Time Synchronization in WSN: A survey Vikram Singh, Satyendra Sharma, Dr. T. P. Sharma NIT Hamirpur, India

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1 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: Tme Synhronzaton n WSN: A survey Vkram Sngh, Satyendra Sharma, Dr. T. P. Sharma NIT Hamrpur, Inda Abstrat: Wreless sensor networks have emerged as an mportant and promsng researh area n the reent years. They are a speal type of ad-ho networks, where wreless deves ollaborate wth other deves to send data to the destnaton. The nodes ollet the sensed data, proess them, and transmt that data over the ommunaton hannel, whh s broadast by nature. Synhronzaton s an mportant ssue for wreless sensor networks beause temporal oordnaton s requred for many of the ollaboratve tasks they perform. E.g. For the task of Data Fuson, n objet trakng and veloty estmaton, n settng the sleep modes of the varous nodes so that the battery lfe s prolonged, et.. There are several synhronzaton shemes whh have been put forward tll date. In ths paper, the shemes exstng presently have been desrbed. I. INTRODUCTION The advanement n the tehnology has enabled the development of tny deves whh are apable of sensng, proessng, and ommunatng wth eah other. Wreless Sensor Networks are a speal type of ad-ho networks, where tny wreless deves ollaborate wth other deves to send data to the destnaton n a mult-hop ommunaton envronment. These deves or nodes have ther own haratersts suh as energy onstrants, nexpensve, small n sze, unrelable, et. These nodes ollet the sensed data, proess them, and transmt that data over the ommunaton hannel, whh s broadast by nature. Synhronzaton s an mportant ssue for wreless sensor networks beause temporal oordnaton s requred for many of the ollaboratve tasks they perform. E.g. For the task of Data Fuson, n objet trakng and veloty estmaton, n settng the sleep modes of the varous nodes so that the battery lfe s prolonged, et.. There are several requrements that determne what knd of synhronzaton tehnque to use. Some of them are: Energy effeny: The synhronzaton tehnque should take nto onsderaton the lmted energy resoures avalable. Energy s the most mportant fator beause the sensors work on batteres and the replaement of the batteres s dffult, or even mpossble n many stuatons. Salablty: Synhronzaton sheme should also sale well wth the network sze. As the sensor nodes nreases/dereases the synhronzaton sheme should be able to sustan the hange n topology. Preson: Some applatons mght need mroseond auray whle others may just requre the orderng of the events. So, aordng to the requrement, the approprate sheme an be used. Robustness: Synhronzaton sheme should be robust aganst the lnk and node falures. For ths purpose, several sensor nodes are deployed n a relatvely smaller area as ompared to other networks. Lfetme: Lfetme dedes whether the synhronzaton s needed for an nstant, or for the entre lfetme of the network. Synhronzaton for the longer perod of tme mght requre regular synhronzaton. Sope: The sope dedes whether the sheme provdes the network wde External Synhronzaton or only loal synhronzaton among the nodes whh are spatally lose. Cost: How muh ost s nurred whle deployng the sheme. The ost should not exeed too muh beause of the reason that the sensors may be deployed n harsh ondtons, and thus may be prone to falures. Sne the sensor networks are often deployed n remote areas, t s better not to rely on the sophstated hardware nfrastrutures lke GPS reevers. Rather, an nternal synhronzaton s enough f mplemented approprately. But, there are some hallenges to the wreless synhronzaton suh as nondetermnst delays. When a node sends a tmestamp value to another node, the paket may fae a varable amount of delay before t reahes the reever. Ths delay mght prevent the two nodes to be synhronzed aurately. Some of the soures of errors are: Send Tme: It onssts of the tme spent n onstrutng the message and transferrng to the network nterfae. Also nludes the kernel proessng, operatng system overheads lke ontext swth, et. Aess Tme: It s the tme taken to aess the transmt hannel. The reasons for the delay ould be watng for the hannel to be dle, watng for ts slot to transmt, et. Ths depends on the type of MAC sheme used. Transmsson/Reepton Tme: Tme taken to send/reeve the message bt-by-bt at the physal layer. Depends on Page 61

2 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: the paket sze and the baud rate of the transmsson. Propagaton Tme: Tme taken to propagate the message from the sender to the reever. Ths s small enough n most ases to be gnored from lateny estmatons. Reeve Tme: Tme taken to notfy the host about the reepton of the message by the network nterfae. II. SYNCHRONIZATION TYPES By tme synhronzaton, we mean that eah and every lok n the network shows the same tme. There are two approahes to aheve ths goal. One s the External Synhronzaton and the other s the Internal Synhronzaton. In the External Synhronzaton, the nodes get the tme from some external soure of the standard tme. Ths type of synhronzaton s also known as Global Synhronzaton. But ahevng suh knd of synhronzaton an be expensve beause t wll requre that some sophstated deves, lke GPS reevers, be attahed to the sensor nodes. And ths requrement an lead to a very expensve deployment of the network. So, another type of synhronzaton an be employed, whh s the Internal Synhronzaton. In ths, the nodes agree on a partular tme for a network, although they may not agree to the global tme. Ths method s muh eonomal and doesn t even requre ostly deves attahed to the nodes. III. CLOCK MODEL Tme Synhronzaton ams at provdng a ommon tme sale for the loks of the nodes n the network. But the loks are not perfet and so the keep on drftng away from eah other over the tme. Therefore the loks that have even been synhronzed one may show dfferent tme after some tme. In software, a lok C (t) s desrbed as: C t where, s the skew rate of the lok and determnes how muh tme the lok wll gan or lose over a gven perod and s the offset, whh determnes the varaton from the real tme. For a perfet lok, the skew rate s 1. In realty, skew rate s not stat over tme. Therefore a synhronzaton sheme should equalze the lok rates as well as offsets, and then should repeatedly orret the offsets to keep the loks synhronzed over a tme perod. A tolerane value n parts per mllon (PPM) s also spefed by the manufaturers, whh determnes the maxmum amount that the skew rate wll devate from Referene Broadast Synhronzaton IV. EXISTING SCHEMES Ths sheme was proposed by Elson, Grod and Estrn n the year In ths sheme, the nodes transmt referene beaons to ther neghbors va physal-layer broadast. Ths broadast doesn t ontan any tmestamp expltly. In fat the arrval tme of the beaon s onsdered as the pont of referene for omparng the loks. Dong so removes the Send Tme and the Aess Tme from the rtal path. In RBS, the rtal path length s the tme from the njeton of the paket nto the hannel to the last lok read. Thus, ths sheme removes the sender s nondetermnsm from the rtal path as shown n the fg 1. Any extant broadast an be used to get the tmng nformaton. No dedated tmesyn paket broadast s needed. Page 62

3 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: Estmaton of Phase Offset: The smplest form of RBS s the broadast of a sngle pulse to two reevers, allowng them to estmate ther relatve phase offsets. That s: 1. A transmtter broadasts a referene paket to two reevers ( and j). 2. Eah reever reords the tme that the referene was reeved, aordng to ts loal lok. 3. The reevers exhange ther observatons. Advantages: 1. The largest soures of nondetermnst lateny an be removed from the rtal path by usng the broadast hannel to synhronze reevers wth one another. Ths results n better preson. 2. Multple broadasts allow tghter synhronzaton. 3. RBS an work usng loal tmestamps. Absolute tme referene s not requred. Ths sheme has some lmtatons too. Ths sheme works n a network whh has a physal broadast hannel. But t annot be used n the networks whh employ pont-to-pont lnks. 2. Tmng-syn Protool for Sensor Networks Ths sheme was proposed by Ganerwal et.al. n the year Ths protool ams at provdng network-wde synhronzaton. The dfferene between RBS and Tmng-syn Protool for Sensor Networks (TPSN) s that TPSN uses the lassal approah of sender-reever synhronzaton whereas RBS uses the approah of reever-reever synhronzaton. In TPSN, ntally a herarhal topology s reated. A level s assgned to eah node n that herarhal struture. The node assgned the level 0 s known as root node. Ths establshng of the herarhal struture s alled level dsovery phase. The root node then ntates the seond stage of the sheme whh s known as synhronzaton phase. In ths phase, a level node wll synhronze to a level -1 node. Eventually all the nodes n the herarhal struture wll be synhronzed. The root node an be hosen usng some leader eleton algorthm. Level Dsovery Phase: Ths s the frst phase and starts when the network s deployed. The root node s assgned the level 0 and t then ntates ths phase by broadastng a level_dsovery paket. Ths paket ontans the dentty and the level of the sender. The mmedate neghbors reeve ths paket and assgn themselves one level greater than the sender s level. After assgnng themselves a level, they too broadast the level_dsovery paket ontanng ther own level. The proess ontnues and eah node s assgned a level. One assgned a level, suh pakets are negleted to hek the ongeston. Synhronzaton Phase: A two-way message exhange s used n ths phase. Here, T1, T4 represent the tme measured by loal lok of A. Smlarly T2, T3 represent the tme measured by loal lok of B. At tme T1, A sends a synhronzaton pulse paket ontanng ts level number and the value of T1 to B. Node B reeves the paket at tme T2 where T2 s equal to T1 + + d. Here and d represents the lok drft between the two nodes and propagaton delay respetvely. At tme T3, node B sends an aknowledgement paket ontanng the level of B and the values of T1, T2, and T3 to node A. Node A reeves the paket at tme T4. Now node A an alulate the lok drft and propagaton delay as follow: ( T 2 T1) ( T 4 T3) ( T 2 T1) ( T 4 T3) ; d 2 2 Calulatng the drft, node A an orret ts lok aordngly, and an get synhronzed to node B. Ths approah s alled sender-ntated approah beause the sender synhronzed tself to the reever. Page 63

4 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: The message exhange handshake begns after the root node ntates the phase by broadastng a synhronzaton paket. Nodes at level 1 on reevng the paket ntates the two-way message exhange. They may take some random tme before ntatng the two-way message exhange to avod the ontenton n medum aess. On reevng bak an aknowledgement, these nodes adjust ther loks to the root node. On reevng the message exhange, the level 2 nodes bak off for some random tme and then ntate the message exhange wth nodes n level 1. Ths proess ontnues and eventually all the nodes get synhronzed to the root node. If an eleted root node des, the level 1 nodes won t reeve any aknowledgement paket and hene tmeout wll our. A leader eleton s run and a new root s eleted. And then the level dsovery phase s restarted. 3. Lghtweght Tree-based Synhronzaton (LTS) Ths sheme was proposed by Greunen and Rabaey. The am s not to maxmze the auray, but to mnmze the omplexty of synhronzaton. The authors beleve that the auray needed n sensor networks s relatvely low. Two LTS algorthms have been proposed for multhop synhronzaton of the network. Both these algorthms requre the nodes to synhronze to some referene ponts suh as a snk node. Frst algorthm: It s a entralzed algorthm. A spannng tree s onstruted frst and then the nodes are synhronzed along the (n-1) edges of the spannng tree. The root of the spannng tree s the referene node and s responsble for ntatng the synhronzaton. The depth of the spannng tree affets the tme to synhronze the whole network. Therefore, the depth s ommunated bak to the root node so that s an use ths nformaton n ts resynhronzaton tme deson. Seond algorthm: The seond algorthm works n a dstrbuted manner. Eah node an dede the tme for ts own synhronzaton. Spannng tree struture s not used n ths algorthm. When a node A needs to be synhronzed, t sends a synhronzaton request to the losest referene node. All the nodes along the path from the node A to the referene node must be synhronzed before the node A s synhronzed. The advantage of ths sheme s that some nodes may not need frequent synhronzaton. And sne n ths sheme, the nodes dede of ther own synhronzaton, unneessary synhronzaton efforts are saved. Also t may boost the number of parwse synhronzatons, sne for eah request, all nodes along the path from referene node to the ntator of resynhronzaton need to be synhronzed. The synhronzaton requests an be aggregated also to redue the wastage of resoures. 4. Floodng Tme Synhronzaton Protool (FTSP) Ths sheme was proposed by Marot et.al n the year Ths sheme provdes mult-hop synhronzaton. The root node, whh s dynamally eleted node, mantans the global tme and all other nodes synhronze ther loks to that of the root. An ad-ho struture s formed by the node to transfer the global tme from the root to all the nodes. Spannng tree s not formed n ths sheme. Ths saves the ntal phase of establshng the tree and s more robust aganst node and lnk falures and dynam topology hanges. Mult-hop Synhronzaton: Every node n the network has a unque ID. A node broadasts a synhronzaton message n order to synhronze other nodes. Ths synhronzaton message ontans three felds: the tmestamp, the rootid, and the seqnum. The tmestamp ontans the global tme estmate of the transmtter when the message was broadasted. The rootid feld ontans the ID of the root, as known by the sender of the message. The seqnum s a sequene number set and nremented by the root when a new synhronzaton round s ntated. Eah node mantans a hghestseqnum for the purpose of adng message flterng. Eah node has one more varable, known as myrootid. Ths varable ontans the rootid as know by the node. The synhronzaton message whh s then reeved an be used to reate a referene pont only f the rootid feld of the message s less than or equal to myrootid, and the seqnum feld s greater than the hghestseqnum n ase the rootid=myrootid. 5. Global Clok Synhronzaton n Sensor Networks Ths sheme was proposed by Q. L et al. n the year Ths paper ams at the global synhronzaton n a sensor network. Four methods have been dsussed: 1. The all-node-based method 2. The luster-based method 3. The fully loalzed dffuson-based method 4. The fault-tolerant dffuson-based method. Page 64

5 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: The all-node-based method: Ths method s used on all the nodes n the system and t s most effetve when the sze of the sensor network s relatvely small. The key dea s to send a message along a loop and reord the ntal tme and the end tme of the message. Then, by usng the message travelng tme, the tme n dfferent segments of the loop an be averaged whh wll smooth over the error of the loks. In order to synhronze the entre network, paths need to be desgned so that they ontan all the nodes. The synhronzaton s dvded nto two phases. In the frst phase, a synhronzaton paket s sent along a rng. The ntatng node of the paket reords ts loal startng tme and the endng tme of the paket. Eah other node smply forwards the paket and reords how many hops the paket had traveled thus far. In the seond phase, a lok orreton paket s sent along the same rng. Ths paket nforms eah node of the paket startng and endng tme for the ntatng node and the total hops n the yle. Eah node then omputes ts lok adjustment. The luster-based synhronzaton: In ths, same method s used to synhronze all the luster heads by desgnng a message path that ontans all the luster heads. Then, n the seond step, the nodes n eah luster an be synhronzed wth ther head. The dffuson method: The man dea here s to average all the lok tme readngs and set eah lok to the average tme. A node wth hgh lok tme readng dffuses that tme value to ts neghbors and levels down ts lok tme. A node wth low tme readng absorbs some of the values from ts neghbors and nreases ts value. After a ertan number of rounds of the dffuson, the lok n the network wll have the same value. The fault-tolerant method: In ths some nodes are onsdered as tamper-proof (alled N nodes), and other normal nodes are alled M nodes. A tamper-proof node wll destroy tself one t s ompromsed. Ths guarantees that an N node an always be trusted. Four bas operatons are there: 1. Neghbor dsovery: Neghbor dsovery for an N node means fndng all the neghbors that are shared wth another N node. 2. Beaon broadast: In beaon broadast operaton, an N node A broadasts a synhronzaton message to all ts neghbors so that eah of ts neghbors wll reord the urrent lok readng. 3. The ollet operaton: The ollet operaton s a omposte proess n whh all the neghbors that reeve the broadast send A ther lok readngs. 4. Broadast of the average value: In the last step, A broadasts the average value to all the neghborng nodes and all good neghborng nodes wll have a new value after they authentate the message. 6. Fault-Tolerant FTSP Protool for Wreless Sensor Networks Ths protool, proposed by L. Gheorghe et al. n the year 2010 s a modfaton of the Floodng Tme Synhronzaton Protool so that t provdes the synhronzaton even n the presene of malous nodes. It nludes three steps: Fault deteton, askng for help, and reevng help and deson. Fault Deteton: In ths proess the node beomes aware that t has reeved an nonsstent lok value, whh mght be very dfferent than the other prevously reeved values. Ths value s represented by havng a onstant value alled the TIME_ERROR_LIMIT that s the maxmum dfferene between sequental reeved global tmes. Askng for help: Ths s the seond step of ths protool. When a sensor node reeves a referene pont that s not ompatble wth ts prevous tme estmates, t stores the loal tme and sends a broadast message that ontans a request for the latest global tme reeved from a synhronzed node. The neghbors reply wth the global tme from the last stored referene pont. Reevng help and deson: Ths step dedes whether the value was atually faulty or not. If the value was not faulty, the regresson table s erased and the new value s stored. Otherwse, a new omputed referene pont wll be nserted nto the regresson table. The synhronzaton message here nludes the followng felds: tmestamp, rootid, seqnum, type and fronterid. The type for normal synhronzaton message s 0. The synhronzaton root sends synhronzaton messages wth a fronterid of 1, meanng that t s transmttng to nodes n the frst fronter. The nodes n the frst fronter send synhronzaton messages wth a fronterid of 2, and so on. The nodes on a ertan fronter store the fronterid reeved n a varable alled my FronterID and they send synhronzaton messages wth the value of (myfronterid+1). Page 65

6 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: 7. Consensus Clok Synhronzaton for Wreless Sensor Networks Ths paper was proposed by M. K. Maggs et al. n the year Instead of tryng to synhronze to an external referene lke absolute tme t or UTC, the CCS protool ams to aheve an nternal onsensus wthn the network on what tme s, and how fast t travels. Wth eah synhronzaton round the CCS algorthm updates the ompensaton parameters for eah node and over tme the network loks onverge to a onsensus. lm Cˆ C t Ths onsensus lok s not a physal lok. It s a vrtual lok that s generated from the network of nodes runnng the CSS algorthm. Ths lok has ts own skew rate and offset relatve to the absolute tme. By expandng the lok funtons from both sdes we get the ompensaton parameters that all nodes must obtan n order to synhronze to the onsensus lok. lm t ˆ t t ˆ lm ˆ t lm ˆ t The CCS algorthm s repeated n synhronzaton rounds, whh basally onssts of two man tasks: offset ompensaton, and skew ompensaton. Offset ompensaton: In ths phase, nodes exhange loal lok readngs whh are used to synhronze nodes to a ommon tme. The goal s to remove the offset error from all the loks n the network. For ths, eah node tres to aurately estmate the nstantaneous average tme of all the loks n the network, as gven below, and set ther loks to ths tme. N 1 C C N 1 Skew ompensaton: In ths, the nodes teratvely ompare the results from the urrent and prevous synhronzaton round n order to mprove ther skew ompensaton parameter. The goal here s to ensure all ompensated loks n the network tk at the same rate. That means lm, t ˆ For perfet offset ompensaton the skew rate of the onsensus lok s gven by: N 1 N 1 However n realty, paket losses & the random transmsson order of the nodes affets the fnal onsensus skew rate Table 1: A omparson hart of varous shemes s shown above Shemes Energy effeny Complexty Salablty Fault Tolerane RBS hgh hgh good no TPSN hgh low poor no LTS hgh low average no FTSP hgh hgh average no Global lok synhronzaton n sensor networks All-node-based low low very poor no Dffuson-based average hgh good yes Fault tolerant FTSP average hgh average yes Consensus lok synhronzaton for WSN hgh low good yes Page 66

7 Internatonal Journal of Enhaned Researh n Sene Tehnology & Engneerng, ISSN: Vol. 2 Issue 5, May-2013, pp: (61-67), Avalable onlne at: CONCLUSION Synhronzaton n a dstrbuted system suh as Wreless Sensor Network s neessary for ts effetve funtonng. The problem of synhronzaton s not new. Several shemes have been proposed tll now. Ths paper presents some of the synhronzaton shemes whh have been proposed tll date. REFERENCES [1]. M. K. Maggs, S. G. Keefe, and D. V. Thel, Consensus Clok Synhronzaton for Wreless Sensor Networks, n Pro. IEEE Sensors Journal, vol. 12, no. 6, June [2]. W. Su and I. Akyldz, Tme-dffuson synhronzaton protool for wreless sensor networks, IEEE/ACM Trans. Netw., vol. 13, no. 2, Apr [3]. L. Shenato and G. Gamba, A dstrbuted onsensus protool for lok synhronzaton n wreless sensor network, n Pro. 46th IEEE Conf. Des. Control, New Orleans, LA, De [4]. Q. L and D. Rus, Global lok synhronzaton n sensor networks, n Pro. IEEE Conf. Comput. Commun., vol. 1. Hong Kong, Chna, Mar. 2004, pp [5]. M. Marot, G. Smon, B. Kusy, and A. Ledez, The floodng tme synhronzaton protool, n Pro. 2nd Int. Conf. Embed. Netw. Sensor Syst., Baltmore, MD, Nov. 2004, pp [6]. J. Elson, L. Grod, and D. Estrn, Fnegraned network tme synhronzaton usng referene broadasts, n Pro. 5th Symp. Operat. Syst. Desgn Implement., vol. 36. De. 2002, pp [7]. M. Shtu and C. Veerarttphan, Smple, aurate tme synhronzaton for wreless sensor networks, n Pro. IEEE Wreless Commun.Netw. Conf., Mar. 2003, pp [8]. S. Ganerwal, R. Kumar, and M. Srvastava, Tmng-syn protool for sensor networks, n Pro. 1st ACM Conf. Embed. Netw. Sensor Syst., Nov. 2003, pp [9]. F. Svrkaya and B. Yener, Tme synhronzaton n sensor networks: Asurvey, IEEE Netw., vol. 18, no. 4, pp , Jul. Aug [10]. H. Kopetz and W. Ohsenreter, Clok synhronzaton n dstrbuted real-tme systems, IEEE Trans. Comput., vol. 36, no. 8, pp Aug [11]. J. Elson and D. Estrn, Tme synhronzaton for wreless sensor networks, n Pro. 15th Int. Parallel Dstr. Proess. Symp. Workshops, vol , pp b b-6. [12]. S. Ganerwal, R. Kumar, S. Adlakha, M. B. Srvastava, Network-wde tme synhronzaton n sensor networks, NESL Tehnal Report, [13]. S. Ganerwal, Man B. Srvastava, Tmng-syn Protool for Sensor Networks (TPSN) on Berkeley Motes, NESL [14]. J. Elson, K. Romer, Wreless Sensor Networks: A New Regme for Tme Synhronzaton, Proeedngs of the Frst Workshop on Hot Tops In Networks (HotNets-I), Prneton, New Jersey. Otober [15]. D. L. Mlls, Internet tme synhronzaton: The Network Tme Protool In Z. Yang and T.A. Marsland, edtors, Global States and Tme n Dstrbuted Systems. IEEE Computer Soety Press, Page 67