1 A Study of Medium Access Control Protocols for Wireless Body Area Networks Sana UllahΨ, Bin Shen, S.M.Riazul Islam, Pervez Khan, Shahnaz Saleem, and Kyung Sup Kwak Abstract—The seamless integration of low-power, miniaturised, invasive/non-invasive lightweight sensor nodes have contributed to the development of a proactive and unobtrusive Wireless Body Area Network (WBAN). A WBAN provides long-term health monitoring of a patient without any constraint on his/her normal dailylife activities. This monitoring requires low-power operation of invasive/non-invasive sensor nodes. In other words, a power-efficient Medium Access Control (MAC) protocol is required to satisfy the stringent WBAN requirements including low-power consumption. In this paper, we first outline the WBAN requirements that are important for the design of a low-power MAC protocol. Then we study low-power MAC protocols proposed/investigated for WBAN with emphasis on their strengths and weaknesses. We also review different power-efficient mechanisms for WBAN. In addition, useful suggestions are given to help the MAC designers to develop a low-power MAC protocol that will satisfy the stringent WBAN requirements. Index Terms—WBAN, MAC, Low Power, Survey, IEEE 802.15.6. F 1 INTRODUCTION Some of the common objectives in WBAN are A Wireless Body Area Network (WBAN) al- to achieve maximum throughput, minimum lows the integration of intelligent, miniatur- delay, and to maximize the network lifetime by ized, low- power, invasive/non-invasive sensor controlling the main sources of energy waste, nodes in/around a human body that are used i.e., collision, idle listening, overhearing, and to monitor body functions and the surrounding control packet overhead. A collision occurs environment. Each intelligent node has enough when more than one packet is transmitted at capability to process and forward information the same time. The collided packets have to to a base station for diagnosis and prescription. be retransmitted, which consumes extra energy. A WBAN provides long term health moni- The second source of energy waste is idle toring of patients under natural physiological listening, meaning that a node listens to an states without constraining their normal activ- idle channel to receive data. The third source arXiv:1004.3890v2 [cs.NI] 31 Jul 2010 ities. It is used to develop a smart and afford- is overhearing, i.e., to receive packets that are able health care system and can be a part of destined to other nodes. The last source is diagnostic procedure, maintenance of a chronic control packet overhead, meaning that con- condition, supervised recovery from a surgical trol information are added to the payload. A procedure, and can handle emergency events. minimal number of control packets should be used for data transmission. Medium Access • S.Ullah, B.Shen, S.M.R. Islam, P. Khan, and K.S. Kwak are with Control (MAC) protocols play an important the Graduate School of Telecommunication Engineering, Inha role in solving the aforementioned problems. University, Incheon (402-751) South Korea. Generally they are grouped into contention- • Ψ. Author to whom correspondence should be addressed E-mail: based and schedule-based MAC protocols. In [email protected] contention-based MAC protocols such as Car- • S. Saleem is with the Graduate School of Computer Science and rier Sense Multiple Access/Collision Avoid- Engineering, Inha University, Incheon (402-751) South Korea. ance (CSMA/CA) protocols, nodes contend for Manuscript received November 26, 2009; revised December 7, 2009; the channel to transmit data. If the channel published (Sensors, vol. 10, no. 1: 128-145.) December 20, 2009. is busy, the node defers its transmission until 2 TABLE 1 are given to validate the discussion. CSMA vs. TDMA Protocols The rest of the paper is categorized into four Performance Metric CSMA/CA TDMA sections. Section 2 presents the WBAN require- Power consumption High Low ments. A study of different low-power MAC Traffic level Low High protocols proposed/investigated for WBAN is Bandwidth utilisation Low Maximum given in Section 3. Section 4 reviews various Scalability Good Poor power-efficient mechanisms for WBAN with Effect of packet failure Low Latency useful guidelines. The final section concludes our work. Synchronisation Not Applicable Required 2 WBAN MAC REQUIREMENTS it becomes idle. These protocols are scalable The most important attribute of a good MAC with no strict time synchronization constraint. protocol for WBAN is energy efficiency. In However, they incur significant protocol over- some applications, the device should support head. In schedule-based protocols such as Time a battery life of months or years without inter- Division Multiple Access (TDMA) protocols, vention, while others may require a battery life the channel is divided into time slots of fixed of only tens of hours due to the nature of the or variable duration. These slots are assigned to applications. For example, cardiac defibrillators nodes and each node transmits during its slot and pacemakers should have a lifetime of more period. These protocols are energy conserving than 5 years, while swallowable camera pills protocols. The duty cycle of radio is reduced have a lifetime of 12 hours [6]. Power-efficient and there is no contention, idle listening and and flexible duty cycling techniques are re- overhearing problems. These protocols, how- quired to minimize the idle listening, overhear- ever, require frequent synchronization. Table 1 ing, packet collisions and control packet over- compares CSMA/CA and TDMA protocols. head problems. Furthermore, low duty cycle The development of a low-power MAC pro- nodes should not receive frequent synchroniza- tocol for WBAN has been a hot research tion and control information (beacon frames) if topic for the last few years. Considerable re- they have no data to send or receive. search efforts have been dedicated to pro- The WBAN MAC should support simulta- pose/investigate new MAC protocols that neous operation on in-body (Medical Implant could satisfy the crucial WBAN requirements. Communications Service, also called MICS) A number of researchers have considered IEEE and on-body frequency bands/channels [In- 802.15.4 [1] for WBAN since it supports low dustrial, Scientific and Medical (ISM) or Ultra data rate applications, but it is not enough to Wide Band (UWB)] at the same time. In other support high data rate applications (data rate words, it should support Multiple Physical lay- > 250 Kbps). Other protocols such as Heartbeat ers (Multi-PHYs) communication. Other impor- Driven MAC (H-MAC) [2], Reservation-based tant factors are scalability and adaptability to Dynamic TDMA (DTDMA) [3], Preamble- changes in the network, delay, throughput, and based TDMA (PB-TDMA) [4], and BodyMAC bandwidth utilization. Changes in the network [5] protocols have been proposed/investigated topology, the position of the human body, and in the existing literature. In this paper, an the node density should be handled rapidly overview of the aforesaid protocols with focus and successfully. The MAC protocol for WBAN on their strengths and weaknesses is presented. should consider the electrical properties of the Useful suggestions are given to overcome the human body and the diverse traffic nature of weaknesses of these protocols. Then, a compar- in-body and on-body nodes. For example, the ative analysis of many power-efficient mech- data rate of in-body nodes varies, ranging from anisms such as Low-power Listening (LPL), few kbps in pacemaker to several Mbps in scheduled-contention, and TDMA mechanisms capsular endoscope. Fig. 1 shows some of the is presented in the context of WBAN. Examples potential issues of a MAC protocol for WBANs. 3 Fig. 1. Potential issues of a MAC protocol for WBAN The Quality of Service (QoS) is also an im- tocol, the nodes are required to perform Clear portant factor of a good MAC protocol for Channel Assessment (CCA) before transmis- WBAN. This includes point-to-point delay and sion. However, the CCA is not always guar- delay variation. In some cases, real-time com- anteed in the MICS band since the path loss munication is required for many applications inside the human body due to tissue heating such as fitness and medical surgery monitoring is much higher than in free space. Bin et.al applications. For multimedia applications, the studied the unreliability of a CCA in WBAN latency should be less than 250 ms and the and concluded that for a given -85 dBm CCA jitter should be less than 50 ms [6]. How- threshold, the on-body nodes cannot see the ever, the reliability, latency, and jitter require- activity of the in-body nodes when they are at ments depend on the nature of the applications. 3 meters distance away from the surface of the For emergency applications, the MAC protocol body [8]. Sana et al. have studied the behavior should allow in-body or on-body nodes to of the CSMA/CA protocol for WBAN and con- get quick access to the channel (in less than cluded that the CSMA/CA protocol encounters one second) and to send the emergency data heavy collision problems for high traffic nodes to the coordinator. One such example is the [9]. TDMA-based protocols provide good solu- detection of irregular heartbeat, high or low tions to the traffic correlation, heavy collision, blood pressure or temperature, and excessively and CCA problems. These protocols are energy low or high blood glucose level in a diabetic conserving protocols because the duty cycle is patient. Another example is when the node reduced and there are no contention, idle listen- is dying. Reporting medical emergency events ing, and overhearing problems. However, com- should have higher priority than non-medical mon TDMA needs extra energy for periodic emergency (battery dying) events. time synchronization. All the sensors (with In WBAN, most of the traffic is correlated, and without data) are required to receive the i.e., a patient suffering from fever triggers tem- periodic packets in order to synchronize their perature, blood pressure, and respiration sen- clocks.