2008 Adaptive Medium Access Control (MAC) for Heterogeneous Mobile Wireless Sensor Networks (WSNs). Giorgio Corbellini 1
Challenges of the Ph.D. Study of urgency in sensed data Study of mobility in WSNs Definition of heterogeneity-awareness and its introduction at MAC layer 2
Adaptability in WSN An adaptive MAC protocol is able to modify in real time some of its parameters with the scope of matching some network requirements. Several existing MAC protocols for WSN can adapt themselves, but they are adaptive to what? Energy-adaptive, Mobility-adaptive.. 3
Adaptability in WSN Different perspective, the MAC protocol must be: Reactive to the evolving heterogeneity state of the network, Able to auto-configure the network for the new state. Heterogeneity-adaptive 4
Heterogeneity in WSN WSNs are heterogeneous by nature. Some examples: Heterogeneity of sources of information; Heterogeneity of applications; Heterogeneity of mobility of nodes; Heterogeneity of quantity and frequency of information provided Heterogeneity of urgency of the information provided Energy heterogeneity (battery level of nodes, linepowered nodes ) Computational heterogeneity of nodes (type of nodes, density per area) Link heterogeneity (some nodes have long-distance highly reliable links) 5
WSN limitations Most of the times nodes of a WSN have not direct connection with line-power and take energy from batteries. Provided that recharging batteries is often not an option, battery life duration is the main limitation of WSN. Consume of energy must be optimized while wastages must be minimized. Sources of energy consumption are: Processing part (CPU), The wireless interface (radio) that consumes energy. Major sources of Energy wastage are: Idle listening - waste of energy in keeping the radio on when it is not necessary Collisions - waste of energy when two packets collide Overhearing - waste of energy in receiving a packet destined to another node Protocol overhead - MAC headers and excessive control messages are considered energy wastage 6
Medium Access Control The Medium Access Control (MAC) layer sits directly on top of the physical layer and controls the radio (when to send a packet and when to listen for a packet). Unlike from other wireless technologies in which QoS constraints are considered in MAC protocols, MAC protocols for WSNs are mainly focused on energy efficiency. Indeed, most of the protocol propositions in the literature neglect aspects as: fairness between nodes, latency of data, throughput. 7
Medium Access Control Access to the medium can be: Centralized: a base-station manages the access. Distributed: the access is granted by an accessing rule shared by nodes Centralized access: No collisions Simple to implement Most of the complexity load is charged by the base-station Distributed access: Necessary when network size is great. Both techniques can be adopted in WSN but most of the MAC protocols in the literature use distributed rules (lower energy consumption). 8
Medium Access Control Both centralized and distributed MAC protocols for WSN can further be classified according to the wireless access scheme in: Schedule-base protocols, Contention-based protocols, Hybrid-protocols. 9
Medium Access Control: the MAC soup [1] 10
Schedule-based protocols In such protocols a schedule exists, regulating which node may use which resource at which time. Schedule can be fixed or variable Advantages: Collision-free protocols No idle listening or overhearing Weaknesses: Low scalability, Complex to manage in very populated networks because time synchronization is needed Examples: TDMA, FDMA TRAMA[2] LEACH[3] 11
Contention-based protocols In contention-based protocols nodes that want to transmit compete each other for accessing the media. Advantages: Simple to setup, No synchronization needed, Very scalable. Weaknesses: Probability of packets collisions is always not null, Main sources of energy wastage (collisions, idle listening) must be addressed. Examples: MACAW[4] S-MAC[5], T-MAC[6] Preamble sampling[7], LPL[8], B-MAC[9] PAMAS[10] 12
Hybrid protocols Hybrid approaches combine random access methods with frame-based scheduling. Advantages: Flexibility to adapt to traffic fluctuations. Weaknesses: The use of slotting concentrate most of the connection attempts to the beginning of slots. Examples: Z-MAC[11], Crankshaft[12] 13
Mobility Different levels of mobility Static network Mobile sensors with immobile sink(s) Mobile sensors with mobile sink(s) Mobile sink(s) with immobile sensors Different types fo mobility Passive mobility Active mobility Effective mobility models for WSN are needed Examples A doctor that walks inside the hospital Herding and wildlife monitoring Mobility models for WSNs will be addressed during an Internship at CEA/LETI starting in February/March 2009. 14
Sensor-MAC (S-MAC)[5] Advantages: idle listening is reduced by cycling sleep/wakeup periods Use of virtual clusters Use of in-channel signaling Weaknesses: Does not guarantee high throughput Latency is increased because of Sleep/wakeup cycles Sleep/wakeup cycles duration is fixed (TMAC[6]) 15
Sensor-MAC (S-MAC) MACA s idle listening is particularly unsuitable if average data rate is low Most of the time, nothing happens Idea: Switch nodes off, ensure that neighboring nodes turn on simultaneously to allow packet exchange (rendez-vous) Only in these active periods, packet exchanges happen Need to also exchange wakeup schedule between neighbors When awake, essentially perform RTS/CTS 16
References [1] The MAC Alphabet Soup. URL: http://www.st.ewi.tudelft.nl/~koen/macsoup/ [2]V. Rajendran, K. Obraczka, et J.J. Garcia-Luna-Aceves, Energy-efficient collision-free medium access control for wireless sensor networks, Proceedings of the 1st international conference on Embedded networked sensor systems, Los Angeles, California, USA: ACM, 2003, pp. 181-192. [3] W. Heinzelman, A. Chandrakasan, et H. Balakrishnan, Energy-efficient communication protocol for wireless microsensor networks, System Sciences, 2000. Proceedings of the 33rd Annual Hawaii International Conference on, 2000, p. 10 pp. vol.2. [4]V. Bharghavan, A. Demers, S. Shenker, et L. Zhang, MACAW: a media access protocol for wireless LAN's, Proceedings of the conference on Communications architectures, protocols and applications, London, United Kingdom: ACM, 1994, pp. 212-225. [5] Wei Ye, J. Heidemann, et D. Estrin, An energy-efficient MAC protocol for wireless sensor networks, INFOCOM 2002. Twenty-First Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, 2002, pp. 1567-1576 vol.3. [6]T.V. Dam et K. Langendoen, An adaptive energy-efficient MAC protocol for wireless sensor networks, Proceedings of the 1st international conference on Embedded networked sensor systems, Los Angeles, California, USA: ACM, 2003, pp. 171-180. 17
References [7]A. El-Hoiydi, Aloha with preamble sampling for sporadic traffic in ad hoc wireless sensor networks, Communications, 2002. ICC 2002. IEEE International Conference on, 2002, pp. 3418-3423 vol.5. [8]W. Ye, F. Silva, et J. Heidemann, Ultra-low duty cycle MAC with scheduled channel polling, Proceedings of the 4th international conference on Embedded networked sensor systems, Boulder, Colorado, USA: ACM, 2006, pp. 321-334. [9]J. Polastre, J. Hill, et D. Culler, Versatile low power media access for wireless sensor networks, Proceedings of the 2nd international conference on Embedded networked sensor systems, Baltimore, MD, USA: ACM, 2004, pp. 95-107. [10] S. Singh et C.S. Raghavendra, PAMAS - power aware multi-access protocol with signalling for ad hoc networks, SIGCOMM Comput. Commun. Rev., vol. 28, 1998, pp. 5-26. [11]Injong Rhee, A. Warrier, M. Aia, Jeongki Min, et M. Sichitiu, Z-MAC: A Hybrid MAC for Wireless Sensor Networks, Networking, IEEE/ACM Transactions on, vol. 16, 2008, p. 511-524. [12]G. Halkes et K. Langendoen, Crankshaft: An Energy-. Efficient MAC-Protocol For Dense Wireless Sensor Networks,. 18
S-MAC: simple idea S-MAC with priority enhancement when a node needs to send critical sensed data must have priority with respect to the others Introducing a priority contention period in which the RTS/CTS handshake for urgent data is addressed a node that is conscious of the importance of the data could send an Urgent-RTS (URTS) flollowed by a U-CTS Challenges: heterogeneity-awareness of the nodes consciousness of nodes data processing (information fusion) 19