Dynamic TDMA Slot Assignment in Ad Hoc Networks

Transcription

1 Dynmic TDMA Slot Assignment in Ad Hoc Networks Akimitsu KANZAKI, Toshiki UEMUKAI, Tkhiro HARA, Shojiro NISHIO Dept. of Multimedi Eng., Grd. Sch. of Informtion Science nd Technology, Osk Univ. Dept. of Informtion Systems Eng., Grd. Sch. of Engineering, Osk Univ. Astrct In this pper, we propose TDMA slot ssignment protocol to improve the chnnel utiliztion, which controls the excessive increse of unssigned slots y chnging the frme length dynmiclly. Our proposed protocol ssigns one of the unssigned slots to node which joins the network. If there is no unssigned slots ville, our proposed protocol genertes unssigned slots y depriving one of the multiple slots ssigned to node, or enlrging frme length of nodes which cn cuse collision with ech other. Moreover, y setting frme length s power of slots, our proposed protocol provides the collision-free pcket trnsmission mong nodes with different frme length. The simultion results show tht our proposed protocol improves the chnnel utiliztion drmticlly s compred with the conventionl protocols. Introduction Recent dvnces in hrdwre nd wireless communiction technologies hve ledto n incresing interest in dhoc networks which re temporrily constructedy only moile hosts. TDMA (Time Division Multiple Access), which is conventionl wireless communiction technique, hs the ility to provide the collision-free pcket trnsmission regrdless of the trffic lod. There hs een mny studies for pplying TDMA to n dhoc network. However, most of them do not tke into considertion utonomous ehviors of moile hosts, ndthus they cnnot ssign time slots for new coming moile hosts. A few conventionl protocols [, 8] tht ssign time slots for new coming moile hosts show poor chnnel utiliztion ecuse they must provide enough time slots for new coming moile hosts nd this cuses lrge numer of unssignedslots. In this pper, we propose new TDMA slot ssignment protocol to improve the chnnel utiliztion. Our proposed protocol chnges the frme length dynmiclly ccording to the numer of moile hosts in the contention re nd controls the excessive increse of unssignedslots. Here, N- N frmes = cycle Slot M- M- M- For NMOP time Figure. TDMA formt in USAP the contention re is defined for ech moile host s the set of moile hosts tht cn cuse collisions of sending pckets with ech other, i.e., moile hosts within two hops from the host. The reminder of the pper is orgnized s follows. In section, we explin conventionl TDMA protocols for d hoc networks. In section, we explin the proposedslot ssignment protocol which improves the chnnel utiliztion. In section, we show the results of simultion experiments. Finlly, in section 5, we conclude this pper. Relted Work Young [9, ] hs proposedtdma slot ssignment protocols in dhoc networks, clledunifying Slot Assignment Protocol (USAP) nd USAP-Multiple Access (USAP- MA), tht tke into ccount utonomous ehviors of moile hosts. In this section, we explin these protocols.. Unifying Slot Assignment Protocol (USAP) Figure shows the TDMA formt in USAP, which consists of N frmes nd frme consists of M slots. Here, N nd M re fixednumers. The first slot in ech frme is reservedfor prticulr node to trnsmit control pcket, cllednmop (Net Mnger Opertionl Pcket). Thus, USAP llows N nodes to exist in the network, nd ech node gets the opportunity to trnsmit NMOP every N Proceedings of the th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA ) / $. IEEE

2 Frme length = 8 Frme length = 8 Frme length = e 5 f c 6 d g h i j Frme length = 5 6 Figure. ABC (Adptive Brodcst Cycle) frmes, which is clled cycle. NMOP contins the following informtion. Here, suffix i denotes the node tht trnsmits the NMOP nd s ( <s<m) denotes slot in the frme: STi(s) : : if node i trnsmits dt pcket to its neighors in slot s, : otherwise. SRi(s) ::ifnodei receives dt pcket from one of its neighors in slot s, : otherwise. NTi(s) ::ifnodei hs neighors which trnsmit dt pckets to their neighors in slot s, : otherwise. By exchnging these informtion mong neighors, ech node in the network knows unssigned slots in the frme ndcn ssign one of the slots to itself. Becuse ech node updtes its own USAP informtion every time when it receives new NMOP, USAP copes with the topology chnge of the network cusedy movement of nodes. In this pper, we cll node which newly joins the network s new node. In USAP, t the moment when new node joins the network, it is not ssignedny slots nddoes not hve informtion out neighors. Thus, it firstly collects NMOPs y listening the network chnnel for cycle ndrecognizes the slot ssignments in its contention re. Then, the new node selects n unssignedslot ndssigns it to itself. Finlly, the new node nnounces the selected slot to its neighors. Since USAP needs to provide enough slots to ssign to ll nodes in the network, N nd M shoulde lrge enough considering the cse of existing the mximum numer of nodes. As result, the chnnel utiliztion usully ecomes low due to lrge numer of unssigned slots.. USAP Multiple Access (USAP-MA) To solve the prolem of USAP, USAP-MA, which is n extension of USAP, introduces ABC (Adptive Brodcst Cycle). Figure shows the outline of ABC. In ABC, the frme length ndthe frme cycle cn e chngeddynmiclly depending on the numer of moile hosts nd the network topology. They cn lso e chngedin prts in the F : Frme length F = F = 8 F = 6 For request only Figure. TDMA formt in our protocol network. Since it is not necessry to provide enough slots for new nodes in dvnce, the chnnel utiliztion of USAP- MA ecomes higher thn tht of USAP. In ddition, y setting the frme length s power of, ABC cn voidpcket collision t node tht is oundry etween sunetworks with different frme lengths. For exmple, in Figure, node h, which elongs to two sunetworks whose frme lengths re 8 nd, respectively, cn trnsmit pckets without collision in oth sunetworks y setting its frme length s 8. USAP-MA improves the chnnel utiliztion y reducing unssigned slots with considertion of the numer of nodes ndthe network topology. However, USAP-MA does not offer neither when ndhow to chnge the frme length nor how to select slot ssignedto new node. Moreover, ecuse unssignedslots pper in the ltter prt of the frme when the frme length is douled, the chnnel utiliztion is still low. Proposed Protocol In this section, we explin our TDMA slot ssignment protocol. Our protocol sets the frme length for new node sedon the numer of nodes in its contention re nd minimizes the numer of unssignedslots to improve the chnnel utiliztion.. Frme Formt Figure shows the TDMA formt in our protocol. Similr to ABC, the frme length in our protocol is set s power of. The first slot in the frme is reservedfor new nodes to trnsmit control pckets for requesting slot ssignment. Thus, no dt pckets re trnsmitted in this slot.. Pcket Formt There re two modes, trnsmit mode nd control mode, nd ech node ehves differently in different modes. Pckets trnsmittedin ech mode re s follows:. Trnsmit mode Dt pcket (DAT) contins the informtion on the frme length ndslots ssignedto the sender, ndthe Proceedings of the th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA ) / $. IEEE

3 mximum frme length of the sender nd its neighors. Of cource, dt is lso contined in DAT.. Control mode Request pcket (REQ) is trnsmittedy only new node. By sending this pcket to neighors, new node requests the informtion on the frme length ndssignedslots of ll nodes in its contention re. Informtion pcket (INF) contins the informtion on the frme length of the sender nd slots ssigned to the sender nd its neighors. Suggestion pcket (SUG) is trnsmittedy only new node similr to REQ. By sending this pcket to the neighors, the new node nnounces the frme length ndits ssignedslot. Reply pcket (REP) is trnsmittedfor the confirmtion of receiving SUG.. Slot Assignment A new node selects slot ssigned to itself y the following four steps. () Requesting the informtion on slot ssignment in the contention re At the moment when new node joins the network, it knows neither the informtion on network topology nor slots ssignedto other nodes in its contention re. To get these informtion, the new node listens the network chnnel ndchecks pckets trnsmittedfrom the neighors for certin period. A DAT from neighor contins the informtion on its frme length, ssignedslots, ndthe mximum frme length mong itself ndits neighors. From these informtion, the new node knows the position of the first slot in frme ndthe mximum frme length mong ll nodes in its contention re. Then, the new node sends REQ in the first slot of the next frme. Neighors tht hve receivedthe REQ trnsmittedfrom the new node trnsit to the control mode. Then, ech neighor of the new node gives informtion on its frme length ndslots ssignedto itself ndits neighors y trnsmitting n INF in its ssignedslot. The trnsmission of the INF lso hs the mening of declring to its neighors tht the node hs trnsitedto the control mode. After ll neighors of the new node hve trnsmitted INFs, ll nodes in the contention re of the new node cn recognize its ppernce. () Setting the frme length nd grsping slot ssignment After collecting INFs from ll neighors, the new node sets its frme length. If ll nodes in its contention re hve the sme frme length, the new node sets its own frme length to this length. Otherwise, the new node dopts the mximum frme length mong ll nodes in the contention re. Then, from the receivedinfs, the new node knows the Informtion from node () Informtion of the new node () () () () () () () () (5) (6) () slots slots Figure. Copying informtion of node informtion on the slot ssignment in the contention re. In the former cse, this is done y only merging the informtion of ll receivedinfs. In the ltter cse, the following process is requiredto get the slot ssignment informtion. First, the new node cretes its own slot ssignment informtion of frme length, M o, Here, M o denotes the frme length tht is set to the new node. Then, if the frme length of neighor is sme s M o, the slot ssignment informtion of the neighor in the receivedinf is copiedto tht of the new node. Otherwise, when M o = αm i, the slot ssignment informtion of the neighor is copiedrepetedly to every M o /α slots from the first entry in tht of the new node. Here, M i is the frme length of the neighor nd α is n integer of power of. In this wy, the new node merges the informtion from ll neighors ndcretes its own slot ssignment informtion. For exmple, when the new node sets its frme length s 8, the slot ssignment informtion in the INF receivedfrom node whose frme length nd ssignedslot is 8 nd respectively, is copiedrepetedly to every slots in tht of the new node, s shown in Figure. () Selecting n ssigned slot Bsedon the slot ssignment informtion cretedt the new node, it selects slot ssigned to itself y the following three procedures. In Figure 5, Figure 6 nd Figure, lck ndwhite slots denote the first slot ndunssignedslots, respectively, ndshdedslots denote slots ssignedto other nodes in the contention re. Moreover, the numer nd the letter in ech slot denote the slot numer nd the node to which the slot is ssigned, respectively.. Getting n unssigned slot (GU) If some unssignedslots re foundin the slot ssignment informtion, the new node ssigns one of them to itself. For exmple, s shown in Figure 5, when unssined slots nd re found, the new node ssigns slot either or to itself.. Relesing multiple ssigned slots (RMA) If no unssigned slot is found, the new node checks whether some nodes in the contention re re ssigned multiple slots. If such nodes re found, the new node releses one of these slots nd ssigns it to itself. If there re more thn one node to which multiple slots Proceedings of the th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA ) / $. IEEE

4 () () () () () () c (5) (6) () Unssigned slots Figure 5. Getting n unssigned slots () () () d () c (5) f (6) g () e () node c d e () () () d () c (5) (6) () e slot, 5, 6 Multiple slots Figure 6. Relesing multiple ssigned slots c d New node e 5 Figure 8. Exmple of conflicting n ssigned slot f g () () () () d () c (5) f (6) g () e (8) (9) () () d () c () f () g (5) e Unssigned slot Figure. Douling the frme re ssigned, the node with the lrgest numer of ssignedslots mong them is chosen to relese slot. For exmple, s shown in Figure 6, when node nd re ssignedmultiple slots, the new node selects slot from slots,,5, nd6 which re ssignedto nd, ndssigns the selectedslot to itself.. Douling the frme (DF) If no unssignedslot is foundndno node hs multiple ssignedslots which re le to e ssignedto the new node, the new node doules the frme length of the slot ssignment informtion ndcopies the ssignment informtion to oth of the former hlf ndthe ltter hlf of the douled frme. As descried ove, the first slot in the frme is not ssignedto ny nodes. Therefore, fter douling the frme length, the first slot in the ltter hlf ecomes n unssignedslot. The new node ssigns this slot to itself. For exmple, s shown in Figure, when the new node doules the frme length (i.e., 6), slot 8 is ssignedto itself. () Announcement nd Confirmtion After selecting slot ssignedto itself, the new node sends SUG to its neighors which contins informtion on the frme length ndthe ssignedslot. When neighor receives this pcket, it updtes its slot ssignment informtion. If the frme length of the neighor is different from tht in SUG, its frme length is chngedin the sme wy in () Setting the frme length nd grsping slot ssignment. After updting the informtion sed on the received SUG, ech neighor sends REP to its neighors. Sending the REP implies the confirmtion of the SUG for the new node nd the nnouncement of updting the slot ssignment informtion ndexiting from the control mode for ll neighors of the sender. The sender nd receivers of the REP dopt the new slot ssignment nd restrt dt trnsmission from the next frme. After receiving the REPs from ll neighors, the new node trnsits to the trnsmit mode.. Detection of Conflict nd its Solution In our protocol, conflict of slot ssignment occurs when new node connects to two or more nodes to which the sme slots re ssigned. In the exmple shown in Figure 8, conflict occurs t new node etween node nd in slot. When new node detects the conflict, it solves the conflict in the following procedure:. Deleting conflicting slot If there re some un-conflicting slots ssignedto nodes cusing the conflict, the conflicting slot is relesed from ll the nodes except for tht with the smllest numer of ssignednodes. For exmple, s shown in Figure 9, when nodes nd re ssignedunconflicting slots,, nd5, conflicting slot is relesedfrom node to which more slots re ssigned thn node.. Dividing the ssignment If multiple slots re conflicting t the new node, these slots re divided to the nodes which hve cused the conflict. For exmple, s shown in Figure, conflicting slot nd re divided to nodes nd, respectively.. Douling the frme nd dividing the ssignment If conflict occurs mong nodes to which only one slot is ssigned, the conflict cnnot e solved with the current frme length. In this cse, the frme length of these nodes is douled nd the slot ssignment is divided in the douled frme. For exmple, s shown in Figure, the spce for conflicting slot is douled y douling the frme length, nd it is divided to nodes nd. Proceedings of the th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA ) / $. IEEE

5 ,,5,,5,,, Delete slot from node Figure 9. Deleting conflicting slot (Frme length : ) Doule the (Frme length : 8) frme length (Frme length : 8) Divide the slots,, Divide the slots Figure. Dividing the ssignment After reconfigurting the slot ssignment, the new node sends SUG with the informtion on the reconfigured slot ssignment ndthe selectedslot. Neighors which hve receivedthe SUG lso reconfigure their slot ssignment nd sendreps with the reconfiguredinformtion. However, in this sitution, the new node my fil to collect the informtion on the slot ssignments correctly due to the collisions of INFs. If such cse occurs, the new node sends the informtion on the slot in which collisions hve occuredto ll neighors instedof the SUG. Neighors of the new node, which hve sent the INFs in the conflicting slot, retrnsmit the INFs fter witing for certin frmes determined t rndom. This opertion is repeted until the new node completes the collection of INFs from ll neighors..5 Relesing Slot Assignment When node exits from the network, it releses slots ssignedto itself ndstops trnsmitting DATs. Neighor nodes detect the exit of the node when no pckets from the exited node hve een received during the time of the frme length of the exitednode. Then, they relese the slots ssignedto the exitednode from their slot ssignment informtion. In ddition, they lso relese the slots ssigned to nodes tht hve gone out of their contention res due to the node s exit. After reconfiguring the slot ssignment, neighors of the exited node send the updted informtion to their neighors. The nodes which hve received this informtion reconfigure the slot ssignment y relesing the slots ssignedto the exitednode. Performnce Evlution In this section, we show simultion results regrding performnce evlution of our proposedprotocol. In the simultion experiments, we compre the performnce of our protocol with USAP.. Simultion Environment For simplicity, it is ssumedtht ech node in the network does not move. A new node ppers t rndom posi- Figure. Douling the frme nd dividing the ssignment tion within the re in which the node cn connect to t lest one node. After the new node hs ppered, no other node ppers or exits from the network until the slot ssignment to the new node is completedndll nodes trnsit ck to the trnsmit mode. The numer of nodes is initilly, nd increses one y one until the whole numer of nodes in the network ecomes 5. For USAP, we set the frme length 5, in order to enle ll nodes to e ssignedslots certinly. Bsedon the slot ssignment informtion mde from the received NMOPs, the unssignedslot with the smllest slot numer mong ll unssignedslots is ssignedto the new node. On the other hnd, in our protocol, the new node is ssigned the slot with the smllest slot numer mong ll unssignedslots when using GU. When using RMA, new node is ssigned the slot with the smllest slot numer mong the possile cndidtes.. Evlution Criteri In the simultion experiments, we evlute the verge chnnel utiliztion [] of ech protocol. The chnnel utiliztion of node i, ρ i, represents the frequency tht node i cn trnsmit pckets ndis expressedy the following eqution: ρ i = the numer of slots ssigned to node i. () the frme length of node i The verge chnnel utiliztion is the verge of chnnel utiliztion in whole network. The higher the chnnel utiliztion of node is, the more frequent the node cn trnsmit pckets. However, even if the verge chnnel utiliztion of the entire network is high, the end-to-end communiction throughput my not e high due to some intermedite nodes with low chnnel utuliztions. This is ecuse pcket trnsmission etween the source ndthe destintion is delyed t such intermedite nodes. Therefore, we lso evlute the firness of slot ssignment y compring the vrince of the chnnel utiliztion of ll nodes.. Simultion Results Figure ndfigure show the simultion results. The verticl xis in the grph indictes the verge chnnel uti- Proceedings of the th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA ) / $. IEEE

6 Averge chnnel utiliztion Our protocol USAP 5 Numer of nodes Figure. Averge chnnelutiliztion Vrince of chnnel utiliztion Our protocol..5 USAP 5 Numer of nodes Figure. Vrince of chnnelutiliztion liztion in Figure ndthe vrince of the chnnel utiliztion in Figure. The horizontl xis in oth grphs indictes the numer of nodes in the network. From the result in Figure, the verge chnnel utiliztion in USAP is very low ecuse the fixedfrme length cuses mny unssignedslots. On the other hnd, the verge chnnel utiliztion in our protocol is lwys higher thn tht in USAP. This result shows tht the chnnel utiliztion is much improvedy our protocol. Although the chnnel utiliztion decreses s the numer of nodes increses, it is converging to certin vlue (lmost.5). Therefore, it cn e concluded tht our protocol cquires stlely high chnnel utiliztion. However, s shown in Figure, the vrince of the chnnel utiliztion of our protocol is high, contrry to tht of USAP eing lwys zero. As mentioned ove, such unfirness of slot ssignment my cuse the deteriortion of end-to-end communiction throuput. Thus, ddressing this prolem is significnt issue open to our future work. 5 Conclusion In this pper, we hve proposed TDMA slot ssignment protocol to improve the chnnel utiliztion. Our protocol voids the increse of unssigned slots y minimizing the frme length of ech node. Moreover, it provides the collision-free pcket trnsmission mong moile hosts with different frme lengths y using the frme length of power of. We lso hve conducted simultion experiments to evlute the performnce of our protocol. The results show tht our protocol improves the chnnel utiliztion. Our protocol cn enlrge the frme length, ut cnnot minify it. Thus, s prt of our future work, we pln to extendour protocol to hve fcility of minifying the frme length effectively when node exits from the network. We will lso extendour protocol to hve fcility of ressigning the slot for eliminting the unfirness of the chnnel utiliztion. In ddition, we consider how to ccommodte the movement of nodes. Acknowledgement This reserch ws supportedin prt y Specil Coordintion Funds for promoting Science nd Technology of the MEXT ndy Grnt-in-Aidfor Encourgement of Young Scientists numered8 from the JSPS. References [] L. Bo ndj. J. Grci-Lun-Aceves: Chnnel ccess scheduling in d hoc networks with unidirectionl links, in Proc. Interntionl Workshop on Discrete Algorithms nd Methods for Moile Computing nd Communictions, pp. 9-8 (July ). [] R. Bittel, E. Cples, C. D. Young, ndf. Loso: Soldier phone: n innovtive pproch to wireless multimedi communictions, in Proc. IEEE MILCOM 98 (Nov. 998). [] J. Broch, D. A. Mltz, D. B. Johnson, Y.-C. Hu, nd J. Jetchev: A performnce comprison of multi-hop wireless dhoc network routing protocol, in Proc. MOBICOM 98, pp (Oct. 998). [] A. Ephremides nd T. V. Truong: Scheduling rodcsts in multihop rdio networks, IEEE Trnsctions on Communictions, vol. 8, no., pp (Apr. 99). [5] S. Even, O. Goldreich, S. Morn, nd P. Tong: On the NP-completeness of certin network testing prolems, Networks, vol. (98). [6] F. A. In, J. McNir, L. Crrsco, ndr. Puigjner: Medium ccess control protocols for multimedi trffic in wireless networks, IEEE Network Mgzine, vol., no., pp. 9- (July/Aug. 999). [] H. Lee, J. Yeo, S. Kim, nds. Lee: Time slot ssignment to minimize dely in d-hoc networks, IST Moile Communictions Summit (Sept. ). [8] L. C. PondndV. O. K. Li: A distriutedtime-slot ssignment protocol for moile multi-hop rodcst pcket rdio networks, in Proc. IEEE MILCOM 89, vol., pp. - (Nov. 989). [9] C. D. Young: USAP: unifying dynmic distriuted multichnnel TDMA slot ssignment protocol, in Proc. IEEE MILCOM 96, vol. (Oct. 996). [] C. D. Young: USAP multiple ccess: dynmic resource lloction for moile multihop multichnnel wireless networking, in Proc. IEEE MILCOM 99 (Nov. 999). Proceedings of the th Interntionl Conference on Advnced Informtion Networking nd Applictions (AINA ) / $. IEEE