Synchronous Distributed Wireless Network Emulator for High-Speed Mobility: Implementation and Evaluation

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1 Synchronous Dstrbuted Wreless Network Emulator for Hgh-Speed Moblty: Implementaton and Evaluaton Mnoru Kozum, Tomoch Ebata Yokohama Research Laboratory, Htach, Ltd., 292 Yoshda-cho, Totsuka-ku, Yokohama, Japan E-mal: {mnoru.kozum.ts, Tomoak Tsutsum, Kohta Ohshma and Matsuak Terada Department of Computer and Informaton Scences, Tokyo Unversty of Agrculture and Technology, Naka-cho, Kogane, Tokyo, Japan Emal: {kohta, Abstract Wreless network emulators provde varous moble node dynamc condtons and wreless lnks for real mplementaton of protocols to be evaluated based on userdefned moblty scenaros. The target of our proposed emulator s IP-based protocols for large-scale moble wreless networks composed of a large number of hgh-speed vehcles. The requrements mposed by our target are hgh scalablty and hgh-speed moblty emulaton whle mantanng hgh accuracy for many hours. To satsfy ths, we frst ntroduce a dstrbuted archtecture then propose an approach called tme-based wreless lnk emulaton. In ths approach, the moblty scenaro s converted nto a wreless lnk schedule, and each dstrbuted processor emulates a wreless lnk based only on the schedule and ts clock, whch s precsely synchronzed wth the other processor clocks by usng IEEE 1588 PTPv2. Intensve experments on the performance of the latest verson of our prototype emulator show that () hgh-speed moblty emulaton up to 200 m/sec wth 0.1 m accuracy, () packet delay emulaton wth 100 μsec accuracy, and () tme-stamps based measurement of elapsed tme spent by test nodes wth 100 μsec accuracy have been acheved. Our proposed emulator functoned well n an experment wth a smple protocol mplementaton for car-to-car/nfrastructure communcaton. Keywords-dstrbuted wrelese network emulator; hgh-speed moblty; clock synchronzaton; IEEE 1588 PTPv2 I. INTRODUCTION A wreless network emulator s a powerful tool for evaluatng moble communcaton protocols that generates varous condtons of moble nodes dynamcs and wreless communcaton lnks based on user-defned moblty scenaros [1] [9]. A wreless network emulator s especally useful when conductng ntensve tests and performance evaluatons of real protocol mplementaton at the last stage of system ntegraton because we do not have to force moble communcaton nodes to move around by usng actual trans or cars. The targets of our proposed emulator are the moble communcaton protocols that enable IP connectvty servces for hgh-speed vehcles such as bullet trans and hgh-speed cars. Moble IP and moble ad-hoc network protocols are such target examples. Based on these IP connectvty servces, we can ntegrate varous applcatons such as Internet access servces for passengers on bullet trans and car-to-nfrastructure communcaton systems to enhance safety on freeways. When we apply a wreless network emulator to these knds of moble communcaton protocols, the followng requrements have to be satsfed: () the scalablty by whch the emulator can handle the large number of test nodes that run the target protocol, () the ablty to emulate the moblty wth a wde range of speed untl several hundred klometers per hour, and () the ablty to mantan emulaton functons over a long perod to enhance the relablty of the protocol mplementatons. Other fundamental requrements of wreless network emulators are hgh accuracy of moblty emulaton, wreless lnk qualty emulaton, and packet loggng wth tmestamps so that the target protocol mplementatons can be correctly tested and analyzed. Varous dstrbuted emulators have been proposed [5][6][7] to acheve ths type of scalablty n whch the locaton nformaton for emulatng moblty s shared among dstrbuted processors va a hgh-speed LAN. However, none of these wreless network emulators satsfy the requrements noted above at the same tme. To satsfy the requrements noted above, we developed a synchronous dstrbuted wreless network emulator that uses our tme-based wreless lnk emulaton approach. Wth ths approach, a moblty scenaro s converted nto a tme-based wreless lnk schedule, and each processor emulates wreless lnks based only on the schedule and ts clock, whch s precsely and stably synchronzed wth the other processors clocks by usng IEEE 1588 PTPv2 [10]. The synchronzed clock s also used by wreless lnk qualty emulaton and packets loggng wth tme-stamps to enhance ther accuraces. Furthermore, we mplemented a prototype that uses the proposed emulaton approach to confrm that our emulator can satsfy the accuracy requrements. The rest of ths paper s organzed as follows. Secton II descrbes the requrements of wreless network emulator based on our targets as well as basc one. The archtecture of the proposed emulator and the tme-based wreless lnk emulaton approach are also descrbed. Secton III descrbes the overvew and features of latest verson of the prototype emulator. Secton IV gves measurement results whch show that our prototype can satsfy the accuracy requrements. Secton IV also descrbes a use case n whch a smple protocol mplementaton for car-to-car/nfrastructure (C2X) communcaton s tested /12/$ IEEE

2 usng our prototype emulator. Secton V dscusses related work, and Secton VI presents our conclusons. II. OVERVIEW OF SYNCHRONOUS DISTRIBUTED WIRELESS NETWORK EMULATOR A. Basc functonal requrements Our wreless network emulator s categorzed as a MAC layer emulator [1], whch mmcs the wreless lnks among the test nodes that mplement real communcaton protocols except the Mac and physcal layers. The test nodes are connected to the emulator by Ethernet *1 cables rather than wreless lnk, as shown n Fg. 1. The hardware of the test nodes can be temporal devces that provde the protocol mplementaton wth the same software envronment as those of the target devces. The test nodes may also be the fnal target devces. The basc functons of a wreless network emulator are as follows. Home agent and applcaton servers Real world moble communcaton BST 1 BST 2 BST 3 system Physcal and MAC MN 1 layer Propagaton MN 2 range MN : Moble node gap overlappng BST : Base staton Wreless network emulator Moblty Manager scenaro Vrtual space Moblty Lnk qualty Packet Propagaton range emulator emulator logger Wreless lnk swtcher TN1 TN3 TN4 TN5 TN2 Ethernet cables L TN 1 TN 2 TN 3 TN 4 TN 5 TN TN : Test node L: Propagaton dstance Protocol mplementaton Test nodes Fgure 1: General structure and functons of wreless network emulator. 1) Moblty emulaton based on moblty scenaro: An emulator must provde a vrtual space n whch test nodes move around based on user specfcatons, whch defne the number of test nodes and the way each test node moves by specfyng moblty nformaton such as velocty and ntal locaton. We call these specfcatons a moblty scenaro. Most exstng emulators mplement moblty by perodcally updatng test nodes locatons based on the moblty scenaro. 2) Wreless lnk swtchng based on emulated locaton: After emulaton starts, an emulator must swtch the packets transmtted by the test nodes to the other ones based on ther emulated locatons. Though actual wreless communcaton lnks depend on varous factors, we use a smple and commonly used dstance-based propagaton model [5] [8] n whch the transmtted packets are receved by all the other test nodes located wthn a pre-defned propagaton dstance, as shown n Fg. 1. 3) Wreless lnk qualty emulaton: For wreless lnks between test nodes located wthn propagaton range, an emulator must be able to control the qualty of wreless lnk communcaton, whch s affected by a rado wave propagaton envronment. However, we use a smple, commonly used model n whch the qualty of the wreless lnk s emulated by packet delay, packet loss, and bandwdth lmtaton. Furthermore, we start wth the packet delay as the frst step n the development of our wreless network emulator. 4) Packet loggng: An emulator must provde a packet loggng functon wth tme-stamps so that the behavor and performance of the target protocol mplementaton can be correctly measured and analyzed after emulaton. 5) Management functons for emulaton: An emulator must provde users wth functons to execute emulaton such as the commands to start, suspend, resume, stop, and montor the emulaton process. B. Target communcaton systems and ther requrements A typcal example of our target communcaton system s shown n Fg. 1. The moble communcatons nodes embedded n hgh-speed vehcles and base statons communcate wth each other based on the IP-based protocol mplementatons that are tested by our emulator. Examples of these protocols nclude a moble IP wth hgh-speed handover protocols and moble adhoc network protocols. We have to satsfy the followng requrements based on the characterstcs of our target communcaton systems. 1) Hgh scalablty of confguraton: Snce the number of moble communcaton nodes ranges from a few to several hundred, the emulator must be scalable n the sense that t can provde ts functons to a large number of test nodes. To be scalable, the emulator must be able to connect hundreds of test nodes from a physcal pont of vew. 2) Hgh-speed moblty emulaton: Snce one of our target moble nodes s bullet trans, the test nodes n the vrtual space can be moved at hgh speed. Our target speed s 100 m/sec, whch s almost ten tmes faster than that assumed wth exstng emulators. 3) Long-term emulaton: Snce the target protocols and the systems bult on top of them are used for socal nfrastructure, tests must be conducted on a long-term bass, mtatng actual tme, so that all the bugs and defects can be found and hgh relablty can be acheved. To satsfy ths requrement, the emulator must contnue workng for at least several hours whle also satsfyng the other requrements descrbed above and the requrements on accuraces descrbed n the next secton. C. Accuraces to be assured and acheved as goals Snce an emulator s testng tool of target protocol mplementaton, an emulator tself must accurately provde ts basc functons, otherwse the test become meanngless. So, the followng accuraces are what we have to ensure. 1) Accuracy of moblty emulaton: Accuracy of moblty means the fdelty of emulated test node moblty to the userdefned moblty scenaro n terms of tme and locatons of the test nodes. The extent of ths accuracy depends on the resolutons of the moblty scenaro that the emulator can ensure. For our target communcaton systems, the propagaton ranges must be set at a resoluton of 1.0 m to set ther overlappng and/or gap ranges, whch means the *1: Ethernet s a regstered trademark of Xerox Corporaton /12/$ IEEE

3 accuracy of moblty emulaton must be 0.1 m (10% of resoluton). The test node speed must be ensured up to 200 m/sec, whch s the relatve speed when two test nodes pass each other at a maxmum speed 100 m/sec. Hence, locatons of test nodes must be updated at the nterval of less than 500 μsec f we use exstng moblty emulaton approach. 2) Accuracy of wreless lnk qualty emulaton: The accuracy of wreless lnk qualty emulaton means the fdelty of emulated lnk qualty to user settngs. Snce we start wth the mplementaton of packet delay emulaton, the accuracy of packet delay was set as our target. By takng nto account the accuraces of exstng wreless network emulators [9], our goal of packet delay settng tme resoluton s 1 msec, whch means accuracy must be less than 100 μsec. 3) Accuracy of elapsed tme measurement: One of the man usages of logged packet data s to trace the protocol behavor based on the tme-stamps. Another mportant usage s a measurement of the elapsed tme spent by test nodes for ther communcaton and applcaton processes to evaluate ther performance. The start and end of the processes are specfed by two packets pcked up from the logged packets. By takng nto account of the goal of the packet delay emulaton accuracy, 1 msec resoluton wth 100 μsec accuracy s our goals of the measurement. D. Archtecture and features of proposed wreless network emulator To acheve hgh scalablty, we use a dstrbuted archtecture n whch the processes for emulatng moblty and wreless lnks are shared by dstrbuted processors connected by a hgh-speed LAN. The ssue we face when applyng the exstng dstrbuted emulaton approach [5][6][7], whch we call a locaton-based approach, to our target of hgh-speed moblty s the load ncurred by the process of updatng and sharng locaton nformaton of test nodes. If the speed of test nodes ncreases, the frequency of updatng and sharng must ncrease to mantan emulaton accuracy. However, the hgh frequency of the updatng and sharng of locaton nformaton ncreases the steady load of the emulator processors, whch n turn affects the scalablty of the system. To solve ths ssue, we propose a tme-based wreless lnk emulaton approach. Ths approach requres no addtonal process for dstrbuted processors to update and share the locaton nformaton. Instead, we need only precse and stable clock synchronzaton among dstrbuted processors to make our approach work correctly and accurately for a long perod to satsfy long-term emulaton. 1) System archtecture and clock synchronzaton mechansm: As shown n Fg. 2, the proposed emulator conssts of two types of processors, agent and manager wth a user nterface termnal, both of whch are connected to two dfferent networks. The frst one, a backbone network, s manly used for forwardng packets as well as command and data exchange between manager and agents. The second one, a clock synchronzaton network, s used to synchronze the slave clocks of all the processors wth the Grand master clock based on IEEE 1588 PTPv2, whch s a standardzed open protocol that can ensure sub-μsec accuracy of clock synchronzaton. Ths backbone network structure ensures that the traffc for clock synchronzaton s not affected by other traffc caused by packets forwarded by agent processors. Through ths archtecture, the network emulator can manage the ncrease n the number of test nodes by just addng the agent processors. Synchronous dstrbuted wreless network emulator processor Manager processor processor Grand master clock Processors Clock synchronzaton network Backbone network Slave clock processor Promscuous mode s Ethernet cable Test nodes Network for confgurng test target communcaton system Protocol mplementaton Fgure 2: Structure of proposed dstrbuted wreless network emulator. 2) Tme-based wreless lnk emulaton: a) Moblty scenaro and wreless lnk schedule: The moblty scenaro of each test node s generally defned by specfyng ther trajectores n vrtual space, as shown n Fg. 3, where the trajectores of test node (TN ) and test node j (TN j ) are defned. The manager generates the wreless lnk schedule table that shows the lst of perods when a par of nodes can communcate wth each other va a wreless lnk. In Fg. 3, the dstance of propagaton range s assumed as L, and the generated lst of perods are [t 1, t 2 ], [t 3, t 4 ], and [t k-1, t k ], whch are regstered to the wreless lnk schedule table. Moblty scenaro x (vrtual space) TN j propagaton range L TN t 1 t 2 t 3 t 4 t k-1 t k t TN1 TN TNj TNm Wreless lnk schedule table TN1 TN TNj TNm start tme end tme t 1 t 2 start tme end tme t 3 t 4 start tme end tme t k-1 t k Fgure 3: Moblty scenaro and structure of wreless lnk schedule table. b) Tme-based lnk swtch: The agent processors swtch the flow of packets transmtted from test nodes based on the wreless lnk schedule, as shown n Fg. 4. processor Wreless lnk schedule TN 1 Tmebased lnk swtcher TN 2 TN 1 Data Grand master clock TS Data TS Self-corrector TN 2 Data Fgure 4: Tme-based wreless lnk swtch and self-correcton mechansm. j TS sc Slave clock Tmebased j Lnk qualty TS emulator rel lnk k TS swtcher Logger (Packet delay) Logger Logger Protocol mplementaton j k Applcaton processor TN m /12/$ IEEE

4 When one of the test nodes, TN 1, transmts a packet to TN 2 located wthn propagaton range based on ther protocol, the agent processor tures the packet at the L2 layer wth promscuous mode, and a tme-stamp, whch we call a ture tme-stamp TS, read from the slave clock s lnked to the tured packet wth test node ID (TN 1 ). Next, the wreless lnk schedule s searched based on TN 1 and TS, to fnd other test nodes that can be reached by wreless lnk. Then, the tured packet wth TS, TN 1 and found test node ID (TN 2 ) s forwarded va the backbone network to the agent processor j that connects the found test nodes. c) Self-correctng mechansm for accurate packet delay emulaton: The tured packet forwarded to the agent processor j s nput to the lnk qualty emulator module, whch delays the packet. To make the lnk qualty emulaton more precse, the self-corrector module corrects the delay tme by subtractng the resdence tme, whch s calculated by TS j sc TS where TS j sc s the tme-stamp obtaned just before the lnk qualty emulator,.e., self-correcton tme-stamp. d) Accurate packet trace and elapsed tme measurement: The lnk swtcher and packet delay generator stores the user specfed porton of turng and releasng packets wth TS and TS rel where TS rel s the tme-stamps obtaned just before packet release,.e., release tme-stamp. Based on these tme-stamps, we can accurately trace the tme of each packets transmsson and measure the elapsed tme spent by test nodes for ther communcaton and applcaton processes. For example, n Fg. 4, the elapsed tme spent by the processes among TN 2, TN m, and an applcaton processor can be measured based on TS k and TS j rel. These usages of logged packets tme-stamps enable precse analyss of performance and behavor of target testng protocol mplementaton. III. PROTOTYPE IMPLEMENTATION A. System hardware structure Our current prototype system conssts from one manager processor and four agent processors whch are connected by Ethernet swtch (SW), as shown n Fg. 5. Ethernet Swtch (1Gbps) PTPv2- Manager processor (Core 2Duo, 2.66GHz 2GB Mem) *TC: Transparent Clock PTPv2 TC* (100Mbps Ethernet SW ) Fgure 5: Hardware structure of prototype. PTPv2 Grand master clock PTPv2- PTPv2- processor 1 processor 4 (Core 2Duo, 2.66GHz (Core 2Duo, 2.66GHz 2GB Mem) 2GB Mem) to test nodes The hardware specfcatons of each processor and Ethernet SW are also shown n Fg. 5. We use the PTPv2-enablng network nterface cards (PTPv2-) so that the load for processng PTPv2 would not affect the emulaton processes of the agents. The slave clocks n the PTPv2-s (PTPv2-clocks) are precsely synchronzed wth the grand master clock va 100 Mbps Ethernet SW, whch has a PTPv2 transparent clock functon. The vendor s specfcaton of slave clock offset aganst the grand master s ±20 nanosec. B. System software structure and features of mplementaton Fgure 6 shows the software structures of the manager and agent processors of the latest prototype verson, whch are mplemented on the top of Fedora13 *2 (kernel ) and have the followng features. movng patterns XML-based moblty scenaro x x x <emulaton area="1000"> <node d= 1" loc = 1000" speed= 0" tme="00:00" L= 200" /> t t t <node d= 2" loc = 500" speed= 60" tme="00:00" L= 200" /> <node d= 3" loc = 350" speed= 60" tme="00:00" L= 200" /> Scenaro Unt m <node d= 4" loc = 200" speed= 60" tme="00:00" L= 200" /> Scenaro Unt 2 Scenaro Unt 1 </emulaton> pattern 1 PTPv2- Socket/UDP/TCP/IP processor Socket/UDP/TCP/IP PTPv2- MM I/F MGR Man Lnk schedule generator & loader Executon manager Packet logger manager Manager processor Wreless Lnk schedule table Tmebased lnk swtcher Self-corrector Deadlne checker Watng Raw Socket/Ethernet drver queue from/to test nodes Packet delay generator Fgure 6: Software structure of manager and agent processors. Packet Logger 1) XML-based moblty scenaro defnton: We start wth a smple 1-D lnear model for vrtual space and the moblty scenaro n whch vehcles move at a constant velocty snce our frst testng target s the IP communcaton system for hgh-speed vehcles to and from fxed base statons. The moblty scenaro s specfed by defnng the length of the emulated area, node ID, ntal locaton, and speed and start tmes for each test node. Users can edt these parameters by usng XML, as shown n Fg. 6. 2) Lnk schedule generator and loader: Based on the XML-based moblty scenaro, the wreless lnk schedule s generated by solvng a lnear nequalty equaton. Users can generate the lnk schedule whch repeats and/or combnes predefned scenaros wth a user specfed order and nterval tme. Ths functon s helpful when long-term emulaton s requred. The generated lnk schedules can be dstrbuted to the agent nodes va the backbone network before emulaton starts. 3) PTPv2--clock-based emulaton processng: The accuracy of tme-based wreless lnk emulaton depends on TS, TS sc, and TS rel, obtaned for each packet n agent processors. To make best use of the precsely synchronzed PTPv2--clock, the lnk swtcher and self-corrector n the agent processors access the PTPv2--clock nstead of accessng the clocks n the PC. The nterfaces of the PTPv2- -clock s the memory mapped I/F (denoted as MM I/F n Fg. 6), whch s provded by the PTPv2- vendor, wth low latency (several μsec). The accuracy of these tme-stamps are evaluated n the next secton. 4) PTP--clock-based packet delay generator: Other than the key modules of agent processor noted above, the *2: Fedora s a regstered trademark of RedHat, Inc.. *3: Intel Core s a regstered trademark of Intel Corporaton n the USA and other countres /12/$ IEEE

5 PTPv2--clock s also accessed by the packet delay generator for the watng tme to be closer to the specfed delay tme. The packet delay generator perodcally checks the deadlne of packets n the queue based on the PTPv2-clock, where the deadlne s set by the self-corrector when the packet s put nto the watng queue. We use the Lnux mcrosleep functon usleep( ) just for mplementng the packet delay generator s perodc access to the watng queue, and we do not use t for mplementng packet delay because usleep( ) does not ensure sleep tme. IV. EVALUATION A. Analyss on accuracy of moblty emulaton Snce our proposed tme-based wreless lnk emulaton approach converts the moblty scenaro nto a tme-doman schedule, moblty emulaton accuracy depends on the accuracy of TS and the speed of the test node. The ture tme-stamp error TS at agent processor, whch we defne as the dfference between the tme the packet s transmtted by the test node and TS, s calculated as follows: TS (1) off ptp where T, T off, and T ptp are packet ture latency, offset of the slave clock aganst the grand master clock, and access latency of the PTPv2--clock, as shown n Fg. 7. Grand Master clock processor PTPv2- MM I/F Tme-based lnk swtcher T Fgure 7: Capture tme-stamp and related parameters. The moblty emulaton error at agent processor, denoted as E m, s calculated as follows: E TS v v (2) m Raw Socket/Ethernet drver test packets test node where v s the speed of the test node connected to the agent processor. Snce our goal of moblty emulaton accuracy s to make E m less than 0.1 m for a test node at 200 m/sec, TS must be less than 500 μsec from Equaton (2). Through prelmnary measurements, the latency data, T and T ptp have been obtaned for the prototype as follows: 130 T T rel 170 sec (3) T ptp 5 sec (4) where T rel s packet release latency and nequalty (3) s vald up to a 1024 Byte packet sze. Snce the PTPv2--clock offset T off s ±20 nanosecond as noted above, TS can be less than 180μsec from equaton (1) and nequalty (3) (4), whch mean we can satsfy the condton for achevng the goal of moblty emulaton accuracy. off T off TS ptp Slave clock T ptp Trel B. Evaluaton of ture tme-stamp synchronzaton For accurate wreless lnk swtchng, TS must be synchronously obtaned for packets sent from dfferent test nodes at the same tme. The analyss n the prevous secton shows that TS are less than 180 μsec. Ths means that the dfferences n TS among agent processors are less than 180 μsec. To confrm ths, we conducted experments to evaluate the accuracy of TS synchronzaton among agent processors. 1) Measurement setup: Two agent processors, and j, were connected wth one test node va 1 Gbps L2 SW as Fg. 8 shows. The test node perodcally broadcast a test packet contanng a sequence number, k, to two agent nodes. k: sequence number of test packet Grand Master Slave clock processor PTPv2- processor j PTPv2- MM I/F MM I/F Tme-based TS k Tme-based TS k lnk swtcher lnk swtcher Logger Logger Raw Socket/Ether drver Raw Socket/Ether drver 10 Gbps t Packet (k) t j (k) 10 Gbps Analyzer 1 Gbps test packets :L2 SW test node Fgure 8: Experment setup for measurng accuracy of ture tme-stamps. A packet analyzer was also connected wth mrrorng ports of 10 Gbps L2 SWs to measure the actual arrval tme of test packet to each agent processor. Each test packet wth sequence number k was logged at the two agent processors and the packet analyzer wth tme-stamps whch are denoted as TS (k), TS j (k), t (k), and t j (k), as shown n Fg. 8. The test packet sze vared from 64 Bytes up to 1024 Bytes and 300 hundred tme trals were executed for each packet sze. 2) Results: Fgure 9 plots the average, maxmum, and mnmum values of (TS j (k) TS (k)) (t j (k) t (k)), whch express the dfferences between ture tme-stamps calbrated by (t j (k) t (k)) that means the dfference n packet arrval tme to each agent processor. Capture tme-stamp dfference (μsec) Packet sze (Byte) Fgure 9: Packet sze vs. ture tme-stamp dfference. These results show that the tme dfferences range wthn ±10 μsec for all packet szes, whch means the relatve error of moblty emulaton between two agent processors s 0.02m at 200 m/sec. Another pont to be noted s that the above results /12/$ IEEE

6 of synchronzaton accuracy s vald for same packet sze and we need further evaluaton when packets szes are dfferent, whch s one of our future works. C. Evaluaton of accuracy of packet delay emulaton 1) Measurement setup: As shown n Fg. 10, test node TN 1 transmtted test packets contanng sequence number to test node TN 2 located wthn propagaton range of TN 1. A packet analyzer tured each packet wth a tme-stamp, t (k) and t j (k), va 10 Gbps L2 SW mrrorng ports to measure the emulated delay tme whch was calculated as t j (k) t (k). The packet delay settng vared from 1 to 20 msec and the packet szes vared from 64 Bytes to 1024 Bytes. For each packet delay and packet sze settng, emulated packet delay was measured 300 tmes at 1 sec nterval. k: sequence number of test packet Grand Master PTPv2- processor processor j Socket/TCP/UDP/IP MM I/F Socket/TCP/UDP/IP MM I/F Tme-based Self-correcton TS Wrelss lnk schedule lnk swtcher TS sc k k Delay generator TS j rel k Raw Socket/Ether drver Raw Socket/Ether drver 10 Gbps t Packet (k) t j (k) test packet analyzer 10 Gbps TN 1 LAN dummy packets TN 2 Fgure 10: Measurement setup for evaluatng accuracy of packet delay emulaton and elapsed tme measurement. 2) Results: Fgure 11 shows the average, maxmum, and mnmum values of packet delay emulaton error for each packet sze where the delay settng was 1 msec and the delay emulaton error was calculated as (t j (k ) t (k)) 1.0 msec. Packet delay emulaton error (μsec) Delay settng = 1 msec Packet sze (Byte) Fgure 11: Packet sze vs. packet delay emulaton error. Although the average error ncreased up to 30 μsec n proporton to the packet sze, the maxmum errors were stll less than 100 μsec; hence, the goal on accuracy of packet delay emulaton was acheved for 1 msec delay settng. The reason of the ncrease n error s the ture and release latences caused by memory copy overhead n socket and Ethernet drver layer. We can calbrate these latences by estmatng them based on the packet sze just before callng the Packet delay generator module. In ths experment, we also measured resdence tmes, T res = TS sc TS, for each packet sze, whch ranged from 146 μsec to 170 μsec. Ths means that the self-correctng mechansm effectvely works to make the emulated delay error less than 100 μsec. Fgure 12 plots the packet delay emulaton error for 1 msec to 20 msec delay settngs where packet sze was 1024 Byte. Ths shows that the goal on accuracy s acheved as the emulated delay error s less than 100 μsec for all delay settng, though the average error slghtly ncreases as delay settng tme ncreases. We confrmed that the goal was also acheved for smaller packet szes. Packet delay emulaton error (μsec) Packet sze = 1024 Byte Packet delay settng (msec) Fgure 12: Packet delay settng tme vs. packet delay emulaton error. D. Evaluaton of the accuracy of elapsed tme measurement 1) Measurement setup: We used the same setup shown n Fg. 10. To mmc an applcaton process, whch s the target of elapsed tme measurement, we connected TN 1 and TN 2 by a LAN as shown n Fg. 10. When TN 2 receve a test packet from TN 1 va the emulator, TN 2 sends a dummy packet to TN 1 va the LAN, then TN 1 sends a next test packet wth ncremented sequence number after 10 msec delay. Each test packet was logged by the agent processors and packet analyzer wth TS (k), TS j rel(k), t (k), and t j (k). The elapsed tme spent by the mmcked applcaton process was calculated as TS (k+1) TS j rel(k), whereas the actual elapsed tme, whch was almost 10 msec, was calculated as t (k+1) t j (k). The test packet sze vared from 64 Bytes to 1024 Bytes and packet delay settng was 0. Three hundred tme trals were executed for each packet sze. 2) Results: Fgure 13 plots the average, maxmum, and mnmum values of the elapsed tme measurement error calculated as (TS (k+1) TS j rel(k)) (t (k+1) t j (k)). Elapsed tme measurement error (μsec) Delay settng = Packet sze (Byte) Fgure 13: Packet sze vs. elapsed tme measurement error /12/$ IEEE

7 The results show that the error was less than 100 μsec, whch means that the resoluton of elapsed tme measurement based on TS and TS rel, was 1 msec. Another pont to be noted s that we need further evaluaton when the target elapsed tme s less than 10 msec and packet szes are dfferent. E. Use of emulator for testng C2X communcaton One of our man targets s the protocols for C2X communcaton systems, whch have been developed to enhance the safety of road traffc. To demonstrate how the proposed emulator works for evaluatng target protocol mplementaton, the key functons of the prototype emulator were tested usng the C2X communcaton protocol as a target protocol example. 1) Target C2X communcaton and moblty scenaro: As shown n Fg. 14, multple vehcles move through the wreless propagaton range of a road sde unt that broadcasts traffc nformaton data at a fxed tme nterval. The traffc nformaton data s shared among the movng vehcles by mult-hop-based C2X communcaton. Although repeated tests wth varous movng patterns are requred to evaluate C2X communcaton protocol mplementatons, we can break them down nto basc patterns. Users frst defne a scenaro unt by usng XML based on the basc movng pattern, then create a moblty scenaro and lnk schedules by usng the Lnk schedule generator, whch combnes scenaro unts. We defne a smple movng pattern as an example of a scenaro unt; three vehcles move n the same drecton at the same nterval of dstance l and at speed v, as shown n Fg. 14(a). TN 3 v : C2X communcaton program : Traffc nfo. delvery program l TN 2 v l Road sde unt TN 1 v Emulaton parameters TN 4 Propagaton range (L): 200m Packet delay settng tme: 1msec Inter-vehcle dstance (l): 150m Speed of vehcle (v): 120km/hr Traffc nfo. delvery nterval: 100msec Propagaton range Traffc nfo. packet sze: 128Byte x L L (a) Intal state at t = 0 and emulaton parameters ntervals were calculated based on TS rel set on the logged data collected from agent processors 1 and 3. processor1 Manager processor Grand Master TN 1 TN 2 TN 3 TN 4 communcaton programs for vehcles communcaton programs runnng on one PC for road sde unt Fgure 15: Experment set up for C2X communcaton program evaluaton. Interval of receved packets (msec) C processor2 TN1 TN3 D processor3 PTPv2- processor Sequence number Fgure 16: Sequence number vs. tme nterval of receved packets. Ths plot shows two spkes for each test node. These spkes means that () the communcaton route from TN 4 to TN 1 changed from drect to one hop at A and to two hops at B, () the communcaton route from TN 4 to TN 3 changed from two hops to one hop at C and to drect at D. Ths s an example of a frst step analyss on the target protocol. To further analyze the target protocol, users can synthesze the communcaton sequences among the test nodes based on the TS and TS rel set on the collected log data. Fgure 17 shows an example of a sequence chart that descrbes the communcaton among test nodes around spke A n Fg. 16. A B (b) Sequence No. = 261 (c) Sequence No. = 262 Fgure 14: Example of moblty scenaro for C2X communcaton and the communcaton route between road sde unt and movng vehcles TN 4 sequence no. 261 * TN 1 TN 2 Emulated delay (1msec) ) Experment setup: Fgure 15 shows the system structure of the experment setup, whch had four agent processors and two PCs. One PC was used for TN 4 and another was used for TN 1, TN 2, and TN 3. All the test nodes ran the C2X communcaton program for a road sde unt and vehcles. We used a smple packet floodng protocol as the target C2X communcaton program n whch we embedded a 10 msec delay to smulate the packet forwardng overhead. The emulaton parameters were set as shown n Fg. 14(a). 3) Results: Fgure 16 plots the tme ntervals of the successve two packets receved at TN 1 and TN 3. These tme t (sec) sequence no. 262 * Packet copy & forwardng delay (10msec) sequence no. 262 * Fgure 17: Example of sequence chart made from collected logged data from agent processors (sequences of redundant packets are omtted) /12/$ IEEE

8 The one way delay tme of packet transmsson from TN 4 to TN 1 can be calculated from tme-stamps, and the results show that the delay tme changed from 1 msec to 12 msec. Although the chart s manually created, t s stll helpful for us to analyze the test target protocol mplementaton because we do not need to ntroduce a clock synchronzaton mechansm among the test nodes to measure the delay tme. V. RELATED WORK Exstng dstrbuted emulaton approaches amed at scalablty can be categorzed nto two types dependng on the way the dstrbuted processors share moblty emulaton. The frst type s the locaton-based approach [5][6][7] whch has an ssue on the ncreases n the load of the emulator processors when we use ths approach for our target as noted n Secton II-D. The second type s our tme-based lnk swtchng approach, n whch the packets sent from test nodes are swtched based only on the pre-computed wreless lnk schedule. Nether updatng nor sharng the locaton nformaton s necessary after emulaton starts. Zheng and Lonel developed a dstrbuted emulator called EMWIN [8][9], whch uses an approach smlar to ours based on a synchronzed clock by usng NTP. Ther system generates a wreless lnk schedule based on a moblty scenaro and shares t among emulator processors before emulaton starts. After emulaton starts, each emulator processor updates the lst of neghbors that stores the ID of other test nodes to be lnked based on the schedule. Ths approach s smlar to our tme-based approach, but even f no test node emulator processor transmts any packets, EMWIN has to update the lst of neghbors, whch creates addtonal and steady loads of emulator processors. On the other hand, the tme requred to fnd lnked test nodes mght be shorter than wth our approach. Although we do not expect a large tme dfference between the approaches used n EMWIN and our proposed emulator, we need to evaluate the tme requred to fnd lnked test nodes. Another pont to be clarfed s the accuracy of clock synchronzaton n EMWIN because Zheng and Lonel presented no evaluaton [8][9]. They only reported that ts NTP-based clock synchronzaton consderably ncreased overhead traffc [9]. VI. CONCLUSIONS AND FUTURE WORK We proposed a dstrbuted wreless network emulator for a large-scale wreless moble network comprsed of a large number of hgh-speed vehcles. To satsfy hgh scalablty and hgh-speed moblty emulaton whle mantanng hgh accuracy for many hours, we developed the tme-based wreless lnk emulaton approach. In ths approach, the moblty scenaro s converted nto a wreless lnk schedule, and each dstrbuted processor emulates a wreless lnk based only on the schedule and ts clock, whch s precsely synchronzed wth the other processors clocks by usng IEEE 1588 PTPv2. We developed a prototype that uses PTPv2-enabled s to effectvely mplement our proposed approach. We also developed wreless lnk schedule generator and dstrbuted packet logger & collector functons, whch can smplfy longterm scenaro generaton and enable more precse analyss of emulaton results. We conducted ntensve experments of our latest verson of the prototype emulator to evaluate our proposed approach and functons. The results proved that we could acheve () hghspeed moblty emulaton up to 200 m/sec wth 1 m space resoluton, () packet delay emulaton wth 1 msec resoluton and 100 μsec accuracy, and () measurement of elapsed tme spent by test nodes and applcatons wth 1 msec resoluton and 100 μsec accuracy. Fnally, we demonstrated the usefulness of our emulator by descrbng the use case for testng and analyzng the C2X communcaton protocol mplementaton. In the future, we plan to evaluate the prototype s scalablty as well as the accuraces aganst hgh-traffc test packets. To enhance and extend the functons to make our emulator more useful, we wll focus on the moblty scenaro, whch s now lmted to lnear moblty. For emulaton of wreless lnk qualty, we currently use a smple dstance-based propagaton model, whch needs to be mproved. REFERENCES [1] W. Kess and M. Mauve, A Survey on Real-world Implementatons of moble ad-hoc Networks, Elsever s Ad Hoc Networks, vol. 5, no. 3, pp , [2] E. Hernandez and A. Helal, RAMON: Rapd-Moblty Network Emulator, proc. of 27th Annual IEEE Internatonal Conference on Local Computer Networks (LCN'02), pp.0809, [3] K. Lan, E. Perera, H. Petander, C. Dwertmann, L. Lbman and M. Hassan, MOBNET: The desgn and mplementaton of a network moblty testbed for NEMO protocol, Proc. of the 14th IEEE Workshop on Local and Metropoltan Area Networks (LANMAN 2005), Chana, Greece, 1821 September [4] K. Mtan, R. Shbu and F. Teraoka, Expermental Implementaton of the Wreless Envronment Emulator for IP Moblty Experment, IPSJ SIG Techncal Report, DPS-118, pp , June, [5] W. Chao, J. P. Macker and J. Weston, NRL Moble Network Emulator, Naval Research Lab Formal Report, ,054, [6] P. Mahadevan, A. Rodrguez, D. Becker, A. Vahdat, MobNet: a scalable emulaton nfrastructure for ad hoc and wreless networks, Proc. of WTMeMo: workshop on Wreless Traffc Measurements and Modelng [7] D. Kasamatsu and N. Shnomya, Implementaton and Evaluaton of Emulator for Testng Servce Programs n MANET, proc. of 2010 IEEE 24 th Internatonal Conference on Advanced Informaton Networkng and Applcatons(AINA) Workshops,, pp , Perth Australa, Aprl [8] P. Zheng and L. M. Lonel, EMPOWER: A Cluster Archtecture Supportng Network Emulaton, IEEE Trans. On Parallel and Dstrbuted Systems, pp , [9] P. Zheng and L. M. Lonel, EMWIN: Emulatng a Moble Wreless Network, Proc. of the 5th ACM nternatonal workshop on Wreless moble multmeda Atlanta, Georga, USA, pp.64 71, [10] IEEE Std IEEE-1588 Standard for a Precson Clock Synchronzaton Protocol for Networked Measurement and Control Systems. Avalable /12/$ IEEE

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