Research Article A Hybrid and Secure Priority-Guaranteed MAC Protocol for Wireless Body Area Network Sana Ullah, Muhammad Imran, and Mohammed Alnuem College of Computer and Information Sciences, King Saud University, Riyadh 11543, Saudi Arabia Correspondence should be addressed to Sana Ullah; sanajcs@gmail.com Received 26 November 2013; Accepted 7 January 2014; Published 23 February 2014 Academic Editor: Honggang Wang Copyright © 2014 Sana Ullah et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Tis paper introduces a hybrid and secure MAC protocol (PMAC) for WBAN. Te PMAC protocol uses two contention access periods (CAPs) for accommodating normal and life-critical trafc and one contention-free period (CFP) for accommodating large amount of data packets. Te priority-guaranteed CSMA/CA procedure is used in the CAP period, where diferent priorities are assigned to WBAN nodes by adjusting the backof window size. In addition, a set of security keys is used to prevent illegal access to the network. Analytical expressions are derived to analyze the average delay, power consumption, throughput, and packet loss probability of the PMAC protocol. Results derived from these expressions are validated by computer simulations. 1. Introduction Wireless body area networks (WBANs) are captivating sig- nifcant attention from research and industrial communities. Tey provide unprecedented and limitless opportunities for medical and nonmedical applications [1, 2]. Medical appli- cations employ implantable or wearable tiny devices in, on, or around the human body for continuous health moni- toring such as predicting contingency abnormal condition and treatment of chronic diseases including diabetes and hypertension. Non-medical applications include interactive body computing, information exchange, social networking, monitoring forgotten things, entertainment, monitoring of athletes, education, or assessing soldier fatigue and battle readiness. Most of these applications require WBAN to stay operational for a longer period of time which necessitates power-efcient network operation. For example, a diabetic patient requires round-the-clock real-time glucose monitor- ing over a period of years to decades in order to avoid catastrophic consequences such as stroke or a heart attack that may result in death. Tis requires WBAN node to continuously monitor and report glucose level for timely action. Nodes of WBAN primarily rely on low power batteries that hinder their operation over longer period of time. Terefore, power-efcient network operation is extremely crucial for prolonging network lifetime. Medium access control (MAC) protocols play a criti- cal role in extending network lifetime by controlling the dominant sources of energy waste such as packet collisions, overhearing, idle listening, and control packet overhead. Most of the existing MAC protocols for WBAN are catego- rized into contention- and schedule-based MAC protocols. In contention-based MAC protocols, nodes compete for the channel to transmit the data; carrier sensor multiple access with collision avoidance (CSMA/CA) protocol is the best example of contention-based MAC protocols where the nodes contend for the channel using a random backof period. Tese protocols are scalable and do not require estab- lishing infrastructure. However, they are unable to handle emergency situations where multiple nodes (with the same priorities) are triggered to send life-critical data. Schedule- based MAC protocols divide the channel into multiple fxed or dynamic slots, which are used for data transmission. Tese protocols reduce idle listening and overhearing; however, they incur signifcant protocol overhead. Tis paper proposes a hybrid and secure MAC pro- tocol (PMAC) for WBAN. Te PMAC protocol supports two contention access periods (CAPs) and one contention free period (CFP). In the CAPs, the operation is mainly based on a priority-guaranteed CSMA/CA procedure, where diferent WBAN nodes are assigned diferent priorities. Tese priorities are adjusted by tuning the contention window Hindawi Publishing Corporation International Journal of Distributed Sensor Networks Volume 2014, Article ID 481761, 7 pages http://dx.doi.org/10.1155/2014/481761