An Energy-Efficient MAC Protocol for Wireless Sensor Networks Speaker: hsiwei Wei Ye, John Heidemann and Deborah Estrin. IEEE INFOCOM 2002 Page

Outline  Introduction  S-MAC protocol  Evaluation  Conclusion

Introduction  Wireless sensor network  Large number of densely distributed nodes  Battery powered  Multi-hop wireless ad hoc network  Node positions and topology dynamically change  Self-organization  Reducing power consumption  Nodes are normally battery operated and power limited  Its very difficult to change or recharge batteries  Prolonging network lifetime

Introduction  Important attributes of MAC protocols 1. Collision avoidance 2. Energy efficiency 3. Scalability in node density 4. Latency 5. Fairness 6. Throughput 7. Bandwidth utilization Primary Secondary

Sources of Energy Waste  Idle listening-consumes % of the energy required for receiving  Collision - corrupted packets must be retransmitted and it increases energy consumption  Overhearing - node picks up packets that are destined to other nodes  Control Packet Overhead

S-MAC Protocol Four major components of to save energy  Idle listening – periodic listen and sleep reduces energy consumption by avoiding idle listening. Collision – Using RTS and CTS Overhearing – by switching the radio off when the transmission is not meant for that node Control overhead – Message passing

Periodic Listen and Sleep  Basic scheme  Each node go into periodic sleep mode during which it switches the radio off and sets a timer to awake later  When the timer expires it wakes up and listens to see if any other node wants to talk to it

Main progress of listen and sleep  Synchronization  The duration of time for listening and sleeping can be selected according to different application scenarios  To reduce control overhead, neighboring nodes are synchronized (i.e. Listen and sleep together)  Not all neighboring nodes can synchronize together  Nodes exchange their schedules by broadcasting it to all its immediate neighbors. §After they start data transmission, they do not go to periodic sleep until they finish transmission

Choosing and Maintaining Schedules  Each node maintains a schedule table that stores schedules of all its known neighbors.  To establish the initial schedule (at the startup) following steps are followed: 第一 : A node first listens for a certain amount of time.  If it does not hear a schedule from another node, it randomly chooses a schedule and broadcast its schedule immediately.  This node is called a SYNCHRONIZER.

Choosing and Maintaining Schedules 第二 :  If a node receives a schedule from a neighbor before choosing its own schedule, it just follows this neighbor ’ s schedule.  This node is called a FOLLOWER and it waits for a random delay and broadcasts its schedule. 第三 :  If a node receives a neighbor ’ s schedule after it selects its own schedule, it adopts to both schedules and broadcasts its own schedule before going to sleep. (i.e., it schedules itself to wake up at the times of both is neighbor and itself).

Maintaining synchronization  Time synchronization among neighbors are needed to prevent clock drift  Each node periodically broadcasts SYNC packets to its neighbor  Receivers adjust their timers immediately after they receive the SYNC packet Sender Node IDNext-Sleep Time SYNC Packet

Time relationship Randomly

Collision Avoidance  Similar to IEEE using RTS/CTS mechanism  Address the hidden terminal problem  All senders perform carrier sense before initiating a transmission, if a node fails to get the medium, it sleeps and wakes up when the receiver is free and listening again

Overhearing Avoidance  Overhearing problem in IEEE  In IEEE , each node keeps listening all transmissions from its neighbors in order to perform effective virtual carrier sensing, which is a significant waste of energy.(each node overhears a lot of packets that are not directed to itself) Solution to avoid overhearing  Let interfering nodes go to sleep after they hear an RTS or CTS packet,

Overhearing Avoidance  all immediate neighbors of both the sender and the receiver should sleep after they hear the RTS or CTS packet until the current transmission is over  network allocation vector (NAV):virtual carrier sense, The node records this value in an variable  a node should sleep to avoid overhearing if its NAV is not zero. It can wake up when its NAV becomes zero

Message passing  Transmitting a long message as a single packet  the re-transmission cost is high (if only a few bits have been corrupted)  問題 ( IEEE fragmentation) If we fragment the long message into many independent small packets, we have to pay the penalty of large control overhead and longer delay.

Message passing 解決方法 :  Fragment long message into many small fragments and transmit them in a burst Only one RTS and one CTS packet are used to reserve the medium for entire message  Neighboring node hearing a RTS or CTS will go to sleep for the time needed to transmit all the fragments  Each fragment needs ACK, if no ACK is received, it will extend the reserved transmission time for one more fragment and re-transmit the current fragment immediately

Protocol implementation Testbed –Used Rene Motes as the development platform and testbed –TinyOS –3 working modes: receiving, transmitting and sleep Topology used in the experiment –3 MAC modules on the mote and TinyOS platform 1.Simplified IEEE DCF 2.Message passing with overhearing avoidance 3.The complete S-MAC

Average energy consumption in the source nodes

Percentage time source nodes in sleep

Energy consumption in the intermediate node

Conclusions  S-MAC has good energy conserving properties comparing with IEEE