wih differen prioriies. I defines a service for ime-criical frames o suspend he ransmission of a nonime-criical frame and resume is ransmission aferwards. 8.Qbu can be only used when boh, sender and receiver of a link, are aware of he proocol. In PAR 8.Qbv Enhancemens for Scheduled Traffic [3], he sandardisaion of ime-riggered raffic was sared. The main goal of 8.Qbv is o provide deerminisic communicaion in so called engineered LANs. An engineered LAN is a nework in which schedules can be offline designed. 8.Qbv is an exension for he scheduled ransmission of frames based on iming derived from IEEE 8.AS (see Secion II-A). The proposal uses prioriy values encoded ino he VLAN ag o deermine beween scheduled and credi-based raffic. Hillebrand e al. [] provide a general analysis of he impac of ime-riggered and even-riggered raffic in swiched neworks. The auhors focus on he performance evaluaion of ime-riggered messages, assuming even-riggered frames are less criical. In conras, his paper analyses he impac of imeriggered communicaion on compeing raffic of oher classes on he same infrasrucure, as he achievable performance of ime-riggered proocols is already well known. The simulaion models for IEEE 8. and Eherne are boh based on he INET-work for OMNeT++ (hp: //ine.omnepp.org) and have been inroduced and validaed in previous work []. OMNeT++ is an open-source even-based nework simulaion oolchain. The source code of he real-ime Eherne models is published (hp://eine.realmv6.org) and can be used free of charge for simulaion-based analyses. III. EXTENDING WITH TIME-TRIGGERED TRAFFIC As he laes draf of 8.Qbv (Draf.) is in a sage oo early o implemen, we provide a sraigh forward concep for he inegraion of ime-riggered raffic in Eherne. Our approach combines IEEE 8.Qav raffic wih imeriggered Eherne (AS68). Due o he naure of imeriggered raffic, he resuls of he simulaion-based assessmen are easily ransferrable o fuure 8.Qbv implemenaions. Time-riggered () raffic requires o never be delayed by any oher frame. Though, frames mus have he highes prioriy when ime-riggered,, and bes-effor frames have o be sen over he same physical nework infrasrucure. This addiional requiremen conflics wih he original sandard, ha demands o assign he highes prioriy o SR class- A frames. This definiion allows for he iming guaranees in Eherne. Due o he modified prioriies in our approach, exensions in queueing and scheduling of frames mus be made. A. Queueing In AS68, ime-riggered frames are sen over saically configured mulicas pahs, so called Virual Links (VL). The ime-riggered scheduler sends frames according o a saic schedule, based on a domain-wide synchronised clock. Due o his saic schedule, frames are never allowed o be delayed by any oher frame. The schedule defines a se of windows for each line card. During such a window, only frames are sen. In he proposed ime-aware shaping concep, hese windows and he domain wide synchronised clock are used as inpu for he Credi Based Shaping (CBS) algorihm (see Figure 3). The algorihm checks for each frame wheher he ransmission can be finished before he nex window sars. If no frame fis, he ransmier remains idle unil he ransmission of he nex scheduled frame begins. This idle ime someimes referred o as guard band guaranees ha and bes-effor frames will never inerfere wih frames.

4 3 3 3 Credi Credi Queue Size Ougoing Linecard BE Queue Size Credi Ougoing Linecard Queue Size Ougoing Linecard BE new BE sen sen (a) Sandard IEEE 8.Qav Credi is window Queue empy (b) Shaping wih ime-riggered frames Credi is sen window sen (c) Wors case wih ime-riggered frames Fig.. Credi Based Shaping (CBS) algorihm behaviour in implemenaion wih and wihou ime-riggered raffic B. Scheduling The sandard CBS algorihm for SR classes A and B is based on a credi value. The credi sars wih. Whenever he credi is greaer or equal o, an frame is allowed o be ransmied. When no frame is sen, he credi increases according o he idle slope. If he ransmission of an frame is blocked by anoher frame, he credi increases above (see in Figure a). Since SR class raffic has he highes prioriy for sandard, he frame will be ransmied as soon as he line card ges idle (see in Figure a). Now he credi decreases according o he send slope. If he credi is negaive when he ransmission of he frame is finished ( 3 in Figure a), he credi increases according o he idle slope again unil i is ( in Figure a). The proposed ime aware shaper may inroduce furher delays for boh Classes: If an frame is ready o be ransmied and oo large o be sen before he sar of he nex window, he frame will be queued and he credi will be increased according o he idle slope (see [, ] in Figure b). When he window passed, he queued messages are ransferred as long as he credi is greaer or equals. C. Analysis of he Wors Case for he New Time Aware Shaper The proposed ime aware shaper causes a new wors case scenario. When an frame is ready o be ransmied, he line card can be already occupied by a BE frame. When he line card becomes idle again, he frame may no be ransmiable as i would inerfere wih a following imeriggered frame. This case is visualized in figure c. A, jus afer he ransmission of he BE frame has been sared, an frame ges ready o be sen. Since he line card is used, he frame is being queued for he ime span [, ] and he credi increases. If he ransmission ime of he frame would be longer han [, ], he frame mus be furher delayed unil he window finishes ( 3 ). In he whole ime, he credi rises o preserve he bandwidh coningen of he queued sream. For his reason, laency guaranees for mus be recalculaed: For he calculaion he maximum ransmission ime of hree consecuive Eherne frames (M) and he inermediae iner frame gaps (T ifg ) is used. Hence he maximum inerference ime rise by facor 3. In case of a Mbi/s (R) nework, he highes inerference imespan (T mi ) now is: T mi = 3 M R + T ifg = 3 3Bye () Mbi/s +.96µs = 369µs The proocol guaranees ha frames will no be delayed more han µs (class measuremen inerval (T ms )) for each swiching hop. The oucome of his is he new maximum laency per nework device (T mdl ): T mdl = T mi + T ms = 369µs + µs = 9µs µs Now he maximum laency over 7 hops (T ml (7)) can be calculaed for a nework wih concurren ime-riggered raffic: T ml (N hop ) = ( + N hop ) T mdl T ml (7) = 8 µs = ms The resul shows a duplicaion ( ms) of he maximum laency guaranee of ms when adding ime-riggered raffic. IV. EVALUATION & CASE STUDY This secion analyses he inegraion of ime-riggered and raffic by simulaing differen nework configuraions. The simulaion resuls lead o design sraegies for neworks ha compound ime-riggered and raffic on he same physical infrasrucure. A. Nework Topology & Traffic Configuraion The evaluaions are based on a nework ha generaes worscase scenarios. I consiss of en nodes inerconneced via hree swiches (see Figure ) o produce a boleneck wih high probabiliy of inerference. For all raffic classes, he longes pah beween sender and receiver is four swiching hops. In he nework all links are configured wih a bandwidh of Mbi/s. The configuraion uses nodes and as alkers, while node 8 is he lisener for boh sreams. Each sream is configured o use 3 B payload in a µs class measuremen inerval, resuling in a oal bandwidh of Mbi/s ( Mbi/s). The ime-riggered messages use a cycle ime of ms and full size frames o generae maximum () (3)

5 Node Node Node 3 Node 8 Node 9 Node Node Node 6 Node 7 Node BE Fig.. Nework opology and raffic sreams for he analyses inerference. For each roue, wo messages are configured, resuling in a oal bandwidh of Mbi/s for ime-riggered raffic. Node broadcass full size bes-effor cross-raffic in a ms o 3 ms inerval. All nodes also reply wih full size bes-effor messages o generae raffic burss. The case sudy uses differen parameers for ime-riggered and bes-effor raffic, while he configuraion for remains he same. This allows o uphold he comparabiliy of he resuls hrough he whole evaluaion. In he following, we analyse end-o-end laency, jier, credi and queue lengh behaviour. Over he whole evaluaion, he jier is defined as he maximum difference of end-o-end laency: T jier = T max T min () This definiion of jier is usually used in real-ime sysems, as i can be uilised o calculae various applicaion specific parameers, such as buffer sizes or acion poins of asks. B. Validiy of Time Aware Shaper We firs compare he end-o-end laency of ime-riggered raffic wih and wihou frames. This allows us o assess he validiy of our previously presened ime aware raffic shaping sraegy. The analysis should reveal ha raffic never inerferes wih ime-riggered raffic, bu sill is higher prioriised han bes-effor frames. The simulaion seup is configured as explained in secion IV-A. The reserved bandwidh for ime-riggered messages is Mbi/s and for Mbi/s. Bes-effor raffic uses he remaining bandwidh. Figure 6 shows he end-o-end laency disribuion as cumulaive disribuion funcion (CDF) for a nework wih only ime-riggered and bes-effor messages C D F S a m e T T la e n c y fo r b o h s e u p s E n d -o -E n d L a e n c y [m s ] T T A V B B E Fig. 6. CDF of laency for nework wih and wihou raffic (op) compared wih a nework wih ime-riggered, and bes-effor messages (boom). As expeced, he ime-riggered behaviour remains independen of concurren sreams (see highlighed area in Figure 6). frames show higher jier han ime-riggered messages when added o he configuraion. This is due o heir even-riggered scheduling sraegy. The laency is bounded (<.8 ms) and hough complian wih he specificaion. The bes-effor end-o-end laency significanly suffers when he frames are added o he configuraion. The maximum laency increases o nearly ms due o he sauraed link beween he swiches. Table I shows he resuls in deail. C. Compac Time-riggered Schedule One of he mos challenging aspecs in a nework ha combines ime-riggered and even-riggered (such as ) raffic is he design of a schedule. The scheduling of imeriggered messages will no only have side effecs on end-oend laency and jier, bu also he performance of compeing messages of oher raffic classes. A compac schedule is a schedule wihou gaps beween consecuive ime-riggered frames. The more compac a schedule is, he more oher messages can be delayed. To show he effec of hese compac schedules, we configure a wors-case scenario and evaluae is influence on he communicaion. In he wors case scenario, he four ime-riggered full size messages are sen on a pah ha shares he same link wih he sreams. The ime-riggered messages are sen W ih o u A V B W ih A V B TABLE I LATENCY COMPARISON WITH AND WITHOUT TRAFFIC Nework Class Min [µs] Max [µs] Mean [µs] Wihou Wih BE BE Incoming Incoming Ougoing Delay Fig. 7. Compac scheduling of frames

6 R e la iv e N u m b e r o f P a c k e s p e r C la s s [% ] T T A V B B E E n d -o -E n d L a e n c y [m s ] Fig. 8. Laency disribuion of he differen raffic classes wih compac schedule (Please noe he differen bin sizes in main diagram and inlay) consecuively wih a gap of 3 µs, ha is oo small for he frames (39 B including header and iner frame gap (IFG)) o fi in. Due o he media reservaion for ime-riggered messages, he frames are no allowed o be sen unil all four ime-riggered packes are ransmied. Figure 7 shows he scenario. The simulaion confirms he influence of he ime-riggered scheduling ha is unfavorable for concurren sreams. Figure 8 shows he laency disribuion for all raffic classes. As discussed in he previous secion, he ime-riggered messages provide boh, low laency (<6. µs) and jier ( 9 ns), while bes-effor messages suffer from high laency (<9.6 ms) and jier (<9. ms) due o he low prioriy and heavily sauraed link beween swich and swich 3. The laency of he sreams says below.7 ms, he jier is. ms. For compac schedules he upper bounds for end-o-end laency provided in Secion III canno be achieved. The desired upper bound (Tmax ) is given in Equaion : T max = Tmax Node/ + Tmax Swich/ + Tmax Swich3 = µs + µs + µs =.ms As here is only inerfering ime-riggered raffic on he links beween he swiches, he firs and he las swiching hop mus be calculaed wih he sandard maximum delay of µs. Due o he compac schedule, he simulaed end-oend laency is approximaely µs oo high compared o he value calculaed wihou regarding scheduling. To analyically evaluae he maximum end-o-end laency in compac schedules, he calculaion for he inerference (T I ) wih ime-riggered sreams mus be adaped o include he gaps (T gapi ) beween he ime-riggered frames: T I = 6 M R + T ifg + = 6 3Bye Mbi/s n i= T gapi +.96µs + 7µs = 8.3µs () (6) Q u e u e le n g h [# ] C y c le m s T im e [s ] A V B B E Fig. 9. Queue lengh of and bes-effor frames over simulaion ime. Deail showing cyclic queuing behaviour due o ime-riggered frames. Based on he maximum possible inerference he maximum delay for he swiches wih concurren ime-riggered messages ) can be recalculaed: (T Swich/ max T Swich/ max = T I + T ms = 8.3µs + µs = 936.3µs Equaion 8 gives he wors-case delay for he compac schedule: (7) T max = µs µs + µs =.ms (8) The analyical wors-case of approximaely. ms is no reached in his configuraion (.7 ms). To find he source of he delays, he queue lenghs for and bes-effor raffic of he sauraed ougoing line card of swich o swich 3 are analysed. The lengh of he queue for ime-riggered messages is no of ineres, as i canno exceed by definiion. The queue lengh for bes-effor messages rises up o. The lengh for frames is always below. Figure 9 shows he queue lenghs in deail in he randomly chosen imespan from 6 ms o 6 ms simulaion ime. The spikes in he queue lengh of he buffer are cyclic ( ms cycle ime) and caused by he reservaion mechanism for imeriggered messages. Up o messages can arrive from he previous swich in he reserved ime due o he addiional bursiness inroduced by he ime-riggered shaping. Thanks o he reserved bandwidh, he queue is always processed unil he nex cycle sars. This behaviour can also be observed in he credi of he por, ha rises up o 3 88 credis, allowing he consecuive ransmission of up o frames ( 66 credis for one frame). D. Adjused Time-riggered Schedule To show ha he nework performance of sreams in a nework wih concurren ime-riggered raffic can be significanly improved, he schedule of he previous example is reconfigured. The new schedule conains gaps of 3 µs

7 Incoming Incoming Ougoing Delay Fig.. Scheduling of frames adjused for beween he ime-riggered frames ha are large enough o fi packes in (see Figure ). The resuls show a significan improvemen of he performance for sreams wih he adjused schedule. From he laency disribuion for all raffic classes (see Figure ) a beer performance for boh, and bes-effor sreams can be read off, while he laency for ime-riggered raffic remains he same (6.6 µs o 6. µs) compared o he compac schedule (see Secion IV-C). The adjused schedule reduces he maximum laency of and bes-effor raffic by %. The mean laency of is even reduced by wo hirds (see Table II). The improved schedule is also refleced in he maximum queue lenghs, ha are reduced for from o 7 messages and for bes-effor from o 9 messages. E. Maximum Transmission Uni (MTU) Analysis To furher improve he performance, we applied he recommendaions of Imiaz e al. [], and analysed he impac of a reduced Maximum Transmission Uni (MTU). Exemplarily we changed in he previous configuraion he MTU for and BE messages from B o 7 B. Due o he lower probabiliy of inerference and shorer maximum ransmission ime for concurren BE frames, he laency of he sreams is furher reduced. The end-o-end laency decreases by anoher hird (max..3 µs; mean 3.3 µs). Of course, he laency can be furher reduced wih smaller MTU (see Figure ). Our analysis reveals ha i is imporan o also analyse he impac of he schedule when deermining he bes MTU for a nework. There are frame sizes ha sui beer for a given schedule han ohers, hough only reducing he MTU will no ineviably yield overall performance (see linear fi in Figure ). TABLE II LATENCY COMPARISON OF CONFIGURATION WITH COMPACT AND ADJUSTED SCHEDULE. THE RATIO SHOWS FACTOR OF IMPROVEMENT Nework Traffic Compac Adjused Meric Class Schedule Schedule Raio Min Laency [µs] Max Laency [µs] Mean Laency [µs] :.. : BE.6.6 : : :. BE : : :.37 BE :. R e la iv e N u m b e r o f P a c k e s p e r C la s s [% ] T T A V B B E E n d -o -E n d L a e n c y [m s ] Fig.. Laency disribuion of he differen raffic classes wih adjused schedule (Please noe he differen bin sizes in main diagram and inlay) F. Discussion From he previous evaluaion, several recommendaions for he configuraions of neworks wih ime-riggered and raffic can be made. Scheduling of ime-riggered messages has significan impac on he performance of concurren sreams in such a nework. When a compac schedule wih muliple consecuive messages is configured he end-o-end laency of -frames can become high (see Secion IV-C). This can even compromise he assured maximum end-o-end laency bounds defined in he specificaion and hus endanger reliable applicaion behaviour. Schedules ha are specifically designed for concurren raffic allow o reduce his addiional delay by more han % (see Table II), wihou affecing ime-riggered raffic. When reducing he MTU for concurren raffic (imeriggered and ), he delay ha is caused by congesion can be furher reduced, allowing o achieve almos he same performance in a nework wih ime-riggered frames as wihou he synchronous raffic (see Secion IV-E). In he ime-riggered raffic class usually very small conrol messages are ransmied, and a reducion of he MTU has no influence on he design of hese applicaions. For bes-effor messages, segmenaion echniques of he ranspor layer (layer ) can be uilised o ransfer larger informaion. Table III summarises he relevan nework-merics for he raffic class in all scenarios shown. L a e n c y [µ s ] 8 6 m a x m e a n m in lin e a r fi 6 8 M T U [B y e ] Fig.. laency wih varying Maximum Transmission Uni (MTU)

8 TABLE III COMPARISON OF NETWORK METRICS FOR STREAMS IN THE DIFFERENT CONFIGURATIONS ANALYSED Nework Compac Opimised Half-size Meric Schedule Schedule MTU Laency Queue Lengh Credi Min... Max Mean Max 7 Mean Min Max Mean V. CONCLUSION & OUTLOOK The Eherne Audio/Video Bridging proocol suie is one of he favoured echnologies for a real-ime Eherne based incar neworking infrasrucure. Is dynamic sream reservaion mechanism and low configuraion effor allows for a fas and flexible nework design. Bu for he rigid iming requiremens of conrol raffic for driver assisance, X-by-wire applicaions, and auonomous driving, he guaraneed end-o-end laency of ms over 7 hops is insufficien. New sandardisaion projecs propose he inegraion of ime-riggered communicaion ino. While he achievable performance of ime-riggered raffic in Eherne is already well known, he impac of scheduling on he legacy SR classes is no ye quanified. Wih his work, we presen a simulaion sudy ha analyses he effecs of concurren synchronous (ime-riggered) and asynchronous () raffic. Our resuls show ha an implemenaion of a shaping sraegy wih ime-riggered messages is possible, bu imposes an addiional delay for messages ha can double is end-o-end laency in he wors-case. By evaluaing differen configuraions, we demonsrae he impac of he schedule design on messages. Compac schedules can significanly delay sreams, in conras o schedules ha are specifically designed regarding he concurren frames. Togeher wih a careful reducion of he MTU for conrol raffic and bes-effor messages, wih concurren ime-riggered messages can preserve he same endo-end laency as wihou synchronous raffic. We hope ha our findings will increase he awareness of he imporance of schedule design for he upcoming 8.Qbu sandard. The simulaion models used can be downloaded from our sie (see hp://eine.realmv6.org), o help o assess new nework designs. In our fuure work, we will analyse realisic in-car nework designs based on Eherne exended wih ime-riggered raffic. We will examine how well he recommendaions made in his paper can be applied on he raffic paerns of real auomoive applicaions and evaluae he merics in he ypical opologies of fuure in-car neworks. For real-world ess in our auomoive prooype, we plan on implemening he inroduced raffic shaper in hardware. This includes a clien implemenaion based on a SoC design [6], as well as a FPGA-based swich implemenaion. For furher performance improvemens, we will exend he framework wih frame preempion (as proposed in PAR 8.Qbu [3]) o compare he resuls of boh laency reducion sraegies and furher reduce he necessary guard band. ACKNOWLEDGEMENTS This work is funded by he Federal Minisry of Educaion and Research of Germany (BMBF) wihin he RECBAR projec. REFERENCES [] IEEE 8. Task Group, IEEE 8. Audio/Video Bridging (). [Online]. Available: hp:// hml [] Y. Kim and M. Nakamura, Auomoive Eherne Nework Requiremens, Tech. Rep., Mar.. [Online]. Available: hp:// new-avb-kimnakamura-auomoive-nework-requiremens-3.pdf [3] IEEE 8. TSN Task Group, IEEE 8.Qbv - Enhancemens for Scheduled Traffic. [Online]. Available: hp:// pages/8.bv.hml [] M. D. J. Teener and G. M. Garner, Overview and iming performance of ieee 8.as, in IEEE Inernaional Symposium on Precision Clock Synchronizaion for Measuremen, Conrol and Communicaion (ISPCS 8). Piscaaway, NJ, USA: IEEE Press, 8, pp [] G. M. Garner, A. Geler, and M. D. J. Teener, New simulaion and es resuls for IEEE 8.AS iming performance, in Inernaional Symposium on Precision Clock Synchronizaion for Measuremen, Conrol and Communicaion (ISPCS 9). Piscaaway, NJ, USA: IEEE Press, 9, pp. 7. [6] PROFIBUS & PROFINET Inernaional, Profine, Karlsruhe. [Online]. Available: hp:// [7] W. Seiner, Eherne Specificaion, Tech Compuerechnik AG, Nov. 8. [Online]. 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