1. MAC Protocols for mobile wireless sensor networks Luís Bernardo Miguel Pereira Francisco Ganhão Rodolfo Oliveira Rui Dinis Paulo Pinto July 5, 2010 Ciência 2010 tele1.dee.fct.unl.pt

2. Outline MAC layerPHY layerConclusionMotivation OUTLINE Motivation MAC layer PHY layer Conclusions

3. Outline MAC layerPHY layerConclusionMotivation Critical infrastructure protection with wireless sensor networks Contradictory objectives: Maximize WSN lifetime (minimize energy consumption) Have controlled packet delay and throughput Support mobile and fixed battery powered nodes

4. Outline MAC layerPHY layerConclusionMotivation Layered approach: 6lowPAN, ROLL, 802.15.4, … Cross-layer interfaces to handle hardware/energy limitations MAC layer (Multimode MAC protocols) Adapt operation to application requirements PHY layer (PC H-ARQ / MPD receivers) Reduce energy lost with collisions/interference satisfying app. requirements Routing MAC PHY Application Transport Objective: Save Energy and meet the application’s communication requirements e.g. Vehicle tracking vs. Environment monitoring

5. Outline MAC layerPHY layerConclusionMotivation MAC - Motivation A Wireless Sensor Network (WSN) mobility scenario Mobile nodes moving through static WSN islands Static nodes (single radio) - battery must be saved Mobile nodes - external energy resources High throughput needed during a short connection Standard WSN Medium Access Control (MAC) protocol do not handle the set of requirements mentioned before

6. Outline MAC layerPHY layerConclusionMotivation MAC - Motivation Medium Access Control (MAC) protocols save Energy by turning the radio off Asynchronous MAC protocols (e.g. B-MAC; X-MAC) Low Power Listening bind the receiver and sender using a large preamble Advantages Nodes run independent asynchronous duty-cyles - good for mobility Energy efficient to bursty traffic Disadvantages Limited throughput and high delay for more than one sender

7. Outline MAC layerPHY layerConclusionMotivation MAC - Motivation Synchronous MAC protocols (e.g. S-MAC, LL-MAC, Z-MAC, 802.15.4) Contention protocols using Carrier Sense Multiple Access (e.g. S- MAC) Scheduled protocols using Time Division Multiple Access (e.g. LL- MAC) Use hybrid approach (e.g. Z-MAC) Support both: CSMA and TDMA - changes to TDMA fallback during load peaks, maximizing the throughput Advantages High throughput available for peak periods

8. Outline MAC layerPHY layerConclusionMotivation MAC - Motivation Disadvantages High energy consumption even for idle periods Synchronized duty-cyles - bad for mobility CSMA requires SYNC frame before communication TDMA requires an additional slot allocation algorithm High mobility requires high SYNC rates to keep track from the neighbors

9. Outline MAC layerPHY layerConclusionMotivation MAC - Conceptual Idea Goal Have a low energy asynchronous mode Have a synchronous mode high throughput in the presence of mobile asynchronous nodes Allow shorter connection times than other hybrid protocols Maximize throughput for mobile nodes in the neighborhood of synchronous nodes We propose the Mobile Multimode Hybrid MAC (MMH-MAC) Asynchronous and Synchronous modes

10. Outline MAC layerPHY layerConclusionMotivation Asynchronous Mode Goal Minimize the idle energy consumption MMH-MAC asynchronous mode uses Preamble sampling approach similar X-MAC protocol Two techniques to minimize the interference between synchronous and asynchronous nodes It uses Low Power Listening mechanism Sender sends a sequence of short preambles with duration up to 2*T duty_cycle before the data frames Unicast receivers may send and Early Preamble ACK

11. Outline MAC layerPHY layerConclusionMotivation Asynchronous Mode Passive interference mitigation Alignment of the asynchronous active time with the public slot of the last visited synchronous node Preamble overhead is reduced due to the immediate reception of an early PACK Active interference mitigation Improved Shut-up mechanism

12. Outline MAC layerPHY layerConclusionMotivation Synchronous Mode Slotted scheme - Nodes runs a synchronized duty-cycle period. 11 slots with fixed duration of 100ms each Slots are subdivided in ten 10ms subslots Public Slot (slot 0) Shared by all the nodes, it’s used for broadcast traffic and casual unicast traffic Unicast traffic is acknowledged and run a contention based protocol First 50 ms reserved for MAC signaling (SYNC frames)

13. Outline MAC layerPHY layerConclusionMotivation Synchronous Mode Private slot (slot 1-10) Reserved slots for unicast traffic between two nodes Collision free environment Traffic is acknowledged After 25ms of inactivity nodes go into sleep SYNC frames are used to: Maintain inter-node duty-cycle synchronization Broadcast private slot allocation As beacons to detect neighborhood changes (above an RSSI value)

14. Outline MAC layerPHY layerConclusionMotivation Synchronous Mode MMH-MAC mobility handling features Multiple SYNC frames can be transmitted per duty- cycle Normal SYNC frames are transmitted in a random subslot of public slot 0 Other SYNC frames are sent when an asynchronous node is detected A neighbor SYNC table is kept that measures link stability allowing cluster formations

15. Outline MAC layerPHY layerConclusionMotivation Synchronization Process Goal Guarantee that all neighbors follow the same duty-cycle schedule (synchronous and asynchronous nodes) If all nodes are asynchronous Packet Hello is sent preceded by a sequence of preambles Request/Ok exchange identifies the neighbors and reserves private slots SYNC defines the initial synchronization reference

16. Outline MAC layerPHY layerConclusionMotivation Synchronization Process If at least one node is synchronous, neighbor nodes follow the existing duty cycle Passive approach (classical) Where M node waits for the SYNC packet Active approach (new) M sends preambles to trigger the Shup-Up mechanism in one active slot in one of its neighbors Wait for the SYNC to proceed with the synchronization First empty slot or idle dedicated slot Next public slot

17. Outline MAC layerPHY layerConclusionMotivation Synchronization Process Performance Depends on the number of active private slots more active slots = less time a node takes to listen to M preambles more active slots = more time until finding an idle slot MMH-MAC proposes the use of listening private slots mechanism The node turns on the radio for 10 ms when the slot is free Each listening slot costs 1% of duty-cycle Depends on the preamble starting slot Slot 0 is the optimal case

18. Outline MAC layerPHY layerConclusionMotivation MAC - Results We use TOSSIM simulator Run MMH-MAC nesC code Added the mobility support Additional meters measure active time/sleep time/tx time/receive time Simulated scenario 21 static nodes in synchronous mode organized in 6 static clusters Each dedicated slot has CBR traffic (10 packets/sec and 35 bytes/packet) Each static node sends one SYNC per duty-cycle (1,1s minimum value) Energy estimation: Xbow Telos B current consumption

19. Outline MAC layerPHY layerConclusionMotivation MAC - Results Simulated scenario A mobile node moves randomly on the scattered WSN Connects 120 times to the islands with a variable connection time We evaluate three scenarios [WCNC’2010] Passive syncronization Active synchronization without listening slots Active synchronization with one listening slot (slot 6)

20. Outline MAC layerPHY layerConclusionMotivation MAC - Results Time to synchronize As function of the number of allocated dedicated slots

21. Outline MAC layerPHY layerConclusionMotivation MAC - Results Throughput As function of the connection duration time

22. Outline MAC layerPHY layerConclusionMotivation MAC - Conclusions MMH-MAC significantly reduces the time to an asynchronous node to start communicating to a synchronous node and vice versa Minimize the interference between asynchronous and synchronous nodes We implement the code on TinyOS and we made short tests on real nodes We are implementing a mixed TelosB / SunSPOT scenario

23. Outline MAC layerPHY layerConclusionMotivation PHY - Motivation Classical WSN PHY (e.g. 802.15.4) limit energy efficiency Packets involved in collisions/interference are lost Low complexity H-ARQ may improve energy efficiency WSN applications with hard constraints on: Delay Bitrate

24. Outline MAC layerPHY layerConclusionMotivation PHY - Motivation Using an H-ARQ scheme enhances the throughput, compared to a conventional ARQ scheme; Energy could be saved on subsequent re- transmissions; Depending on the distance and the nodes density: Circuit’s energy consumption ≥ expended energy transmission.

25. Outline MAC layerPHY layerConclusionMotivation PHY - Objectives Analyze the Energy per useful packet (EPUP): Diversity Combining (DC) H-ARQ technique; Conventional ARQ (C-ARQ); Obtain the optimal EPUP for a TDMA access mode considering: Delay constraints Throughput constraints.

26. Outline MAC layerPHY layerConclusionMotivation PHY - System Overview Assumptions: Synchronous TDMA MAC slot on a flat fading scenario; Additive White Gaussian Noise channel (AWGN); Slots of equal length, each equivalent to a packet of M bits; A receiver, holds up to R transmissions of a failed packet; After R transmissions, it gives up.

27. Outline MAC layerPHY layerConclusionMotivation PHY - System Overview Receiver Characterization for DC H-ARQ: Linear Bit Combination; Enhancement of the bit reception. +.......

28. Outline MAC layerPHY layerConclusionMotivation PHY - System Overview Energy Analysis EPUP – Energy per useful packet E[N] – Expected number of retransmissions E p – Energy per Packet(d, E b ) Q R+1 – Probability of packet failure after R transmissions

29. Outline MAC layerPHY layerConclusionMotivation PHY - System Overview System Optimization - minimize EPUP, subject to: A minimum goodput S min ; A maximum delay D max ; A minimum success probability.

30. Outline MAC layerPHY layerConclusionMotivation PHY - Performance C-ARQ vs. DC H-ARQ [ICCCN’2010a]: Analytical and simulated results with the ns-2 simulator; Simulation characteristics: Packet size of M=256 bits; 8 Wireless Terminals; Distances ranging between d=10m and 100m; Retransmissions up to R=10.

31. Outline MAC layerPHY layerConclusionMotivation PHY - Performance EPUP in function of d and Eb/N0.

32. Outline MAC layerPHY layerConclusionMotivation PHY - Performance Success ProbabilityDelay

33. Outline MAC layerPHY layerConclusionMotivation PHY - Conclusions DC H-ARQ can extend the battery of a Wireless Terminal, compared to a conventional TDMA ARQ scheme. Longer distances; Re-transmission tolerance. Future Work: MultiPacket Detection schemes [Globecom’07, TWC09, ICCCN’2010b]

34. Outline MAC layerPHY layerConclusionMotivation PHY – MPD vs DC H-ARQ DelayThroughput

35. Outline MAC layerPHY layerConclusionMotivation Conclusions & Future Work MAC layer approaches adapt radio sleep times and synchronization to the application/routing requirements PHY layer reduce transmission power, or synchronization requirements, by using DC H-ARQ or MPD Future Work: Continue to combine MAC and PHY approaches to improve energy efficiency

36. Thank you for your attention Q & A

37. Outline MAC layerPHY layerConclusionMotivation Specific mobility support enhancements MS-MAC improves S-MAC Use RSSI to detect mobility and adapt SYNC period Reduce the sleep duration for mobile nodes, and always-on for fast moving nodes MobH-MAC improves LL-MAC Use CSMA for mobiles nodes and TDMA for static nodes MA-MAC Broadcast of multiple SYNC frames at multiple schedules (not only the node’s schedule) per duty cycles period, carrying the schedules of neighbor nodes Propagation of outdated schedule

38. Outline MAC layerPHY layerConclusionMotivation MAC - Conceptual Idea The previous protocols handle mobility by: Trading-off the energy or bandwidth for enhanced mobility Have a high synchronization delay - wait for beacon or SYNC Multimode Hybrid MAC (MH-MAC) protocol previously proposed for bursty traffic on a fixed network Handles the energy and bandwidth requirements for packeting radios (e.g. CC2420) 3 working modes controlled by an API (Application Programming Interface) Full-on; synchronous; asynchronous It takes too long to change its state - Bad for mobility

39. Outline MAC layerPHY layerConclusionMotivation Shut-up frame can stop: Preamble collision between two asynchronous nodes Interference between asynchronous and synchronous nodes Can be disabled - creation of multi-hop synchronous sink tree Asynchronous Mode