An Energy Efficient MAC Protocol for Wireless Sensor Networks “S-MAC” Wei Ye, John Heidemann, Deborah Estrin Presentation: Deniz Çokuslu May 2008

Outline Motivation S-MAC Experiments Results Conclusion Questions

Motivation WSN’s are battery operated, disposable(!) devices Energy consumption is an important issue Communication dominates most of the energy consumption Medium Access is very critical

Motivation Medium Access Control (MAC)  Allows accessing transmission media  Should avoid collisions TDMA  Reserves time slices to nodes (Time Synchronization) CDMA  Transmission is coded into signals (Extra computation) FDMA  Nodes are assigned to different frequencies (Complex Xmitters) *CSMA  Media is shared (Collisions)

Motivation Contention-based protocols  MACA + RTS/CTS are used - Hidden & Exposed Terminal Problems  T-MAC + RTS/CTS, TimeOut - Early Sleeping  Preamble sampling Periodic sleeping, Long Preamble to wake up receiver  B-MAC Low Power Listening  PAMAS Use of Control Channel (RTS/CTS/Busy Tone)

Motivation IEEE  RTS/CTS  Fragmentation support  CTS only reserves medium for the first fragment  ACK of the first fragment reserves medium for the second and so forth.  Overhearing problems  Long delays because of the fragmentation faults  Idle listening

Challenges *Energy efficiency *Scalability *Fairness Latency Throughput Bandwidth utilization

Sources of Energy Waste in WSNs Collisions  Causes retransmission of packets Overhearing  Receiving others’ packets Control packet overheads  Forms some part of the transmission Idle listening  Keeping the receiver powered on while the node is idle  Consumes %50 - %100 of energy required for receiving

S-MAC

Contributions of S-MAC Periodic listen/sleep  Nodes listens and sleep periodically  Reduces idle listening Collision and overhearing avoidance  Uses RTS/CTS and carrier sense  Puts a node into sleep if its neighbors are communicating  Eliminates overhearing problem Message passing  Divides the message into fragments and send them in burst  Reduces control overhead

Periodic Listen / Sleep Sleep some time  Turn off the radio  Set the timer Wake up  Listen to see if there is a request for communication Requires periodic synch among neighbors  Relative timestamps  Small time slots

Periodic Listen / Sleep Synchronization  Neighbor nodes exchange their sleep/listen schedules  Two neighbor nodes may have different schedules  All nodes know their neighbors’ schedule  When a node wants to send a message, it waits until receiver wakes up  If multiple nodes want to send, they contend the media using RTS/CTS

Periodic Listen / Sleep Synchronization  Each node maintains a schedule table  Choosing schedule Listens if any neighbor broadcast a schedule If it receives a schedule, it uses as its own schedule (follower node) and broadcasts it as a SYNC message If no schedule is received, choose the schedule randomly and broadcasts it as a SYNC message (synchronizer node) If the node receives a schedule after selecting its own, it uses both (multiple schedule)

Periodic Listen / Sleep Maintaining Synchronization  Periodically send SYNC packets SYNC packets are small, and includes sender’s next sleep time  Listen interval is divided into two: Listening SYNC Listening RTS  If a node wants to send a SYNC or Data Wait until receiver wakes Start carrier sense and sense till random time If no xmission, send RTS and then data

Collision and Overhearing Avoidance Collision Avoidance  Carrier sense, RTS and CTS are used  There is a “duration” field in all packets  If a node receives a packet destined to another node, it records the duration into Network Allocation Vector (NAV)  This value is decremented when the NAV timer is fired  If this value is greater than 0, it means that the medium is busy (virtual carrier sense)

Collision and Overhearing Avoidance Collision Avoidance  Physical carrier sense is performed at the physical layer  If both physical and virtual carrier sense indicates that medium is free than node uses the medium

Message Passing Deals with transfering long messages efficiently Options  Long Messages: + Low control overhead - High cost of retransmitting in case of errors  Fragmented Messages: + Low cost of retransmission - High control overhead

Message Passing S-MAC Approach:  Small fragments of messages Cure to control overhead:  Send fragments burstly  Use only one RTS/CTS packet for the whole message  All fragments are confirmed by ACK messages If any fragment is failed:  Extend the reserved transmission time  Retransmit the current fragment  Hidden terminals learn the extension from the ACK packets  Extensions are limited to a pre-defined number

S-MAC Trade-offs Delays inherent to multi-hop contention based network protocols (i.e. IEEE802.11)  Carrier sense delay  Backoff delay  Transmission delay  Propagation delay  Processing delay  Queueing delay Additional delay of S-MAC  Sleep delay

S-MAC Trade-offs Avg sleep delay  A cycle consisting of sleep and listen is called a frame T frame = T listen + T sleep D s = T frame / 2 Energy saving E s =T sleep / T frame Sleep Listen Frame

Implementation Rene Motes are used  Transmission rate: 19,2 Kbps  Energy consumption: Receiving/Idle: 4,5 mA Transmitting: 12 mA Sleeping: 5 μA Motes use TinyOS Standard Packets in TinyOS  Header: 6B  Payload: 30B  CRC: 2B  Total: 38B

Implementation Additional packet type: Control Packets (RTS, CTS, ACK)  Header: 6B  Payload: NA (Benefit: 30 Bytes)  CRC: 2B  Total: 8B

Implementation 3 MAC Modules are implemented for comparison  Simplified IEEE DCF  Message Passing with overhearing avoidance  S-MAC

Implementation Simplified IEEE  Carrier Sense Random duration  Backoff and retry Random backoff (unlike standard IEEE Exponential)  RTS/CTS/DATA/ACK packet exchange  Fragmentation support (Burst mode)  Nodes are either in listen/receive or transmitting mode

Implementation Message Passing with overhearing avoidance  Reduces control overhead  Eliminates overhearing  No periodic sleep, therefore no additional delay to IEEE  Node goes to sleep if its neighbors are communicating

Implementation S-MAC  Listen time: 300 mSec  Sleep time: 1000 mSec  Schedule update (SYNC): 10 frames (13 sec)

Experiments Used Scenario:  Messages: A  C  D B  C  E  Energy consumption is measured  Inter arrival period of messages varies between 1s – 10s Ex. If period is 5s then a message is generated in every 5 sec by each source node

Experiments Used Scenario:  Each source generates 10 messages  Each message consists of 10 fragments  Total of 200 packets are exchanged in each experiment  In the higher traffic scenario inter-arrival: 1s, the wireless media is fully utilized

Experiments Calculations of energy consumption:  Time to pass packets is measured  Percentage of Listen – Sleep is measured  Consumption is calculated by multiplying the time with the required energy Receive/Listen: R = 13,5 mW Transmit: T = 24,75 mW Sleep: S = 15 μW  Total = (R * T listen ) + (T * T transmit ) + (S * T sleep )

Experiments Measured energy consumption in the source nodes

Experiments Measured percentage of time that the source nodes in the sleep mode

Experiments Measured energy consumption in the intermediate node

Conclusions S-MAC is an energy efficient mac protocol compared to IEEE Trade-off between energy consumption and latency can be easily tuned Experiments show that the aimed results are satisfied

Acknowledgements This work is supported by:  NSF under grant ANI as the SCOWR project (  DARPA under grant DABT as the SCADDS project ( and under contract N C-8066 as the SAMAN project ( via the Space and Naval Warfare Systems Center San Diegohttp://

Questions...