Introduction to Wireless Sensor Networks

1. Introduction to Wireless Sensor Networks Medium Access Control (MAC) 17 February 2005

2. Organizational Class Website www.engineering.uiowa.edu/~ece195/2005/ Class Time Office Hours

3. Medium Access Control (MAC)Introduction • One Approach (Be nice – share) • Avoid interference by scheduling nodes on sub-channels • TDMA (Time-Division Multiple Access) • FDMA (Frequency-Division Multiple Access) • CDMA (Code-Division Multiple Access) • Another Approach (Compete/contend) • Don’t pre-allocate transmission, compete => probabilistic coordination • ALOHA (Transmit. Collision? Yes, discard packet, retransmit later) • Carrier Sense (IEEE 802.11)

4. MAC Attributes • Basic task of MAC protocol • Collision avoidance/minimization • Energy efficiency • MAC layer controls radio. Radio often consume most energy • Scalability and adaptivity • Nodes join, exit, rejoin, die, move to different location • Good MAC should accommodate such changes • Channel utilization • Very important in cellular or wireless LAN • Often secondary in WSNs (Why?) • Latency • Throughput • Fairness • Important in traditional cellular/wireless LAN, less important in WSNs (Why?)

5. MAC Attributes • For WSNs, most important attributes of a good MAC are • Effective collision avoidance • Energy Efficiency • Scalability and adaptivity • Other attributes are normally secondary • Fairness • Latency • Channel utilization

6. Energy Efficiency in MAC Protocols • Motivation – Energy efficiency is very important in WSNs. • Question – what causes energy waste from a MAC perspective? • Collision • Collided packets are discarded, retransmission require energy • Not a big issue in scheduled (TDMA, CDMA, FDMA) MAC protocols, but an issue in contention MAC protocols. • Idle listening • Long distance (500 m or more) Tx energy consumption dominates, but in short-range communication Rx energy consumption can be close to Tx energy consumption • MICA2 idle:receiving:transmission ratio at 1 mW is 1:1:1.41 @ 433 MHz and 1mW • Can be a dominant factor in WSN energy consumption

7. Energy Efficiency in Mac Protocols • Overhearing • When a node receives packets that are destined for another node • Control packet overhead • Sending, receiving, listening, all consumes energy • Adaptation • Reconfiguring when nodes join leave

8. TDMA Overview Channel is divided into N slots (a frame) • Each node gets a time slot • It only transmits in its time slot • It only need listen during its time slot • Frame my be static – fix number of slots • Frames may be dynamically adjusted • Frequently used in cellular communications (i.e., GSM)

9. TDMA for WSNs Often used in WSNs • Typically, nodes communicate with base station • Major advantage of TDMA – supports low-duty-cycle operations on nodes • Large frames • Nodes only have to listen in their time slot • Low duty-cycle =>low energy consumption

10. TDMA Disadvantages for WSNs • Cluster paradigm (analogous to cell phones) • One node is selected as the cluster head and acts as base station • Nodes communicate only with head • Direct peer-to-peer communication not energy efficient • Nodes must listen on all time slots, reducing energy • Inter-cluster communication requires other MAC protocols • Most important issue is limited scaling • When a new nodes joins the base station must reallocate slots and adjusting the frame size • This can take time and energy to propagate

11. Examples of Scheduled Protocols • Sohrabi & Pottie • Homework/Exam (page 79 in book) • Low-Energy Adaptive Clustering Hierarchy (LEACH) • Organize nodes into cluster hierarchies • TDMA within each cluster • Nodes only talk to node head • Position of head is rotated among nodes depending on remaining energy • Node then uses long-range/high-power communication to base • Nodes don’t need to know global topology • Nodes don’t need control information from base station

12. Scheduled Protocols • Bluetooth • Designed for PAN, but attractive for WSNs • Bluetooth organizes itself into clusters, piconets • Frequency-hopping CDMA is used to handle inter-cluster interference • Within cluster, TDMA MAC protocol • Master-slave approach. Cluster head (master), other nodes are slaves. • Master uses polling to decide which salve can transmit • Only communication between master and one or more slaves are possible • Maximum number of nodes in a cluster is 8

13. Bluetooth

14. Bluetooth Power Classes

15. Energy Conservation in Scheduled MAC Protocols • Collision free • No need for idle listening • TDMA naturally support low-duty cycle operation

16. Review Question • True or false – With respect to WSN MACs, effective collision avoidance is less important than fairness. • Describe and elaborate briefly (three sentences) what we mean by the term “latency” as an attribute of a WSN MAC. • True or false – channel utilization is a crucial attribute of as WSN MAC • True or false – energy waste from collisions are less in contention MAC protocols than in scheduled MAC protocols. • List three scheduled MAC protocols. • True or false – in WSN where low-power, short range radio transmission is used, idle listing contributes very little to the energy consumption budget of a node • Briefly explain the term idle listening in WSNs • One major issue in TDMA MAC protocols that of limited scaling. Explain in a 5-6 sentence paragraph. • Describe the LEACH MAC protocol. • Describe the Sohrabi & Pottie MAC protocol. • What is the fundamental purpose of a MAC protocol?