Swayambhoo Jain<br />MSEE, 1st semester<br />University of Minnesota, Twin Cities<br />Media Access Control (MAC) in Wirel...
Outline<br />Traditional MAC families<br />Time Division Multiple Access (TDMA)<br />Carrier Sense Multiple Access (CSMA)<...
Traditional MAC: TDMA<br />TDMA is a reservation based strategy in which medium is accessed in a time slotted fashion.<br ...
Traditional MAC : CSMA<br />CSMA is a contention based strategy<br />Simple probabilistic MAC protocol in which  shared me...
Reservation based v/s Contention based<br />CSMA :<br />Requires no infrastructure<br />No synchronization and robust to c...
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...
Challenges in design of MAC for WSNs<br />High energy efficiency for network longevity <br />Scalability<br />Small footpr...
Questions / Discussion<br />For Wireless Sensor Networks which one is better CSMA or TDMA ?<br />Bad<br />Good<br />Good a...
Sources Energy Wastage in WSNs<br />Can be solved by Contention <br />Energy Efficiency is the prime requirement for MAC d...
Taxonomy of MAC protocols in WSN<br />Scheduling based protocols<br />Protocols with common active period<br />Preamble se...
Scheduled Based MACs<br />Scheduling based protocols<br />Medium is shared based on a schedule (requires synchronization)<...
TSMP (Scheduling based) <br />It is a TDMA based protocol which uses FDMA and frequency hopping. This allows a node to par...
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<...
Scheduled Based MACs – Various Approaches<br />
MACs with Common Active Periods<br />Protocols with common active periods<br />Energy is saved by common active/sleep peri...
SMAC (Common Active Period Protocol)<br />Set of nodes periodically become active/sleep in a synchronized fashion. This se...
SMAC<br />Schedule 1<br />Schedule 2<br />At the start, a node listens to the channel for at least one active period and s...
SMAC<br />The long data packets are broken into small packets and transmitted in a burst (RTS/CTS used only in transmittin...
Common Active Periods – Various Approaches<br />
Preamble Sampling MACs<br />Preamble Sampling Protocols<br />Each node chooses to be active/to sleep independent of others...
Preamble Sampling – Various Approaches<br />
Hybrid MACs<br />Hybrid Protocols<br />Due diverse set of applications the WSNs target Hybrid protocols are needed one par...
Z-MAC (Zebra-MAC) – A hybrid MAC scheme<br />Z-MAC is a hybrid MAC scheme :<br />Uses CSMA for high throughput at low cont...
Z-MAC – Neighbor Discovery<br />The sensor nodeinitially broadcasts periodic pings to its 1-hop neighbors<br />Ping messag...
Z-MAC – The Slot Assignment (DRAND)<br />DRAND :<br />Distributed implementation of RAND<br />Well suited for WSNs, does n...
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...
Z-MAC – Local Frame Exchange<br />Time frame Rule:  <br />“ If for a node i, Maximum Slot Number in  two hop  neighbor is ...
Z-MAC - Transmission Control<br />After slot and frame assignment each node sends these details to its two-hop neighbors.<...
Z-MAC - Transmission Control<br />Busy<br />Owner Accessing Channel<br />Busy<br />Owner Accessing Channel<br />Random Bac...
ZMAC – Transmission Control<br />Explicit Contention Notification (ECN) messages notify the two hop neighbors not to act a...
Z-MAC - Synchronization<br />Z-MAC performs like CSMA with or without synchronization at low traffic.<br />At high traffic...
Z-MAC – Performance Evaluation<br />Performance was measured for single-hop, two-hop, multiple hop configurations.<br />It...
Z-MAC – Performance Evaluation<br />Single hop configuration<br />Nodes are kept equidistant from the receiver in a circle...
Z-MAC – Experiment result multi-hop<br />
Z-MAC- Experiment result multi-hop<br />
Z-MAC – Limitations <br />What are the limitations of Z-MAC ?<br />
Hybrid Protocols : Various Approaches<br />
Zigbee / IEEE 802.15.4 - Introduction<br />A group of companies (Zigbee Alliance) started working on a technology for a lo...
Zigbee <br />Application Interface<br />Network Layer<br />MAC Layer (IEEE 802.15.4)<br />MAC Layer<br />PHY Layer<br />Ap...
IEEE 802.15.4 based LR-WPAN <br />Device Classification:<br />Fully functional device (FFD) :<br />Can talk to FFD as well...
IEEE 802.15.4 based LR-WPAN<br />Two types of Communication modes :<br />Beacon Enabled<br />Beacon is transmitted by FFD ...
IEEE 802.15.4 based LR-WPAN<br />IEEE 802.15.4 MAC provides following services:<br />MAC Data Service – responsible for tr...
IEEE 802.15.4 MAC – Super Frame<br />Superframe consists of :<br />Active period, whose length is defined by SD (Superfram...
IEEE 802.15.4 MAC Layer – The Super Frame<br /> ( Transmitted by PAN Coordinator, is of variable length for GTS allocation...
IEEE 802.15.4 MAC Layer – Data Services<br />Direct Data Transfer<br />Message Sequence Diagram<br />
IEEE 802.15.4 MAC Layer – Data Services<br />Indirect Data Transfer<br />Message Sequence Diagram<br />
IEEE 802.15.4 MAC Layer – MLME<br />
IEEE 802.15.4 - Association<br />Message Sequence Diagram<br />
IEEE 802.15.4 - Dissociation<br />
IEEE 802.15.4 - Orphaning<br />
Slotted CSMA/CA in IEEE 802.15.4<br />SlottedCSMA<br />Delay for random(2BE −1) unit backoff  periods<br /> <br />NB= 0 , ...
Unslotted CSMA/CA in IEEE 802.15.4<br />UnslottedCSMA<br />NB= 0 , BE =macMinBE<br /> <br />Y<br />Delay for random(2BE −1...
WSN MAC protocols summary<br />
References<br />[1]  InjongRhee, AjitWarrier, Mahesh Aia and Jeongki Min, “ZMAC:aHybrid MAC for Wireless Sensor Networks”,...
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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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  • Tell something about MAC… why it is needed …. how important it is .. etc.
  • Give Analogy for different students taking different classes in different days … then doing CSMA for asking doubts.Synchronization : Is the heart of TDMA …. problem is how to achieve this .. generally infrastructure is required for thisSlot Assignment: Optimal Slot Assignment is NP-Hard Problem…. Slot Size should be such that it allows for transmission of few data packets… It generally takes into account the delay, communication strategy (should be large enough to accommodate various packets exchanged in transmission. Frame Size: depends on the number nodes in a communication area.
  • In the ambit of non-persistant CSMA .. a protocol may employ various other techniques to avoid collisions like RTS/CTS exchange. Transmit step differs in CSMA/CA
  • One-hop collisions resolved by Carrier Sensing Two-hop collisions (Hidden node / Exposed Terminal node) resolved by control messaging …. But control messaging leads to energy consumptions.Efficient slot assignment is a NP-hard problem. Wireless feature allows slot reuse across independent clusters of nodes. One fixed schedule might not work in because channel conditions change.No collisions only if perfect synchronization (this requires frequent messaging)INTERFERENCE range v/s Communication range you need to find out independent set of nodes
  • *In general fairness is not a issue because network has one single goal.*Fairness over short duration is allowed again it depends on the application.*Over a longer duration of time we need fairness as distributed computation of functions can get held
  • Overhearing : Nodes receive the packets destined for other nodes. (Especially when routes are found … node discard packets ..)Idle listening : Listening to channel to receive the possible traffic.Overemitting : Transmission of message when destination is not ready
  • Scheduling of communication Link : a pair of node are given a slot … no collision , idle listening and overhearing but large over headScheduling of Senders : Senders are given slots… Avoids idle listening , collisions …. less overhead overhearing remains a problem Scheduling of reciever : Recievers are given slots … avoids idle listening , overhearing .. less overhead but collisions remain a big problem.First two suited for periodic and high traffic load and last one for medium traffic load.
  • The high amount of energy is spent on initial network setup. This is based on the assumption that there will not be drastic change in the topology. High energy initially affects the network life time. One advantage is there that amount of spent in the depends only on the local networks size
  • Global Sync. Leads to high energy drain &amp; local sync leads to slot wastage. But what is more pragmatic wrt to WSNs.Some slots are anyways required for control messages so it does not lead to as much a waste as it appears.
  • Note : Slot size affects the performance only under HCL because under LCL it is CSMA. If slot sizes are large then the large network delays.To is chosen by stochastic analysis to maximize throughput under the assumption that sync error is less than the one slot period.
  • The problem is that the decision based noise does not help when the high contention is with the one hop neighbourhood.Two types of ECN : one hop ECN and two hop ECN…. Similar to kind of RTS/CTS …. but uses topology information to avoid two-hop collision. Far better than RTC/CTS because it uses ECN only during HCL.
  • Another question : When do we need global clock synchronization. ? Answer : Needed only once at the beginning. Even with no sync the performance falls back to CSMA/CA
  • Clock drifts lead to frequent running of DRAND 2) Latency and 3) Does not solve hidden node problem
  • Duty Cycle = SD/BDDuration for radio to be on = ½ BI
  • 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 />

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