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Wireless networks - Lecture 35: MAC protocols for WSN. The main topics covered in this chapter include: challenges in WSNs; attributes of MAC protocol; overview of MAC protocols; energy efficiency in MAC; proposed routing protocol; QoS framework; network monitoring and management;...
Wireless Networks Lecture 35 MAC Protocols for WSN Part II Dr Ghalib A Shah Outlines Challenges in WSNs Attributes of MAC Protocol Overview of MAC protocols Energy Efficiency in MAC Proposed Routing Protocol ► ► ► ► ► ► S-MAC T-MAC DS-MAC Traffic Adaptive MAC DMAC Contention-Free MAC Last Lecture Introduction to WSN Applications of WSN Factors Influencing Performance of WSN ► Power consumption, fault tolerance, scalability, topology, cost Architecture and Communication Protocols Research Directions Topology Control Coverage Data Aggregation Temporal/Spatial Correlation Localization / Synchronization Energy Efficient Data Dissemination QoS Framework Network Monitoring and Management How to integrate WSNs into NGWI ? Simulation for Sensor Networks Simulation provides : Controlled , Reproducible testing environment Cost – effective alternative Means to explore and improve design space TinyOS The role of any operating system (OS) is to promote development of reliable application software by providing a convenient and safe abstraction of hardware resources Wireless sensor networks (WSNs) are embedded but general-purpose, supporting a variety of applications, incorporating heterogeneous components, and capable of rapid deployment in new environments An open-source development environment ► A programming language and model (NesC) TOSSIM for simulating TinyOS TinyDB for Sensor DB in TinyOS Introduction Important attributes of MAC protocols ► Collision avoidance • Basic task — medium access control ► Energy efficiency ► Scalability and adaptivity • Number of nodes changes overtime ► ► ► ► Latency Fairness Throughput Bandwidth utilization Overview of MAC protocols Contention-based protocols ► CSMA — Carrier Sense Multiple Access • Ethernet • Not enough for wireless (collision at receiver) C A B Hidden terminal: A is hidden from C’s CS ► MACA — Multiple Access w/ Collision Avoidance • RTS/CTS for hidden terminal problem • RTS/CTS/DATA Overview of MAC Protocols Contention-based protocols (contd.) ► MACAW — improved over MACA • RTS/CTS/DATA/ACK • Fast error recovery at link layer ► IEEE 802.11 Distributed Coordination Function • Largely based on MACAW Protocols from voice communication area ► TDMA — low duty cycle, energy efficient ► FDMA — each channel has different frequency ► CDMA — frequency hopping or direct sequence Energy Efficiency in MAC Design Energy is primary concern in sensor networks What causes energy waste? ► ► ► ► ► Collisions Control packet overhead Overhearing unnecessary traffic Overemitting Dominant in sensor nets Long idle time • bursty traffic in sensor-net apps • Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten) 10 S-MAC: Coordinated Sleeping (3) Adaptive Lis te ning – Lo wduty c yc le to ac tive mo de * OverhearingListen nodes – wakeup at the end of the Listen current transmission (duration field in RTS/CTS) R Sender Receiver ON RTS DATA CTS ACK Sleep (based on RTS) Overhearing nodes (ON) Sleep (based on CTS) Wakes up even though it is not the correct listeninterval Not all receiver’s nexthop nodes can hear the transmission, if adaptive 15 Drawbacks of S-MAC Active (Listen) interval – long enough to handle to highest expected load ► If message rate is less – energy is still wasted in idle-listening S MAC fixe d duty c yc le – is NOT OPTIMAL Hig h Late nc y Normal S-MAC Sleep Active Sleep Active Sleep 16 Active T-MAC: Preliminaries Adaptive duty cycle: Active Active Active Sleep TA Sleep TA TA A node is in active mode until no activation event occurs for time TA ► Periodic frame timer event, receive, carrier sense, send-done, knowledge of other transmissions being ended Communication ~=S-MAC/802.11 Frame schedule maintenance ~=S-MAC 17 T-MAC: Choosing TA Requirement: a node should not sleep while its neighbors are communicating, potential next receiver TA > C+R+T ► C – contention interval length; ► R – RTS packet length; ► T – turn-around time, time bet end of RTS and start of CTS; TA =1.5 * (C+R+T); 18 Prons ► Performs better under variable traffic load Cons ► Higher overheads than SMAC to maintain variable wakeup schedule ► Unfairness and unpredictable delay 19 Dynamic Sensor-MAC (DSMAC) TMAC improves the latency in SMAC at cost of complexity DSMAC provides simple solution to static duty cycle All nodes start with same duty cycle If one-hop latency is observed higher by receiver, it doubles its duty cycle Nodes share their one-hop latency values with neighbors during SYNC period The transmitter also doubles its duty cycle if the destination reported higher one-hop latency This change will not affect the schedule of other neighbors 20 DSMAC Schduling 21 Traffic-Adaptive MAC (TRAMA) Time is divided into random-access and scheduled-access (transmission) periods The random-access period is used to establish two-hop topology information MAC layer can calculate the transmission duration needed, which is denoted as S CHEDULE_INTER VAL the node calculates the number of slots for which it will have the highest priority among two-hop neighbors The node announces the slots it will use as well as the intended receivers for these slots with a s che dule packe t the node announces the slots for which it has the highest priority but it will not use The schedule packet indicates the intended receivers using a bitmap whose length is equal to the number of its neighbors 22 Ad v antag e s ► Higher percentage of sleep time and less collision probability are achieved, as compared to CSMAbased protocols ► Since the intended receivers are indicated by a bitmap, less communication is performed for the multicast and broadcast types of communication patterns, compared to other protocols Dis ad v antag e s ► Transmission slots are set to be seven times longer than the random-access period This means that without considering the transmissions and receptions, the duty cycle is at least 12.5 percent (idle time), 23 DMAC Supports convergecast communication model, Data-aggregation tree is formed from sources to sink node It is an improved slotted ALOHA algorithm Slots are allotted according to the level of tree from leaf to root It incurs low latency but no collision avoidance for nodes at same level 24 DMAC A minimum period u consists of one packet tx and rx Wakeup period in three is skewed depending on depth d so du is the wakeup time Node at higher layer will be in rx state when lower layer nodes are in tx state Nodes on path wakeup sequentially to forward packet to next hop: low latency 25 with efficient energy consumption Contention-Free MAC protocols for Wireless Sensor Networks Asynchronous Slot Assignment ► ► ► Each node locally dis cre tiz e s its local time The number of slots in a time frame, called the fram e s iz e and denoted by , is set to 2 Having the same frame size at all nodes ensures that overlapping time slots remain the same i i 26 ASAND – Basic Approach Select random slot Transmit beacon at Report conflicts between neighbors Listen for slots YES Conflict NO Nonready Obtain slot Ready 27 ASAND – Conflict Reporting u w v The 2-hop neighbors u and v are unaware that they have selected conflicting time slots (their transmissions collide on w) Having observed a collision in its local time t, node w transmits at time t+ , creating a s purious conflict with both u and v This is called conflict re porting essentially reduces a conflict between hidden terminals to a conflict between neighbor nodes After t+ , u and v will be forced to select new slots 28 Summary Challenges in WSNs Attributes of MAC Protocol Overview of MAC protocols Energy Efficiency in MAC Proposed Routing Protocol ► ► ► ► ► ► S-MAC T-MAC DS-MAC Traffic Adaptive MAC DMAC Contention-Free MAC 29 ... Challenges in WSNs Attributes of MAC Protocol Overview of MAC protocols Energy Efficiency in MAC Proposed Routing Protocol ► ► ► ► ► ► S -MAC T -MAC DS -MAC Traffic Adaptive MAC DMAC Contention-Free MAC Last... forced to select new slots 28 Summary Challenges in WSNs Attributes of MAC Protocol Overview of MAC protocols Energy Efficiency in MAC Proposed Routing Protocol ► ► ► ► ► ► S -MAC T -MAC. .. ► MACA — Multiple Access w/ Collision Avoidance • RTS/CTS for hidden terminal problem • RTS/CTS/DATA Overview of MAC Protocols Contention-based protocols (contd.) ► MACAW — improved over MACA