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2556 IEEE TRANSACTIONS ON VEHI CULAR TECHNOLOGY , VOL. 59, NO. 5, JUNE 2010 Cross-Layer Optimized MAC to Support Multihop QoS Routing for Wireless Sensor Networks Heping Wang, Xiaobo Zhang, Farid Naït-Abdesselam, Member, IEEE , and Ashfaq Khokhar, Fellow, IEEE Abstract—Th is paper pr ese nts an effi cie nt hyb rid med ium- access contr ol (HMA C) prot ocol with an embe dded cro ss-l ayer optimizati on solut ion to pro vide rou ting-l ayer coarse-gr ained end- to-en d quali ty-of -ser vice (QoS ) suppo rt for laten cy-s ensit ive traffi c flows. A novel channel-reservation technique is proposed to signif- icantly reduce the end-to-end delay for delay-sensitive traffic flows by allowing packets to go through multiple hops within a single mediu m-acc ess control (MAC) frame and by also giving them higher priority channel access to reduce possible queuing delay. Our prop osed prot ocol (HMAC) combi nes ener gy-ef ficien t fea- tures of the existing contention-based and time-division multiple- access (TDMA)-based MAC protocols and adopts a short frame structure to expedite packet delivery. Simulation results in ns-2 show that HMAC achieves significant performance improvements in ener gy cons umpti on, laten cy , and thro ughpu t over existing MAC protocols. Index Terms—End -to-end laten cy , ener gy effic ienc y, hybr id medium-access control (HMAC), latency-sensitive traffic, quality of service (QoS). I. I NTRODUCTION N ETWORKING low-cost smart sensors through a wireless medium to form wireless sensor networks (WSNs) has broad applications, such as surveillance, event detection, etc. However, due to battery-constrained computationally and communication-wise less powerful nodes in WSNs, traditional protocols are not suitable for WSNs in terms of energy efficiency, scalability, and design complexity. Current research efforts for WSNs mainly focus on exploring energy-efficient network operations in WSNs. At the network layer, a multihop routing paradigm based on short -range communication is popul arly adopted by existing network layer protocols to impro ve ener gy effic ienc y [20]. However, multih op routin g in wireless en vironments incurs increased end-to-end delivery latency and exacerbates potential hidden/exposed terminal problems [16]. Man usc rip t recei ve d Jan uar y 26, 2009; rev ise d May 27, 2009 and Sep tember 23, 200 9; acc ept ed Nove mber 9, 200 9. Dat e of publi cat ion February 17, 2010; date of current version June 16, 2010. This work was supported in part by the U.S. National Science Foundation under Grant CNS- 0910988. The review of this paper was coordinated by Prof. H. Hassanein. H. Wang is with Lemko Corporation, Schaumburg, IL 60173 USA (e-mail: [email protected]). X. Zha ng is wit h CIS CO Syste ms , San Jose, CA 95134 USA (e-mail : [email protected]). F. Naït-Abdesselam is with the University of Sciences and Technologies of Lille, 59655 Villeneuve d’Ascq, France (e-mail: farid.nait-abdesselam@lifl.fr). A. Khok har is with the Depa rtmen t of Electrical and Compute r Engi - neering, University of Illinois at Chicago, Chicago, IL 60607 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexp lore.ieee.org. Digital Object Identifier 10.1109/TVT.2010.2042185 Existing medium access control (MAC) layer solutions [1]– [6] for WSNs genera lly aim at red uci ng ene rgy wast e due to cha nne l idl e lis ten ing and ov erh ear ing . In mos t of the se solutions, per-hop latency is compromised in favor of energy efficiency, thus deteriorating end-to-end communication delay. Wi thout consideri ng the perfor mance requireme nts at upper layers, an energy-efficient MAC solution for WSN may not be optimal from the holistic networking and applications per- spective, where end-to-end latency is sometimes an important consideration [7]. In this paper, we present a new MAC protocol, which is re- ferred to as hybrid MAC (HMAC), which is suitable for WSNs in terms of energy efficiency, latency, and design complexity. HMA C combi nes chan nel-a lloca tion schemes from exis ting contention-based and time-division multiple-access (TDMA)- bas ed MAC pro toc ols to allow the realization of tradeoffs between different performance metrics. It uses a short slotted frame structure and a novel wakeup scheme to achieve high- energy performance, low delivery latency, and improved channel utili zatio n. Compa red with exis ting TDMA -base d MAC protocols [5], [6], HMAC is simple and scalable since each node does not have to maintain neighborhood information. In addit ion, HMAC prov ides routin g layer coarse-gr ained quali ty-of- servi ce (QoS) support at the MAC layer . T o the best of our knowledge, very few existing MAC layer works handle such QoS issues in WSNs. Quality of service-aware medium access control [17] assigns each flow a channel-access priority to reduce the queuing delay for high-priority flows, bu t it still suf fer s fro m a lon g end-to-en d del ay . The MAC protocols presented in [12]–[15] reduce the end-to-end delivery latency while increasing control overhead without considering diff erent performanc e dema nds betwe en flows. The HMA C design presents an extremely low-cost solution compared with the designs proposed in [12]–[15]. Our simulation results on ns-2 [9] show that HMAC outperforms sensor-MAC (S-MAC) and routing-enhanced MAC (RMAC) in terms of per-hop latency and delivery ratio while still maintaining superior energy performance. II. RELATED WOR K Dif ferent WSN appli catio ns hav e their own performanc e demands for the underlying MAC protocols. A significant body of the existing research in WSNs have mainly explored energy- efficient MAC techniques, such as channel duty cycling [1], [2], low power listening [4], and distributed TDMA-based MAC solutions [5], [6]. 0018-9545 /$26.00 © 2010 IEEE

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