1. Architectures and Applications for Wireless Sensor Networks (01204525) Medium Access Control Chaiporn Jaikaeo chaiporn.j@ku.ac.th Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig

2. 2 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols IEEE 802.15.4 IEEE 802.15.4

3. 3 Difficulties Medium access in wireless networks is difficult, mainly because of Medium access in wireless networks is difficult, mainly because of  Half-duplex communication  High error rates Requirements Requirements  As usual: high throughput, low overhead, low error rates, …  Additionally: energy-efficient, handle switched off devices!

4. 4 Requirements for Energy-Efficient MAC Protocols Recall Recall  Transmissions are costly  Receiving about as expensive as transmitting  Idling can be cheaper but is still expensive Energy problems Energy problems  Collisions  Overhearing  Idle listening  Protocol overhead Always wanted: Low complexity solution Always wanted: Low complexity solution

5. 5 Main Options Wireless medium access CentralizedDistributed Contention- based Schedule- based Fixed assignment Demand assignment Contention- based Schedule- based Fixed assignment Demand assignment

6. 6 Centralized Medium Access A central station controls when a node may access the medium A central station controls when a node may access the medium  E.g., Polling, computing TDMA schedules  Advantage: Simple, efficient Not directly feasible for non-trivial wireless network sizes Not directly feasible for non-trivial wireless network sizes But: Can be quite useful when network is somehow divided into smaller groups But: Can be quite useful when network is somehow divided into smaller groups Distributed approach still preferable Distributed approach still preferable

7. 7 Schedule- vs. Contention-Based Schedule-based protocols Schedule-based protocols  FDMA, TDMA, CDMA  Schedule can be fixed or computed on demand  Usually mixed  Collisions, overhearing, idle listening no issues  Time synchronization needed Contention-based protocols Contention-based protocols  Hope: coordination overhead can be saved  Mechanisms to handle/reduce probability/impact of collisions required  Randomization used somehow

8. 8 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols  MACA  S-MAC, T-MAC  Preamble sampling, B-MAC  PAMAS Schedule-based protocols Schedule-based protocols IEEE 802.15.4 IEEE 802.15.4

9. 9 A Distributed, Contention-Based MAC Basic ideas Basic ideas  Receivers need to tell surrounding nodes to shut up  Listen before talk (CSMA)  Suffers from sender not knowing what is going on at receiver BC D Hidden terminal scenario: Also: recall exposed terminal scenario

10. 10 How To Shut Up Senders Inform potential interferers during reception Inform potential interferers during reception  Cannot use the same channel  So use a different one  Busy tone protocol Inform potential interferers before reception Inform potential interferers before reception  Can use same channel  Receiver itself needs to be informed, by sender, about impending transmission  Potential interferers need to be aware of such information, need to store it

11. 11 MACA Multiple Access with Collision Avoidance Multiple Access with Collision Avoidance Sender B issues Request to Send (RTS) Sender B issues Request to Send (RTS) Receiver C agrees with Clear to Send (CTS) Receiver C agrees with Clear to Send (CTS) Potential interferers learns from RTS/CTS Potential interferers learns from RTS/CTS  Store this information in a Network Allocation Vector (NAV) B sends, C acks B sends, C acks Used in IEEE 802.11 Used in IEEE 802.11

12. 12 RTS/CTS RTS/CTS helps, but do not solve hidden/exposed terminal problems RTS/CTS helps, but do not solve hidden/exposed terminal problems

13. 13 MACA Problem: Idle listening Need to sense carrier for RTS or CTS packets Need to sense carrier for RTS or CTS packets  In some form shared by many CSMA variants; but e.g. not by busy tones  Simple sleeping will break the protocol IEEE 802.11 solution: ATIM windows & sleeping IEEE 802.11 solution: ATIM windows & sleeping  Idea: Nodes that have data buffered for receivers send traffic indicators at prearranged points in time  Receivers need to wake up at these points, but can sleep otherwise

14. 14 Sensor-MAC (S-MAC) MACA unsuitable if average data rate is low MACA unsuitable if average data rate is low  Most of the time, nothing happens Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow packet exchange Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow packet exchange  Need to also exchange wakeup schedule between neighbors  When awake, perform RTS/CTS

15. 15 Listen for SYNC tdtd Schedule Assignment Synchronizer   Listen for a mount of time   If hear no SYNC, select its own SYNC   Broadcasts its SYNC immediately Follower   Listen for amount of time   Hear SYNC from A, follow A’s SYNC   Rebroadcasts SYNC after random delay t d Sleep Listen Go to sleep after time t Sleep Listen Broadcasts A B Go to sleep after time t- t d

16. 16 S-MAC Synchronized Islands Nodes learn schedule from other nodes Nodes learn schedule from other nodes Some node might learn about two different schedules from different nodes Some node might learn about two different schedules from different nodes  “Synchronized islands” To bridge this gap, it has to follow both schemes To bridge this gap, it has to follow both schemes Time AAAA CCCC A BBBB DDD A C B D E EEE EEE

17. 17 Timeout-MAC (T-MAC) In S-MAC, active period is of constant length In S-MAC, active period is of constant length Idea: Prematurely go back to sleep mode after timeout Idea: Prematurely go back to sleep mode after timeout  Adaptive duty cycle One ensuing problem: Early sleeping One ensuing problem: Early sleeping  C wants to send to D, but is hindered by transmission A  B ABCD RTS CTS DATA May not send Timeout, go back to sleep as nothing happened ACK RTS

18. 18 Preamble Sampling Alternative option: Don’t try to explicitly synchronize nodes Alternative option: Don’t try to explicitly synchronize nodes  Have receiver sleep and only periodically sample the channel Use long preambles to ensure that receiver stays awake to catch actual packet Use long preambles to ensure that receiver stays awake to catch actual packet  Example: B-MAC, WiseMAC Check channel Start transmission: Long preambleActual packet Stay awake!

19. 19 B-MAC Very simple MAC protocol Very simple MAC protocol Employs Employs  Clear Channel Assessment (CCA) and backoffs for channel arbitration  Link-layer acknowledgement for reliability  Low-power listening (LPL)  I.e., preamble sampling Currently: Often considered as the default WSN MAC protocol Currently: Often considered as the default WSN MAC protocol

20. 20 B-MAC B-MAC does not have B-MAC does not have  Synchronization  RTS/CTS  Results in simpler, leaner implementation  Clean and simple interface

21. 21 Clear Channel Assessment "Carrier Sensing" in wireless networks "Carrier Sensing" in wireless networks Thresholding CCA algorithm Outlier detection CCA algorithm

22. 22 PAMAS Power Aware Multi-Access with Signaling Power Aware Multi-Access with Signaling Idea: combine busy tone with RTS/CTS Idea: combine busy tone with RTS/CTS  Avoid overhearing  Does not address idle listening  Uses separate data and control channels Time Control channel Data channel RTS A  B CTS B  A Data A  B Busy tone sent by B

23. 23 PAMAS Suppose a node C in vicinity of A is already receiving a packet when A initiates RTS Suppose a node C in vicinity of A is already receiving a packet when A initiates RTS A B C ? Time Control channel Data channel RTS A  B CTS B  A No data! Busy tone by C

24. 24 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols  LEACH  SMACS  TRAMA IEEE 802.15.4 IEEE 802.15.4

25. 25 LEACH Low-Energy Adaptive Clustering Hierarchy Low-Energy Adaptive Clustering Hierarchy Assumptions Assumptions  Dense network of nodes  Direct communication with central sink  Time synchronization Idea: Group nodes into “clusters” Idea: Group nodes into “clusters”  Each cluster controlled by clusterhead  About 5% of nodes become clusterhead (depends on scenario)  Role of clusterhead is rotated

26. 26 LEACH Clusterhead Each CH organizes Each CH organizes  CDMA code for its cluster  TDMA schedule to be used within a cluster In steady state operation In steady state operation  CHs collect & aggregate data from all cluster members  Report aggregated data to sink using CDMA

27. 27 LEACH rounds

28. 28 SMACS Self-Organizing Medium Access Control for Sensor Networks Self-Organizing Medium Access Control for Sensor Networks Assumptions Assumptions  Many radio channels  Most nodes are stationary  Time synchronization Goal: set up directional links between neighboring nodes Goal: set up directional links between neighboring nodes

29. 29 SMACS Links Each link is directional Each link is directional  A pair of nodes needs two links to exchange data  Radio channel + time slot at both sender and receiver  Free of collisions at receiver  Channel picked randomly, slot is searched greedily until a collision-free slot is found Receivers only wake up in their assigned time slots, once per superframe Receivers only wake up in their assigned time slots, once per superframe

30. 30 TRAMA Traffic Adaptive Medium Access Protocol Traffic Adaptive Medium Access Protocol Assume nodes are time synchronized Assume nodes are time synchronized Time divided into cycles, divided into Time divided into cycles, divided into  Random access period  Scheduled access period Random Access Period Scheduled-Access Period time cycle Exchange and learn two-hop neighborsExchange and learn two-hop neighbors Exchange schedulesExchange schedules Used by winning nodes to transmit dataUsed by winning nodes to transmit data

31. 31 TRAMA – Adaptive Election How to decide which slot (in scheduled access period) a node can use? How to decide which slot (in scheduled access period) a node can use?  For node id x and time slot t, compute p = h (x  t)  h is a global hash function  Compute p for next k time slots for itself and all two- hop neighbors  Node uses those time slots for which it has the highest priority t = 0 t = 1 t = 2 t=3 t = 4 t = 5 A1423956326 B3364812446 C5318633572

32. 32 Comparison: TRAMA, S-MAC Comparison between TRAMA & S-MAC Comparison between TRAMA & S-MAC  Energy savings in TRAMA depend on load situation  Energy savings in S-MAC depend on duty cycle  TRAMA (as typical for a TDMA scheme) has higher delay but higher maximum throughput than contention-based S-MAC TRAMA disadvantage: substantial memory/CPU requirements for schedule computation TRAMA disadvantage: substantial memory/CPU requirements for schedule computation

33. 33 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols IEEE 802.15.4 IEEE 802.15.4

34. 34 IEEE 802.15.4 IEEE standard for low-rate WPAN (LR-WPAN) applications IEEE standard for low-rate WPAN (LR-WPAN) applications  Low-to-medium bit rates  Moderate delays without too strict requirements  Low energy consumption Physical layer Physical layer  20 kbps over 1 channel @ 868-868.6 MHz  40 kbps over 10 channels @ 905 – 928 MHz  250 kbps over 16 channels @ 2.4 GHz MAC protocol MAC protocol  Single channel at any one time  Combines contention-based and schedule-based schemes  Asymmetric: nodes can assume different roles

35. 35 868MHz / 915MHz PHY 2.4 GHz 868.3 MHz Channel 0 Channels 1-10 Channels 11-26 2.4835 GHz 928 MHz902 MHz 5 MHz 2 MHz 2.4 GHz PHY IEEE 802.15.4 PHY Overview Operating frequency bands Operating frequency bands

36. 36 IEEE 802.15.4 MAC Overview Device classes Device classes  Full function device (FFD)  Any topology  Network coordinator capable  Talks to any other device  Reduced function device (RFD)  Limited to star topology  Cannot become a network coordinator  Talks only to a network coordinator  Very simple implementation Slide 36Joe Dvorak, Motorola9/27/05

37. 37 Network Topologies

38. 38 Cluster Tree Network A special case of peer-to-peer topology A special case of peer-to-peer topology

39. 39 Coordinators Roles Roles  Manage a list of associated devices  Allocate a short address to each device  Transmit beacons (in beaconed mode)  Exchange data with devices and peer coordinators Devices are associated with coordinators Devices are associated with coordinators  Forming a PAN, identified by a PAN identifier

40. 40 Beaconed Mode Superframe structure Superframe structure GTS assigned to devices upon request GTS assigned to devices upon request

41. 41 Data Transfer Device  coordinator Device  coordinator  If having allocated GTS, wake up and send  Otherwise, send during CAP  Using slotted CSMA Coordinator  device Coordinator  device  If having allocated GTS, wake up and receive  Otherwise, see picture

42. 42 Slotted CSMA

43. 43 Further protocols MAC protocols for ad hoc/sensor networks is one the most active research fields MAC protocols for ad hoc/sensor networks is one the most active research fields  Tons of additional protocols in the literature  E.g., STEM, mediation device protocol, many CSMA variants with different timing optimizations, protocols for multi-hop reservations (QoS for MANET), protocols for multiple radio channels, …

44. 44 Summary Many different ideas exist for medium access control in MANET/WSN Many different ideas exist for medium access control in MANET/WSN Comparing their performance and suitability is difficult Comparing their performance and suitability is difficult Especially, clearly identifying interdependencies between MAC protocol and other layers/applications is difficult Especially, clearly identifying interdependencies between MAC protocol and other layers/applications is difficult  Which is the best MAC for which application? Nonetheless, certain “common use cases” exist Nonetheless, certain “common use cases” exist  IEEE 802.11 DCF for MANET  IEEE 802.15.4 for some early “commercial” WSN variants  B-MAC for WSN research not focusing on MAC