S. Mehta and K.S. Kwak. UWB Wireless Communications Research Center, Inha University Incheon, , Korea

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1 S. Mehta ad K.S. Kwak UWB Wireless Commuicatios Research Ceter, Iha Uiversity Icheo, , Korea ABSTRACT I this paper, we propose a hybrid medium access cotrol protocol (H-MAC) for wireless sesor etworks. It is based o the IEEE 802. s power savig mechaism (PSM) ad slotted aloha, ad utilizes multiple slots dyamically to improve performace. Existig MAC protocols for sesor etworks reduce eergy cosumptios by itroducig variatio i a active/sleep mechaism. But they may ot provide eergy efficiecy i varyig traffic coditios as well as they did ot address Quality of Service (QoS) issues. H-MAC, the propose MAC protocol maitais eergy efficiecy as well as QoS issues like latecy, throughput, ad chael utilizatio. Our umerical results show that H-MAC has sigificat improvemets i QoS parameters tha the existig MAC protocols for sesor etworks while cosumig comparable amout of eergy. KEYWORDS Sesor etworks, MAC protocol, eergy efficiecy.. ITRODUCTIO Wireless sesor etworkig is a emergig techology that has a wide rage of potetial applicatios icludig moitorig, medical systems ad robotic exploratio. Sesor etworks ormally cosists of a large umber of desely deployed distributed odes that orgaize themselves ito a multi-hop wireless etworks. The wireless sesor odes are usually equipped with limited power source. I some applicatio scearios, replacemet of power resource might be ot possible. It is for these reasos that researcher are curretly focusig o the desig of power-aware protocols ad schemes for sesor etworks. These schemes iclude power savig hardware desig, power savig topology desig ad power efficiet MAC layer protocols, to ame a few []. Commuicatio i wireless sesor etworks is divided ito several layers. Oe of those is the Medium Access Cotrol (MAC) layer. MAC is a importat techique that eables the successful operatio of the etwork. MAC protocol tries to avoid collisios so that two iterferig odes do ot trasmit at the same time. There are some MAC protocols that have bee developed for wireless sesor etworks. Typical examples iclude S-MAC ad T- MAC [2][3]. The mai desig goal of the typical MAC protocols is to provide high throughput ad QoS. O the other had, wireless sesor MAC protocol gives higher priority to miimize eergy cosumptio tha QoS requiremets. Eergy gets wasted i traditioal MAC layer protocols due to idle listeig, collisio, protocol overhead, ad over-hearig [2]. S-MAC is proposed to improve eergy efficiecy i wireless sesor etworks. It divides the time ito large frames. Every frame has two parts: a active part (o time) ad a sleepig part. A ode turs off its radio durig the sleep time to preserve the eergy. Durig the active time, a A part of this paper was published i IEEE Seoul Sectio 2008 Poster Compettitio as poster, Seoul,Korea [7]. 0.52/ijcc

2 ode ca commuicate with its eighbors ad trasmits the queued packets durig the sleepig time. Hece, S-MAC saves the uecessary waste of eergy o idle listig. Periodic sleep may result i high latecy (for mutihop sesor etworks) ad low throughput problems [2]. Timeout- MAC protocol is proposed to ehace the performace of S-MAC uder variable traffic coditios. It defies the miimum time out timer TA. T-MAC works similar to S-MAC with oe differece of time out timer. I T-MAC, active time eds whe o activatio has occurred for a time out timer TA. Time-out timer may result i early sleepig ad low throughput problems [3]. Data gatherig MAC falls ito duty cycle based MAC protocols category. The mai aim of D-MAC is to achieve low latecy as well as eergy-efficiecy. D-MAC is a improved slotted aloha protocol where slots are assiged to the sets of odes based o data gatherig tree (Paret-child topology). I D-MAC, low latecy is achieved by assigig subsequet slots to the odes that are successive i the data trasmissio path. However, D- MAC works well oly for tree based structure ad may cause a collisio problem [4]. Patter- MAC is a time slotted protocol. It adaptively determies the sleep-wake up schedules for a ode based o its ow traffic ad the traffic patters of its eighbours. I P-MAC, a ode gets iformatio about the activity i its eighbourhood before had through patters. Based o theses patters, a ode ca put itself ito a log sleep for several time frames whe there is o traffic i the etwork. If there is ay activity i the eighbourhood, a ode will kow this through the patters ad will wake-up whe required. Hece, P-MAC saves more eergy compared to S-MAC without compromisig o the throughput. But it is worthwhile to metio about complexity, collisio, ad overhead problems i P-MAC [5]. To maximize the battery lifetime, sesor etworks MAC protocols implemet the variatio of active/sleep mechaism. This mechaism trades etworks QoS for eergy savigs. However, the propose H-MAC ot oly reduce the comparable amout of eergy but also gets good QoS such as latecy, throughput, ad chael utilizatio. From [2]-[5], we ca compare the previous works o differet poits, ad summarize them as show i table. Table. Compariso table Protocol S-MAC T-MAC D-MAC P-MAC H-MAC Time-Syc. Yes Yes Yes Yes Yes Poit to Poit Suitable Suitable ot- Suitable Suitable Suitable Broadcast ot- ot- ot- Suitable Suitable Suitable Suitable Suitable Covergecast ot- ot- Suitable Suitable Suitable Suitable Suitable Mobility ot- ot- ot- Suitable Suitable Suitable May be May Be Type CSMA CSMA TDMA/S. Slotted CSMA/S. Aloha Aloha Aloha Adaptive to chage Ok Good Weak Good Good Half/Full Duplex Half Half Full Full Full 2. H-MAC PROTOCOL We preset a ew hybrid MAC protocol- H-MAC, for sesor etworks. H-MAC is based o IEEE 802. s PSM mode ad slotted aloha [6]. I H-MAC, time is divided ito large frames, every frame has two parts: a active part (o time) ad a sleepig part. Active part is like ATIM widow i PSM mode ad sleepig part is further divided ito slots, where each slot is bit 09

3 bigger tha data frame. Figure shows the compariso betwee S-MAC ad H-MAC time frames. (a) A time frame of S-MAC protocol (b) A time frame of H-MAC protocol (c) O time frame of H-MAC protocol Figure. Time frame of S-MAC ad H-MAC protocols The odes that have packets to trasmit egotiate slots with the destiatio odes durig active time ad trasmit/receive the data packets i pre-egotiated slots durig sleep time. If the odes do t have to trasmit or receive ay data packets go to sleep durig the sleep-time slots. Figure 2 illustrates the workig of H-MAC. (a) A example topology (b) Data trasmissio i H-MAC protocol Figure 2. Workig of H-MAC protocol If ode A has buffered packets destied for ode B, it will otify ode B by sedig ATIM packet. ode A icludes its preferable slot(s) list i the ATIM packet. ode B, upo receivig the ATIM packet, select slot(s) based o seder s list ad its ow list. The receiver s list has higher priority i selectig the slot(s). After ode B selects a slot(s), it icludes the slot iformatio i the ATIM-ACK packet ad seds it to ode A. Whe ode A receives the 0

4 ATIM-ACK packet, it sees if it ca also select the slot(s) specified i the ATIM-ACK. If ode A selects the slot(s) specified i the ATIM-ACK, ode A seds a ATIM-RES (ATIM- Reservatio) packet to the ode B, with ode A s selected slot(s) specified i the packet. The ATIM-RES is a ew type of packet used i our MAC scheme, which is ot i IEEE 802. PSM. The ATIM-RES packet otifies the odes i the viciity of ode A which slot(s) ode A is goig to use, so that the eighbourig odes ca use this iformatio to update their list. Similarly, the ATIM-ACK packet otifies the odes i the viciity of ode B. After the ATIM (O time) time, ode A ad ode B will trasfer the data packet(s) i selected slot(s). 3. UMERICAL RESULTS I this sectio, we show latecy ad throughput aalysis of S-MAC ad H-MAC protocols. As S-MAC is widely accepted ad popular sesor etworks MAC protocol, we choose S-MAC for compariso with H-MAC [2, 7]. For our calculatio we cosider 0 hops lier topology as show i figure 3. Figure 3. 0 hops lier topology Ad we also cosider 20 slots per cycle, where 8 slots are used for data trasfer ad 2 slots for hadshakig sigals. 3.. Latecy Aalysis: IEEE 802. Protocol [8]: The etire latecy over hops is give by cs, tx () = D( ) = ( t + t ). Where, is the umber of hops. t cs, ad t tx represets backoff ad trasmissio delay respectively. Ad represets the curret hop value, average latecy over hops is give by E[ D( )] = ( t + t ) (2) cs, tx S-MAC Protocol 2 : I S-MAC, a complete cycle is deoted by T f ad has two parts: a active part ad a sleep part. Liste/active time is fixed ad set to 0% of T f (0% duty cycle). The delay at hop is give by As adaptive listeig mode icreases the eergy cosumptio; we are ot cosiderig this mode of S-MAC. However, adaptive listig mode reduces the latecy at the cost of eergy-efficiecy.

5 D = t + t + t (3) s, cs, tx Where, Tf >> ttx ad t s, is the sleep delay. I S-MAC without adaptive listeig, cotetio oly starts at the begiig of each frame. This is give by T = t + t + t (4) f cs, tx s, So the sleep delay at hop is give by t = T ( t + t ). (5) s, f cs, tx Substitutig (5) i to (3) D = T + t t (6) f cs, cs, A packet ca be geerated o the source ode at ay time withi a frame, so the sleep delay o the first hop, t s,, is a radom variable whose value lies i (0, T f ). Suppose t s, is uiformly distributed i (0, T f ). Its mea value is T f 2. Combiig it with (6), the overall delay of a packet over hops is give by D( ) = D + D = 2 ( ) s, cs, tx f cs, cs, = 2 = t + ( ) T + t + t s, f cs, tx = t + t + t + T + t t (7) The average latecy of S-MAC without adaptive liste over hops is give by E[ D( )] = E[ t + ( ) T + T + t ] s, f cs, tx = T 2 + ( ) T + t + t f f cs tx = T T 2 + t + t. f f cs tx (8) From the (8) we ca observe that the multihop latecy liearly icreases with the umber of hops i S-MAC whe each ode strictly follows its sleep schedule(s). H-MAC Protocol: H-MAC is similar to S-MAC with oly oe differece of slotted sleep time. I H-MAC, time frame Tf X is give by Tf x = tactive + Cts, (9) Where C is the umber of equal legth slots ad t active is the liste time same as S-MAC (0% duty cycle). Durig the t active time H-MAC ca reserve the slots for r hops withi the same, so the delay for hop trasmissio over r hops is give by Tf x D T (0) f x r 2

6 From the (0), we ca also calculate the delay for hops as D( ) D () From (0) ad (), we ca calculate the average latecy of H-MAC as = E[ D( )] E[ D ] = ( T ) ( Tf x r ) f x r (2) Where * defie the largest iteger value which is equal to *. From (2) we ca observe that the multihop latecy liearly icreases with the umber of hops i H-MAC as i S-MAC. However, the slop of the lie chages to Tf x r Throughput Aalysis: Here, packet legth is fixed ad represeted by t p. Sleep time is represeted by t Sleep ad equivalet to Ct s. Actual data trasmissio take place oly durig the sleep time ad t Sleep >> t p. Hece, the throughput is give by Th = t t active sleep + t sleep (3) S-MAC Protocol: I S-MAC, a ode ca commuicate throughput is give by p packets to oly oe ode withi a frame time, so the Th s = t active p + t sleep (4) H-MAC Protocol: I H-MAC, a ode ca commuicate p packets to maximum m odes withi a frame time. So the throughput is give by Th x = t = t active active Ct p s + Ct m + Ct s s (5) from (4) ad (5) it is clear that H-MAC gives better throughput coditio compared to S- MAC. 3

7 ow we preset aalysis to fid the ratio of successful trasmitted messages 3 [9]. Here, active time is fixed ad equivalet to S m slots. There are odes to compete for medium/slots. A ode ca trasmit oly oe request. Let be the umber of odes tries to get the same mii-slot amog odes. The request messages are uiformly distributed i a active time. The probability that odes are i a slot is give by biomial distributio as follows P[ X = ] = Sm Sm (6) The above biomial distributio also applies to S m slots, thus the expected value of the umber of slots with odes i a slot is give by E[ X = ] = Sm Sm Sm (7) C represets the umber of slots beig filled with exactly odes. So the average umber of collided messages is give by Sm σ = P[ X = C ] C = E[ X = C ] = 2 c = = 2 = S m (8) = 2 Sm Sm = Sm from (7) ad (8) we ca calculate the ratio of the umber of successfully trasmitted request messages ad the total umber of trasmitted request messages. The ratio is give by σ Ratio = = Sm (9) Figure 4 shows the throughput of the etwork while varyig the umber of packets trasmitted from sik to source. After 8 packets H-MAC s throughput reduces to S-MAC throughput, as H- MAC ca t trasmit the packets simultaeously above 8 packets ad data packets take more cycles to reach sik 4. Figure 5 shows the eergy cosumptio of the etwork, S-MAC ad H- MAC cosumes the same amout eergy. Figure 6 shows the latecy performace of S-MAC ad H-MAC. H-MAC performs otably well compared to S-MAC, as it ca trasmit the data simultaeously to other odes. Figure 7 ad 8 shows the ratio of successful trasmitted message. First result we obtai by varyig the umber of eighbourig odes, ad keepig cycle size of 20 slots. Similarly, secod result we obtai by varyig the umber of slots i give cycle, ad keepig the eighbourig odes costat. Oly for H-MAC protocol. Here, we simplified our result for simplicity; ormally H-MAC will perform better tha S-MAC protocol 4

8 S-MAC H-MAC 0.2 Throughput (Th) umber of Packets Figure 4. Throughput of the etwork x 04 H-MAC S-MAC Total eergy (mj) umber of Packets Figure 5. Total eergy cosumptio 4 2 S-MAC H-MAC 0 Latecy (Cycle) umber of Packets Figure 6. Latecy performace 5

9 0.95 H-MAC Ratio umber of odes Figure 7. Ratio of successful trasmitted messages H-MAC, =0 H-MAC, =5 H-MAC, = Ratio umer of Slots Figure 8. Ratio of successful trasmitted messages 4. COCLUSIOS I this paper, we preset the H-MAC protocol, a hybrid MAC protocol based o IEEE 802. s PSM mode ad slotted aloha, ad utilizes multiple slots dyamically to improve performace. We also preset the umerical results of H-MAC, ad show that H-MAC has sigificat improvemets i QoS parameters tha the existig MAC protocols for sesor etworks while cosumig comparable amout of eergy. REFERECES [] I.F. Akayildiz, W. Su, S. Yogesh, & E. Cayirci, (2008) " A Survey o Sesor etworks, IEEE Commuicatio, August, pp 02-4 [2] W. Ye, J. Heidema, & D. Estri, (2000) A Eergy- Efficiet MAC Protocol for wireless Sesor etworks, i proceedig of IFOCOM 2002, ew York, Jue, pp [3] T. V. Dam & K. Lagedoe, (2003) A Adaptive Eergy-Efficiet MAC Protocol for Wireless Sesor etworks, i proceedig of SeSys 03, Los Ageles, ov., pp

10 [4] G. Lu, B. Krishamachari, & C. Ragavedra, (2004) A Adaptive Eergy-Efficiet ad Low- Latecy MAC for Data Gatherig i Sesor etworks, i proceedig of WMA 04, April [5] T. Zheg, S. Radhakrisha, & V. Saraga, (2005) PMAC: A Adaptive Eergy-Efficiet MAC Protocol for Wireless Sesor etworks, i proceedig of IPDPS 05, April, pp [6] IEEE 802. Workig Group, (999) Wireless LA Medium Access Cotrol (MAC) ad Physical Layer (PHY) specificatios. [7] S.Mehta & K.S. Kwak, (2008) H-MAC: A Hybrid MAC Protocol for Wireless Sesor etworks, i proceedig of IEEE Seoul Chapter Paper Cotest, December, pp:705/706. [8] G.Biachi, Performace Aalysis of IEEE 802. distributed coordiatio fuctio, IEEE JSAC, Vol.8,o-3, pp , Mar [9] R. Rom ad M.Sidi, Multiple Access Protocols: Performace ad Aalysis, Spriger-Verlag Publicatio, 989. Authors S.Mehta received the B.E. ad M.S degrees both i Electroics Egieerig from Mumbai Uiversity, Mumbai, Idia, ad Ajou Uiversity, Korea i 2002 ad 2005, respectively. He is curretly pursuig the Ph.D. degree i Telecommuicatio egieerig from the Iha Uiversity, Korea. His research iterests are i the area performace aalysis of wireless etworks ad RFID systems. K. S. Kwak received the B.S. degree from Iha Uiversity, Korea i 977, ad the M.S. degree from the Uiversity of Souther Califoria i 98 ad the Ph.D. degree from the Uiversity of Califoria at Sa Diego i 988, respectively. From 988 to 989 he was a Member of Techical Staff at Hughes etwork Systems, Sa Diego, Califoria. From 989 to 990 he was with the IBM etwork Aalysis Ceter at Research Triagle Park, orth Carolia. Sice the he has bee with the School of Iformatio ad Commuicatio, Iha Uiversity, Korea as a professor. He had bee the chairma of the School of Electrical ad Computer Egieerig from 999 to 2000 ad the dea of the Graduate School of Iformatio Techology ad Telecommuicatios from 200 to 2002 at Iha Uiversity, Icho, Korea. He is the curret director of Advaced IT Research Ceter of Iha Uiversity, ad UWB Wireless Commuicatios Research Ceter, a key govermet IT research ceter, Korea. He has bee the Korea Istitute of Commuicatio Scieces (KICS) s presidet of 2006 year term. I 993, he received Egieerig College Youg Ivestigator Achievemet Award from Iha Uiversity, ad a distiguished service medal from the Istitute of Electroics Egieers of Korea (IEEK). I 996 ad 999, he received distiguished service medals from the KICS. He received the Iha Uiversity Egieerig Paper Award ad the LG Paper Award i 998, ad Motorola Paper Award i His research iterests iclude multiple access commuicatio systems, mobile commuicatio systems, UWB radio systems ad ad-hoc etworks, high-performace wireless Iteret. 7