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Swayambhoo Presentation (2)


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This is a slide presentation giving a nice overview of MAC techniques used in Wireless Sensor Networks.

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Swayambhoo Presentation (2)

  1. 1. Swayambhoo Jain<br />MSEE, 1st semester<br />University of Minnesota, Twin Cities<br />Media Access Control (MAC) in Wireless Sensor Networks-II<br />
  2. 2. Outline<br />Traditional MAC families<br />Time Division Multiple Access (TDMA)<br />Carrier Sense Multiple Access (CSMA)<br />Challenges in MAC design for Wireless Sensor Networks (WSN)<br />Taxonomy of MAC protocols in WSN<br />Schedule based protocols<br />TSMP<br />Protocols with common active periods<br />S-MAC<br />Preamble sampling protocols<br />Hybrid Protocols<br />Z-MAC<br />IEEE 802.15.4 (Zigbee)<br />
  3. 3. Traditional MAC: TDMA<br />TDMA is a reservation based strategy in which medium is accessed in a time slotted fashion.<br />Critical Requirements:<br />Synchronization <br />Slot Assignment Algorithm (Not easy… Optimal Slot Assignment is NP-Hard problem)<br />Choice of frame size and slot size affects the performance<br /> What should be kept in mind before selecting slot size and frame size ?<br />Slot<br />1<br />2<br />3<br />5<br />4<br />6<br />Frame<br />
  4. 4. Traditional MAC : CSMA<br />CSMA is a contention based strategy<br />Simple probabilistic MAC protocol in which shared medium is sensed before transmitting<br />The rule is “If medium is idle transmit otherwise defer the transmission”.<br />Flavors of CSMA : <br />Non-persistant CSMA<br />p-persistant CSMA <br />CSMA/CA used in wireless networks avoids collision by random backoffs.<br />Start<br />Start<br />No<br /> No<br />Medium idle? <br />Medium idle? <br />Wait for random back off time<br />Wait until medium is free<br />Yes<br />Yes<br />Probability 1<br />Transmit<br />Transmit<br />Probability p<br />End<br />End<br />Non Persistant CSMA<br />CSMA<br />
  5. 5. Reservation based v/s Contention based<br />CSMA :<br />Requires no infrastructure<br />No synchronization and robust to changes in network topology<br />High amount of idle listening and overhearing overhead<br />Prone to collisions<br />Throughput decreases as traffic increases<br />TDMA :<br />Suited for Base Station/Remote-Station architectures<br />Requires synchronization and not robust to topology changes<br />No collisions<br />Has potential for saving energy <br />Low throughput even under low traffic<br />One-hop collisions or two-hop collisions ?<br />What are the conditions for no collisions ?<br />
  6. 6. Reservation based v/s Contention based<br />Reservation Based<br />1.0<br />0.9<br />0.8<br />0.7<br />0.6<br />0.5<br />0.4<br />0.3<br />0.2<br />0.1<br /> 0<br />0.01-persistent CSMA<br />Nonpersistent CSMA<br />0.1-persistent CSMA<br />Throughput<br />0.5-persistent CSMA<br />1-persistent CSMA<br />Slotted Aloha<br />Aloha<br />0 1 2 3 4 5 6 7 8 9<br />Offered Load<br />
  7. 7. Challenges in design of MAC for WSNs<br />High energy efficiency for network longevity <br />Scalability<br />Small footprint<br />Robustness towards :<br />Time varying channel conditions and Dynamic network topology<br />Loss in synchronization<br />Low latency<br />Fairness<br />(How can topology change in WSNs?)<br />(Is Fairness really an issue ? )<br />
  8. 8. Questions / Discussion<br />For Wireless Sensor Networks which one is better CSMA or TDMA ?<br />Bad<br />Good<br />Good at low traffic<br />Good at high traffic<br />Bad<br />Good<br />Bad<br />Good<br />Good<br />Bad<br />Depends on traffic<br />Depends on traffic<br />Bad<br />Good<br />Good<br />Bad<br />
  9. 9. Sources Energy Wastage in WSNs<br />Can be solved by Contention <br />Energy Efficiency is the prime requirement for MAC design in WSN<br />MAC layer is most suitable level to address the issue of energy efficiency<br />First we need to identify the possible sources of energy wastage in WSNs.<br />There are many-many types of MACs primarily designed to tackle one or more sources of energy wastage<br />Can be solved by Reservation <br />
  10. 10. Taxonomy of MAC protocols in WSN<br />Scheduling based protocols<br />Protocols with common active period<br />Preamble sensing MACs<br />Hybrid MACs<br />
  11. 11. Scheduled Based MACs<br />Scheduling based protocols<br />Medium is shared based on a schedule (requires synchronization)<br />Variants of TDMA combined with FDMA <br />Suited for periodic and high load<br />TSMP (Time Synchronized Mesh Protocol ) is an interesting example of this family<br />Types of Scheduling done <br />Scheduling of communication link<br />Scheduling of senders <br />Scheduling of receivers<br />Helps in avoiding collisions , idle listening and over hearing<br />
  12. 12. TSMP (Scheduling based) <br />It is a TDMA based protocol which uses FDMA and frequency hopping. This allows a node to participate in multiple frames at the same time thereby allowing multiple synchronization rates for different tasks.<br />Sink generates the scheduling table based on, the list of the nodes, their neighbors and their requirements.<br />Precise sense of time is maintained and only offset information is exchanged together with usual data and ACK packets<br />ch 1<br />ch 2<br />ch.1<br />ch 3<br />ch 4<br />ch.2<br />ch.3<br />ch 5<br />t1<br />t2<br />t3<br />
  13. 13. Ch. 15<br />Ch. 14<br />Ch. 13<br />E<br />Ch. 12<br />F<br />A<br />Ch. 11<br />To Avoid interference<br />B<br />Ch. 10<br />Ch. 9<br />Sink<br />(G)<br />H<br />Pros:<br />Very less collisions<br />No overhearing<br />Minimized idle listening<br />Cons:<br />Complex<br />Scalability<br />Reduced flexibility<br />Memory footprint<br />Ch. 8<br />C<br />Ch. 7<br />D<br />Ch. 6<br />Ch. 5<br />Ch. 4<br />Ch. 3<br />Ch. 2<br />Ch. 1<br />Ch. 0<br />t1<br />t2<br />t3<br />t5<br />t7<br />t10<br />t4<br />t6<br />t8<br />t9<br />
  14. 14. Scheduled Based MACs – Various Approaches<br />
  15. 15. MACs with Common Active Periods<br />Protocols with common active periods<br />Energy is saved by common active/sleep periods across a set of nodes <br />Suited for periodic traffic<br />SMAC (Sensor MAC) typical example of this family<br />Radio on<br />Radio on<br />Radio on<br />Radio off<br />Radio off<br />
  16. 16. SMAC (Common Active Period Protocol)<br />Set of nodes periodically become active/sleep in a synchronized fashion. This set of nodes is called Virtual Cluster <br />Active periods are divided into two periods, one for exchanging SYNC packets and other for exchanging DATA packets <br />Active periods are fixed to a pre-recalculated size optimized for expected traffic. <br />Collisions are avoided by RTS/CTS mechanism <br />Radio on<br />Radio on<br />Radio on<br />Radio off<br />Radio off<br />Reduces idle listening<br />For Sync<br />For Data<br />Reduces Over hearing<br />RTS/CTS/DATA/ACK<br />SYNC<br />
  17. 17. SMAC<br />Schedule 1<br />Schedule 2<br />At the start, a node listens to the channel for at least one active period and sleep period and; if it does not receive SYNC packet it chooses its own schedule and broadcasts it to its neighbors.<br />There can be different SYNC packets in the network and hence the network is, more often, made up of many virtual clusters.<br />The border nodes have to adapt to the schedule of both the neighboring clusters.<br />
  18. 18. SMAC<br />The long data packets are broken into small packets and transmitted in a burst (RTS/CTS used only in transmitting first fragment) <br />Pros:<br />Saves energy by avoiding Idle listening, Overhearing.<br />Well-designed, complete protocol that addresses deficiencies of 802.11 if applied to a sensor network.<br />Cons:<br />Suited only for the periodic traffic patterns; irregular traffic patterns may lead to collisions.<br />Rigidity due to pre-fixed active periods<br />Sleep Delay <br />
  19. 19. Common Active Periods – Various Approaches<br />
  20. 20. Preamble Sampling MACs<br />Preamble Sampling Protocols<br />Each node chooses to be active/to sleep independent of others<br />Nodes sleep most of the time and wake up periodically to check if there is a transmission<br />BMAC is a typical example of this family (Already Covered in detail)<br />Sender<br />Data<br />Preamble<br />Check Interval<br />Receiver<br />Radio Off<br />Radio On<br />Periodic Channel Sampling<br />
  21. 21. Preamble Sampling – Various Approaches<br />
  22. 22. Hybrid MACs<br />Hybrid Protocols<br />Due diverse set of applications the WSNs target Hybrid protocols are needed one particular approach is not perfect.<br />As WSNs inherently have variable traffic patterns schemes suitable for one traffic type are not sufficient<br />Can’t be classified in any of the above categories as they use the combination of above techniques.<br />ZMAC and IEEE 802.15.4 (Zigbee) are typical examples<br /># of Contenders<br />CSMA<br />TDMA<br />Channel Utilization<br />
  23. 23. Z-MAC (Zebra-MAC) – A hybrid MAC scheme<br />Z-MAC is a hybrid MAC scheme :<br />Uses CSMA for high throughput at low contention and hints from TDMA schedule for better performance at high contention<br />Implemented on the top of B-MAC i.e. uses CCA, LPL etc.<br />CSMA is a baseline scheme :<br />Robust to Synchronization errors and dynamic topology changes<br />At worst it always falls back to CSMA performance<br />The design is best understood by the setup phase Z-MAC.<br />Neighbor discovery<br />Time slot assignment<br />Local frame exchange<br />Global time synchronization<br />The idea is that the high initial setup cost is eventually paid back by improved network performance<br />
  24. 24. Z-MAC – Neighbor Discovery<br />The sensor nodeinitially broadcasts periodic pings to its 1-hop neighbors<br />Ping message contains list of node’s 1-hop neighbors<br />List builds up as time passes<br />Eventually every sensor has the list of their 1-hop as well as 2-hop neighbors<br />Current implementation takes 30 seconds <br />The list is used as an input to slot assignment algorithm<br />
  25. 25. Z-MAC – The Slot Assignment (DRAND)<br />DRAND :<br />Distributed implementation of RAND<br />Well suited for WSNs, does not require any extra infrastructure <br />Two hop list from neighbor discovery is used to come up with a slot assignment such that no two nodes in 2-hop neighborhood have the same slot.<br />Slot number assigned does not exceed the number of nodes in a neighborhood<br />Scalable.<br />Slot assignment is highly efficient.<br />
  26. 26. Z-MAC – The Slot Assignment (DRAND)<br />E<br />A<br />C<br />D<br />B<br />F<br />E<br />E<br />A<br />A<br />D<br />C<br />D<br />C<br />F<br />B<br />F<br />B<br />Radio Interference Map<br />(Energy Cost ) α  (Neighbourhood Size)<br /> <br />DRAND slot assignment<br />1<br />0<br />3<br />2<br />0<br />Time slot<br />1<br />Input Graph<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />How to decide this ?<br />Time Frame<br />
  27. 27. Z-MAC – Local Frame Exchange<br />Time frame Rule: <br />“ If for a node i, Maximum Slot Number in  two hop  neighbor is Fi,<br />     Time frame is 2a where ′a′ satisfies  2a−1 ≤ Fi< 2a ”<br />Time frame decided on the basis of local information. (Advantages / Disadvantages ?)<br />May lead to slot wastage on a global scale<br />Allows for robustness towards local topology changes <br />Only local synchronization is needed<br /> <br />1(5)<br />E<br />C<br />3(5)<br />A<br />F<br />2(5)<br />0(2)<br />B<br />D<br />1(2)<br />5(5)<br />G<br />4(5)<br />5(5)<br />H<br />
  28. 28. Z-MAC - Transmission Control<br />After slot and frame assignment each node sends these details to its two-hop neighbors.<br />Two modes of operation :<br />HCL (High Contention Load)<br />LCL (Low Contention Load)<br />Node is normally in LCL until it receives explicit contention notification message from two-hop neighbors within last tECN period.<br />Rules:<br />Common rule : “Owner of the slot has highest priority”<br />LCL Tx. rule : “Any node can compete for a transmission in a particular slot”<br />HCL Tx. Rule : “Only owners and their 1-hop neighbors can compete for a slot”<br />How these rules are imposed ? …. Answer is in next slide<br /> <br />
  29. 29. Z-MAC - Transmission Control<br />Busy<br />Owner Accessing Channel<br />Busy<br />Owner Accessing Channel<br />Random Backoff (Backoffs within fixed To)<br /> <br />Busy<br />Non-owner Accessing Channel<br />To<br /> <br />Busy<br />Non-owner Accessing Channel<br />To<br /> <br />Random Backoff (Backoffs within Toand Tno)<br /> <br />Owner<br />Non Owner<br />𝒔𝒚𝒏𝒄 𝒆𝒓𝒓𝒐𝒓≤𝑻𝒐⇒𝒂𝒕 𝒎𝒐𝒔𝒕 𝟐 𝒕𝒐 𝟑 𝒐𝒘𝒏𝒆𝒓𝒔 𝒇𝒐𝒓 𝒂 𝒔𝒍𝒐𝒕<br /> <br />Trade off between slot size and network delay <br />𝑻𝒊𝒎𝒆 𝑺𝒍𝒐𝒕>𝒄𝒉𝒆𝒄𝒌 𝒑𝒆𝒓𝒊𝒐𝒅+ 𝑻𝒐+ 𝑻𝒏𝒐+𝑪𝑪𝑨 𝒑𝒆𝒓𝒊𝒐𝒅+𝒑𝒓𝒐𝒑𝒂𝒈𝒂𝒕𝒊𝒐𝒏 𝒅𝒆𝒍𝒂𝒚<br /> <br />
  30. 30. ZMAC – Transmission Control<br />Explicit Contention Notification (ECN) messages notify the two hop neighbors not to act as hidden terminal under high load<br />Nodes make local decision of high contention :<br />By keeping track of ACKs from a particular destination and see packet loss rate<br />Since two hop collision are highly correlated to packet loss rate<br />By checking the noise level of the channel by measuring the average number of noise backoffs before transmitting a packet<br />Based on the fact that there is high correlation between the noise backoffs and traffic<br />Flooding ECN is avoided by selective forwarding of ECNs :<br />Wait for a random period before transmitting ECN messages. <br />
  31. 31. Z-MAC - Synchronization<br />Z-MAC performs like CSMA with or without synchronization at low traffic.<br />At high traffic , Z-MAC behaves as TDMA , synchronization is necessary for improved performance.<br />Synchronization is achieved by sending sync. control message limited to certain fraction of the data sending rate. ( How does this help? )<br />Sync. control messages from unsynchronized node should be given less priority.<br />Cavg=1−𝛽tCavg+ βtCavg , where βt is trust factor<br />Question: In Z-MAC do we need local synchronization or global synchronization ?<br /> <br />
  32. 32. Z-MAC – Performance Evaluation<br />Performance was measured for single-hop, two-hop, multiple hop configurations.<br />It was compared mainly against BMAC (Default MAC in TinyOS) on the following metrics :<br />Throughput<br />Energy Efficiency<br />Platform:<br />ns2<br />Mica2<br />8-bit CPU at 4MHz<br />8KB flash, 256KB RAM<br />916MHz radio<br />TinyOS event-driven<br />
  33. 33. Z-MAC – Performance Evaluation<br />Single hop configuration<br />Nodes are kept equidistant from the receiver in a circle<br />Each node transmits with full transmission power<br />Two-hop configuration:<br />Nodes are organized into two clusters of 7-8 node (Dumb-Bell shaped topology)<br />Aim was to measure performance in presence of a Hidden Terminal<br />Transmission Power was 1 dBm (1.3 mW) to control number of hidden terminals<br />Multihop Configuration:<br />42 Mica2 nodes placed in different rooms of a building<br />Routing paths were fixed after one run of Mint (default routing protocol of TinyOS)<br />
  34. 34. Z-MAC – Experiment result multi-hop<br />
  35. 35. Z-MAC- Experiment result multi-hop<br />
  36. 36. Z-MAC – Limitations <br />What are the limitations of Z-MAC ?<br />
  37. 37. Hybrid Protocols : Various Approaches<br />
  38. 38. Zigbee / IEEE 802.15.4 - Introduction<br />A group of companies (Zigbee Alliance) started working on a technology for a low data rate, low power consumption, low cost, wireless networking protocol targeted towards control and sensor networks.<br />Around same time IEEE 802.15.4 (LR-WPAN MAC Protocol) committee started working on a low data rate standard.<br />IEEE 802.15.4 and Zigbee Alliance joined hands to work on this technology and Zigbee is a commercial name of this technology.<br />
  39. 39. Zigbee <br />Application Interface<br />Network Layer<br />MAC Layer (IEEE 802.15.4)<br />MAC Layer<br />PHY Layer<br />Application<br />Customer<br />802.2 LLC<br />ZigbeeAlliance<br />SSCS<br />IEEE<br />Silicon<br />Zigbee Stack<br />Application<br />
  40. 40. IEEE 802.15.4 based LR-WPAN <br />Device Classification:<br />Fully functional device (FFD) :<br />Can talk to FFD as well as RFD<br />Can function as PAN coordinator or as a device<br />Can be used in any topology <br />Reduced functional device (RFD):<br />Can only talk to FFD <br />Does extremely simple tasks.<br />Limited to Star topology<br />Network is made up many PANs each managed by a PAN Coordinator. PANs are identified by unique PAN ID.<br />Network Topologies :<br />Star<br />Peer to peer<br />Mesh<br />Cluster Tree<br />STAR<br />Mesh<br />Cluster Tree<br /> FFD<br /> PAN Coordinator<br />RFD<br />
  41. 41. IEEE 802.15.4 based LR-WPAN<br />Two types of Communication modes :<br />Beacon Enabled<br />Beacon is transmitted by FFD periodically after each BI (Beacon Interval)<br />Super frame structure is followed<br />Suited for higher data rate kind of applications<br />Non Beacon Enabled<br />Simply reduces to CSMA/CA MAC<br />Suited for very simple applications like periodic sensing<br />Synchronization is achieved by :<br />Beacon tracking mode <br />Node simply synchronizes to first beacon and uses the information to switch on just before the next beacon<br />Non-Tracking mode<br />Sync done only when the data needs to be transmitted<br />
  42. 42. IEEE 802.15.4 based LR-WPAN<br />IEEE 802.15.4 MAC provides following services:<br />MAC Data Service – responsible for transmission and reception of MPDU (MAC protocol data units) across the PHY data service<br />MAC Management Services – interfacing to MLME-SAP (MAC Layer Management Entity – Service Access Points) <br />Features :<br />Beacon and GTS Management<br />Channel Access<br />Frame Validation and acknowledge frame delivery<br />Association & Dissociation<br />IEEE 802.15.4 PHY provides following services :<br />PHY data service – responsible for transmission and reception of PPDU (PHY protocol data units) across the channel <br />PHY management service – interfacing to PLME-SAP (PHY Layer Management Entity – Service Access Point)<br />Features :<br />ED (Energy Detection)<br />LQI (Link quality Indication)<br />CCA (Clear Channel Assessment)<br />
  43. 43. IEEE 802.15.4 MAC – Super Frame<br />Superframe consists of :<br />Active period, whose length is defined by SD (Superframe Duration), is divided in 16 equal slots :<br />Slot zero is reserved for Beacon<br />CAP (Contention Access Period) starts in the next slot after beacon is transmitted and nodes compete using slotted CSMA/CA<br />CFP (Contention Free period) starts in the slot after CAP. GTS (Guaranteed time slots ) are used for data transfer.<br />Inactive period <br />Node sleeps in inactive period<br />
  44. 44. IEEE 802.15.4 MAC Layer – The Super Frame<br /> ( Transmitted by PAN Coordinator, is of variable length for GTS allocation )<br />≥aMinCAPlength<br /> <br />Nodes Sleep here<br />Slotted CSMA if Beacon enabled<br />At MAX 7 GTS of one or more slots. GTS can uplink or downlink<br />𝟎≤𝑺𝑶 ≤𝑩𝑶 ≤𝟏𝟒<br /> <br />Questions: 1) Duty Cycle ? & 2) How should the radio be in sync in non tracking mode ?<br />
  45. 45. IEEE 802.15.4 MAC Layer – Data Services<br />Direct Data Transfer<br />Message Sequence Diagram<br />
  46. 46. IEEE 802.15.4 MAC Layer – Data Services<br />Indirect Data Transfer<br />Message Sequence Diagram<br />
  47. 47. IEEE 802.15.4 MAC Layer – MLME<br />
  48. 48. IEEE 802.15.4 - Association<br />Message Sequence Diagram<br />
  49. 49. IEEE 802.15.4 - Dissociation<br />
  50. 50. IEEE 802.15.4 - Orphaning<br />
  51. 51. Slotted CSMA/CA in IEEE 802.15.4<br />SlottedCSMA<br />Delay for random(2BE −1) unit backoff periods<br /> <br />NB= 0 , CW= 0<br /> <br />Battery Life Extension ?<br />BE = min(2,<br /> macMinBE)<br /> <br />Perform CCA on backoff period boundary<br />Y<br />BE = macMinBE<br /> <br />Channel Idle ?<br />Y<br />N<br />N<br />Locate backoff period boundary<br />CW=2, NB=NS+1, <br />BE=min⁡(BE+1, aMaxBE)<br /> <br />CW=CW−1<br /> <br />N<br />NB > macMacCSMABackoffs ?<br />CW=0 ?<br /> <br />N<br />Y<br />Y<br />Failure<br />Success<br />Used in Beacon Enabled mode<br />
  52. 52. Unslotted CSMA/CA in IEEE 802.15.4<br />UnslottedCSMA<br />NB= 0 , BE =macMinBE<br /> <br />Y<br />Delay for random(2BE −1) unit backoff periods<br /> <br />NB > macMacCSMABackoffs ?<br />N<br />Perform CCA<br />Failure<br />Channel Idle ?<br />Y<br />Success<br />N<br />Used Beacon Disabled Mode<br />NB=NS+1, <br />BE=min⁡(BE+1, aMaxBE)<br /> <br />
  53. 53. WSN MAC protocols summary<br />
  54. 54. References<br />[1] InjongRhee, AjitWarrier, Mahesh Aia and Jeongki Min, “ZMAC:aHybrid MAC for Wireless Sensor Networks”, IEEE/ACM Transactions on Networking (TON) Vol. 16 , Issue 3 (June 2008) <br />[2] Wei Ye, John Heidemann and Deborah Estrin, “An Energy-Efficient MAC Protocol for Wireless Sensor Networks”, INFOCOM 2002. Twenty-First Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE<br />[3] Sunil Kumar , Vineet S. Raghavan and Jing Deng,“Medium Access Control protocols for ad hoc wireless networks: A survey”, Ad Hoc Networks Volume 4, Issue 3, May 2006, <br />[4] Abdelmalik Bachir,   Mischa Dohler, Tomas  Watteyne, and  Kin K. Leung, “MAC Essentials for Wireless Sensor Networks”, IEEE Communication Surveys & Tutorials, Vol. 12, No.2, Second-Quarter 2010<br />[5] IlkerDemirkol, CemErsoy, and FatihAlagöz, “MAC Protocols for Wireless Sensor Networks: A Survey”, IEEE Communication Magazine, April 2006, Vol. 44 Issue 4<br />[6] Anurag Kumar, D. Manjunath and Joy Kuri, “Wireless Networking” , 2008 Edition<br />[7] SinemColeriErgen, “ZigBee/IEEE 802.15.4 Summary”<br />