MAC Layer Protocols for Sensor Networks

Transcription

1 MAC Layer Protocols for Sensor Networks Leonardo Leiria Fernandes Compiled by C. Pham for educational purposes.background color has been modified in this presentation. Some slides were also added

2 l Basic Concepts l S-MAC l T-MAC l B-MAC l P-MAC l Z-MAC Contents

3 S-MAC - Sensor MAC l Nodes periodically sleep l Trades energy efficiency for lower throughput and higher latency l Sleep during other nodes transmissions Listen Sleep Listen Sleep t

4 Periodic Listen and Sleep l If no sensing event happens, nodes are idle for a long time l So it is not necessary to keep the nodes listening all the time l Each node go into periodic sleep mode during which it switches the radio off and sets a timer to awake later l When the timer expires it wakes up and listens to see if any other node wants to talk to it Added slide by C. Pham

5 l If a node A wants to talk to node B, it just waits until B is listening l If multiple neighbors want to talk to a node, they need to contend for the medium l Contention mechanism is the same as that in IEEE (using RTS and CTS) l After they start data transmission, they do not go to periodic sleep until they finish transmission Added slide by C. Pham

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

7 l If a node receives a schedule from a neighbor before choosing its own schedule, it just follows this neighbor s schedule. l This node is called a FOLLOWER and it waits for a random delay and broadcasts its schedule. l 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. Added slide by C. Pham

8 Maintaining Synchronization l Timer synchronization among neighbors are needed to prevent the clock drift. l Done by periodic updating using a SYNC packet. l Updating period can be quite long as we don t require tight synchronization. l Synchronizer needs to periodically send SYNC to its followers. l If a follower has a neighbor that has a different schedule with it, it also needs update that neighbor. Added slide by C. Pham

9 l Time of next sleep is relative to the moment that the sender finishes transmitting the SYNC packet l Receivers will adjust their timer counters immediately after they receive the SYNC packet l Listen interval is divided into two parts: one for receiving SYNC and other for receiving RTS Added slide by C. Pham

10 Timing Relationship of Possible Situations Added slide by C. Pham

11 S-MAC Results Latency and throughput are problems, but adaptive listening improves it significantly

12 S-MAC Results l Energy savings significant compared to non-sleeping protocols

13 T-MAC - Timeout MAC l Transmit all messages in bursts of variable length and sleep between bursts l RTS / CTS / ACK Scheme l Synchronization similar to S-MAC

14 T-MAC Operation

15 T-MAC Results l T-MAC saves energy compared to S-MAC l The early sleeping problem limits the maximum throughput l Further testing on real sensors needed

16 B-MAC - Berkeley MAC l B-MAC s Goals: n Low power operation n Effective collision avoidance n Simple implementation (small code) n Efficient at both low and high data rates n Reconfigurable by upper layers n Tolerant to changes on the network n Scalable to large number of nodes

17 B-MAC s Features l Clear Channel Assessment (CCA) l Low Power Listening (LPL) using preamble sampling l Hidden terminal and multi-packet mechanisms not provided, should be implemented, if needed, by higher layers Sender Receiver Sleep Sleep Preamble Sleep Message Receive t t

18 B-MAC Interface l CCA on/off l Acknowledgements on/off l Initial and congestion backoff in a per packet basis l Configurable check interval and preamble length

19 B-MAC Lifetime Model E = E rx + E tx + E listen + E d + E sleep E rx = t rx c rxb V E tx = t tx c txb V E listen " E sample 1 t i E d = t d c data V E sleep = t sleep c sleep V l E can be calculated if hardware constants, sample rate, number of neighboring nodes and check time/ preamble are known l Better: E can be minimized by varying check time/ preamble if constants, sample rate and neighboring nodes are known

20 B-MAC Results l Performs better than the other studied protocols in most cases l System model can be complicated for application and routing protocol developers l Protocol widely used because has good results even with default parameters

21 P-MAC - Pattern MAC l Patterns are 0*1 strings with size 1-N l Every node starts with 1 as pattern l Number of 0 s grow exponentially up to a threshold δ and then linearly up to N-1 l TR = CW + RTS + CTS + DATA + ACK l N = tradeoff between latency and energy

22 Patterns vs Schedules Local Packet to Receiver Local Pattern Bit Send Pattern Bit Schedule * * 0

23 P-MAC Evaluation l Simulated results are better than SMAC l Good for relatively stable traffic conditions l Adaptation to changes on traffic might be slow l Loose time synchronization required l Needs more testing and comparison with other protocols besides S-MAC

24 Z-MAC - Zebra MAC l Runs on top of B-MAC l Combines TDMA and CSMA features CSMA l Pros n Simple n Scalable l Cons n Collisions due to hidden terminals n RTS/CTS is overhead TDMA l Pros n Naturally avoids collisions l Cons n Complexity of scheduling n Synchronization needed

25 Z-MAC Initialization l Neighborhood discovery through ping messages containing known neighbors l Two-hop neighborhood used as input for a scheduling algorithm (DRAND) l Running time and message complexity of DRAND is O(δ), where δ is the two-hop neighborhood size l The idea is to compensate the initialization energy consumption during the protocol normal operation

26 Z-MAC Time Slot Assignment 2 a"1 # F i < 2 a "1 l2 a + s i ( for : l = 0,1,2...)

27 Z-MAC Transmission Control The Transmission Rule: l If owner of slot n Take a random backoff within To n Run CCA and, if channel is clear, transmit l Else n Wait for To n Take a random backoff within [To,Tno] n Run CCA and, if channel is clear, transmit

28 Z-MAC HCL Mode l Nodes can be in High Contention Level (HCL) l A node is in HCL only if it recently received an Explicit Contention Notification (ECN) from a two-hop neighbor l Nodes in HCL are not allowed to contend for the channel on their two-hop neighbors time slots l A node decides to send an ECN if it is losing too many messages (application ACK s) or based on noise measured through CCA

29 Z-MAC Receiving Schedule l B-MAC based l Time slots should be large enough for contention, CCA and one B-MAC packet transmission l Slot size choice, like in B-MAC, left to application

30 Z-MAC Results l Z-MAC performs better than B-MAC when load is high l As expected, fairness increases with Z-MAC l Complexity of the protocol can be a problem

31 Conclusions l Between the protocols studied, B-MAC still seems to be the best one for applications in general l Application developers seem not to use B-MAC s control interface l Middleware service could make such optimizations according to network status

32 Thank You Questions or comments? Thank you for coming!