An Adaptive Energy-Efficient MAC Protocol for Wireless ：无线自适应能量有效的MAC协议

1、An Adaptive Energy-Efficient MAC Protocol for Wireless Sensor NetworksAvanthi KoneruCSCE 5933October 24, 2005AgendaIntroductionRelated WorkT-MAC Protocol DesignExperimentsReal ImplementationIntroductionSensor Network StackData Link LayerMultiplexing of data streamsData frame detectionError ControlMe

2、dium Access control The needTraditional MAC protocols maximize packet throughput, minimize latency, provide fairness.Sensor networks minimize energy consumptionCommunication patternsLocal uni - / broadcastNodes to sink reporting MSP430F149 processor with 2KB RAM and 60KB Flask memory115 kbps RFM TR1

3、001 radioSimilar to Berkeley nodesSome TerminologyIdle listening problem: Most energy in traditional MAC protocols is wasted by idle listening.Duty Cycle: The fraction of time a system is actually employed in performing its function. The percent of time that the system is active as opposed to the ti

4、me the system is inactive.Energy waste due to idle listening: Existing SolutionsTDMAMaintaining a TDMA schedule increases complexity.Keeping a list of neighbors schedules takes up valuable memory.Allocation of time slots needs coordination.Time synchronization is quintessential.Extra wake-up radioOp

5、erates on different frequency than communicationRequires additional components.Energy waste due to idle listening: Existing Solutions (.contd)S-MAC:Energy waste due to idle listening: Existing Solutions (.contd)S-MACFixed duty cycle :active state, sleep stateThroughput is reduced because only active

6、 part is used for communication.Latency increases because a message-generating event may occur during sleep time.Other forms of energy wasteCollisionsProtocol OverheadOverhearingT-MAC Design DetailsT-MAC EvolutionCSMA/CDCSMA/CASMACTMACIEEE 802.11IEEE 802.3Carrier Sense Multiple Access with Collision

7、 AvoidanceFixed duty cycleAdaptive duty cycleARCAdaptive Rate Control DMAC/MMACDirectional AntennasCarrier Sense Multiple Access with Collision Detection T(Timeout)-MAC Protocol DesignEvery node periodically wakes up to communicate with its neighbors and then goes to sleep again until the next frame

8、. The active time ends by time out when nothing is heard on the medium.Reduce idle listening by transmitting all messages in bursts of variable length and sleeping between bursts.Activation eventAn active period ends when no activation event occurs for a time TA.Activation Events:The firing of a per

9、iodic frame timer;The reception of any data on the radio;The sensing of communication on the radio, e.g. during a collision;The end-of-transmission of a nodes own data packet or acknowledgement;The knowledge, through overhearing prior RTS and CTS packets, that a data exchange of a neighbor has ended

10、.Clustering and SynchronizationNodes exchange their schedule by periodically broadcasting SYNC packetNodes take following 2 steps to choose their scheduleListen for SYNC packets for a fixed amount of timeCase 1: No SYNC packets are receivedCase 2: SYNC packet is received.Case 3: Multiple SYNC packet

11、s are received. Broadcast the chosen schedule by sending out SYNC packet.Schedule 2Schedule 1Border nodes with 2 schedules broadcast twiceCollision AvoidanceNodes communicate with each other using a Request-To-Send (RTS), Clear-To-Send (CTS), Data, Acknowledgement (ACK) scheme which provides both co

12、llision avoidance and reliable transmission.RTSCTSdataACKSenderReceiverRTS OperationFixed Contention IntervalContention for the medium is very high when every node starts transmitting their messages in a burst at the start of a frame.RTS transmission in T-MAC starts by waiting and listening for a ra

13、ndom time within a fixed contention interval. This interval is tuned for maximum load.RTS RetriesWhen a node sends an RTS, but does not receive a CTS back, one of following three things has happened:the receiving node has not heard the RTS due to collision; the receiving node is prohibited from repl

14、ying due to an overheard RTS or CTS; the receiving node is asleep.In cases 1 & 2, it would be wrong if the nsending node goes to sleep.A node should retry by re-sending the RTS if it receives no answer. If there is still no reply after two retries,it should give up and go to sleep.Choosing TATA C +

15、R + T C - length of the contention interval R - length of an RTS packet T - turn-around time (the short time between the end of the RTS packet & the beginning of the CTS packet).Overhearing AvoidanceIn S-MAC, a node sleeps after overhearing an RTS or CTS destined for another node. Since a node is pr

16、ohibited from sending during that time, it can not take part in any communication and may as well turn its radio off to save energy. This decreases the throughput by as much as 25%.But, a node may miss other RTS and CTS packets while sleeping and disturb some communication when it wakes up. Overhear

17、ing avoidance must not be used when maximum throughput is required.Asymmetric CommunicationEarly sleeping problem: a node goes to sleep when a neighbor still has messages. Nodes-to-sink communication patternProposed Solutions for early sleepingFuture request-to-send (FRTS)Taking priority on full buf

18、fersFuture-Request-To-Send (FRTS) The idea is to let other node know that we still have a message for it, but are ourselves prohibited from using the medium.The FRTS packet contains the length of time for which communication is blocked.The node that receives the FRTS packet knows that it will be the

19、 future target of an RTS packet and must be awake by that time.Taking priority on full buffersWhen nodes transmit/routing buffer are almost full, it may prefer sending to receiving.Advantages:Probability of early sleeping problem is reduced.The scheme introduces limited flow control in the network.T

20、-MAC uses a threshold: a node may only use this scheme when it has lost contention at least twice.Simulation ExperimentsSimulation setup and parametersThe simulation was created using OMNeT+ discrete event simulation package.The model was based on the real amount of energy consumed by the EYES nodes

21、.20 A - sleeping4 mA - receiving 10 mA - transmitting A network of 100 nodes in a 10 by 10 grid, with a radio range of 8 neighbors for non-edge nodes.Protocols evaluatedCSMA never sleepsS-MAC protocol with a frame length of one second, and with several lengths of the active time, varying from 75 ms to 915 ms.T-MAC protocol with a frame length of 610 ms (20000 ticks of a quartz crystal) and an interval TA with a length of 15 ms.ResultsHomogeneous local unicast with small messages (msglength = 20)Homogeneous local unicast with large messages (msglength = 100)Nodes to sink commu