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

1. Swayambhoo Jain MSEE University of Minnesota, Twin Cities Media Access Control (MAC) in Wireless Sensor Networks-II

2. Outline Traditional MAC families Time Division Multiple Access (TDMA) Carrier Sense Multiple Access (CSMA) Challenges in MAC design for Wireless Sensor Networks (WSN) Taxonomy of MAC protocols in WSN Schedule based protocols TSMP Protocols with common active periods S-MAC Preamble sampling protocols Hybrid Protocols Z-MAC IEEE 802.15.4 (Zigbee)

3. Traditional MAC: TDMA TDMA is a reservation based strategy in which medium is accessed in a time slotted fashion. Critical Requirements: Synchronization Slot Assignment Algorithm (Not easy… Optimal Slot Assignment is NP-Hard problem) Choice of frame size and slot size affects the performance What should be kept in mind before selecting slot size and frame size ? Slot 1 2 3 5 4 6 Frame

4. Traditional MAC : CSMA CSMA is a contention based strategy Simple probabilistic MAC protocol in which shared medium is sensed before transmitting The rule is “If medium is idle transmit otherwise defer the transmission”. Flavors of CSMA : Non-persistant CSMA p-persistant CSMA CSMA/CA used in wireless networks avoids collision by random backoffs. Start Start No No Medium idle? Medium idle? Wait for random back off time Wait until medium is free Yes Yes Probability 1 Transmit Transmit Probability p End End Non Persistant CSMA CSMA

5. Reservation based v/s Contention based CSMA : Requires no infrastructure No synchronization and robust to changes in network topology High amount of idle listening and overhearing overhead Prone to collisions Throughput decreases as traffic increases TDMA : Suited for Base Station/Remote-Station architectures Requires synchronization and not robust to topology changes No collisions Has potential for saving energy Low throughput even under low traffic One-hop collisions or two-hop collisions ? What are the conditions for no collisions ?

6. Reservation based v/s Contention based Reservation Based 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.01-persistent CSMA Nonpersistent CSMA 0.1-persistent CSMA Throughput 0.5-persistent CSMA 1-persistent CSMA Slotted Aloha Aloha 0 1 2 3 4 5 6 7 8 9 Offered Load

7. Challenges in design of MAC for WSNs High energy efficiency for network longevity Scalability Small footprint Robustness towards : Time varying channel conditions and Dynamic network topology Loss in synchronization Low latency Fairness (How can topology change in WSNs?) (Is Fairness really an issue ? )

8. Questions / Discussion For Wireless Sensor Networks which one is better CSMA or TDMA ? Bad Good Good at low traffic Good at high traffic Bad Good Bad Good Good Bad Depends on traffic Depends on traffic Bad Good Good Bad

9. Sources Energy Wastage in WSNs Can be solved by Contention Energy Efficiency is the prime requirement for MAC design in WSN MAC layer is most suitable level to address the issue of energy efficiency First we need to identify the possible sources of energy wastage in WSNs. There are many-many types of MACs primarily designed to tackle one or more sources of energy wastage Can be solved by Reservation

10. Taxonomy of MAC protocols in WSN Scheduling based protocols Protocols with common active period Preamble sensing MACs Hybrid MACs

11. Scheduled Based MACs Scheduling based protocols Medium is shared based on a schedule (requires synchronization) Variants of TDMA combined with FDMA Suited for periodic and high load TSMP (Time Synchronized Mesh Protocol ) is an interesting example of this family Types of Scheduling done Scheduling of communication link Scheduling of senders Scheduling of receivers Helps in avoiding collisions , idle listening and over hearing

12. TSMP (Scheduling based) 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. Sink generates the scheduling table based on, the list of the nodes, their neighbors and their requirements. Precise sense of time is maintained and only offset information is exchanged together with usual data and ACK packets ch 1 ch 2 ch.1 ch 3 ch 4 ch.2 ch.3 ch 5 t1 t2 t3

13. Ch. 15 Ch. 14 Ch. 13 E Ch. 12 F A Ch. 11 To Avoid interference B Ch. 10 Ch. 9 Sink (G) H Pros: Very less collisions No overhearing Minimized idle listening Cons: Complex Scalability Reduced flexibility Memory footprint Ch. 8 C Ch. 7 D Ch. 6 Ch. 5 Ch. 4 Ch. 3 Ch. 2 Ch. 1 Ch. 0 t1 t2 t3 t5 t7 t10 t4 t6 t8 t9

14. Scheduled Based MACs – Various Approaches

15. MACs with Common Active Periods Protocols with common active periods Energy is saved by common active/sleep periods across a set of nodes Suited for periodic traffic SMAC (Sensor MAC) typical example of this family Radio on Radio on Radio on Radio off Radio off

16. SMAC (Common Active Period Protocol) Set of nodes periodically become active/sleep in a synchronized fashion. This set of nodes is called Virtual Cluster Active periods are divided into two periods, one for exchanging SYNC packets and other for exchanging DATA packets Active periods are fixed to a pre-recalculated size optimized for expected traffic. Collisions are avoided by RTS/CTS mechanism Radio on Radio on Radio on Radio off Radio off Reduces idle listening For Sync For Data Reduces Over hearing RTS/CTS/DATA/ACK SYNC

17. SMAC Schedule 1 Schedule 2 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. There can be different SYNC packets in the network and hence the network is, more often, made up of many virtual clusters. The border nodes have to adapt to the schedule of both the neighboring clusters.

18. SMAC The long data packets are broken into small packets and transmitted in a burst (RTS/CTS used only in transmitting first fragment) Pros: Saves energy by avoiding Idle listening, Overhearing. Well-designed, complete protocol that addresses deficiencies of 802.11 if applied to a sensor network. Cons: Suited only for the periodic traffic patterns; irregular traffic patterns may lead to collisions. Rigidity due to pre-fixed active periods Sleep Delay

19. Common Active Periods – Various Approaches

20. Preamble Sampling MACs Preamble Sampling Protocols Each node chooses to be active/to sleep independent of others Nodes sleep most of the time and wake up periodically to check if there is a transmission BMAC is a typical example of this family (Already Covered in detail) Sender Data Preamble Check Interval Receiver Radio Off Radio On Periodic Channel Sampling

21. Preamble Sampling – Various Approaches

22. Hybrid MACs Hybrid Protocols 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 Can’t be classified in any of the above categories as they use the combination of above techniques. ZMAC and IEEE 802.15.4 (Zigbee) are typical examples # of Contenders CSMA TDMA Channel Utilization

23. Z-MAC (Zebra-MAC) – A hybrid MAC scheme Z-MAC is a hybrid MAC scheme : Uses CSMA for high throughput at low contention and hints from TDMA schedule for better performance at high contention Implemented on the top of B-MAC i.e. uses CCA, LPL etc. CSMA is a baseline scheme : Robust to Synchronization errors and dynamic topology changes At worst it always falls back to CSMA performance The design is best understood by the setup phase Z-MAC. Neighbor discovery Time slot assignment Local frame exchange Global time synchronization The idea is that the high initial setup cost is eventually paid back by improved network performance

24. Z-MAC – Neighbor Discovery The sensor nodeinitially broadcasts periodic pings to its 1-hop neighbors Ping message contains list of node’s 1-hop neighbors List builds up as time passes Eventually every sensor has the list of their 1-hop as well as 2-hop neighbors Current implementation takes 30 seconds The list is used as an input to slot assignment algorithm

25. Z-MAC – The Slot Assignment (DRAND) DRAND : Distributed implementation of RAND Well suited for WSNs, does not require any extra infrastructure 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. Slot number assigned does not exceed the number of nodes in a neighborhood Scalable. Slot assignment is highly efficient.

26. Z-MAC – The Slot Assignment (DRAND) E A C D B F E E A A D C D C F B F B Radio Interference Map (𝐸𝑛𝑒𝑟𝑔𝑦 𝐶𝑜𝑠𝑡 ) α (𝑁𝑒𝑖𝑔h𝑏𝑜𝑢𝑟h𝑜𝑜𝑑 𝑆𝑖𝑧𝑒) DRAND slot assignment 1 0 3 2 0 Time slot 1 Input Graph 1 2 3 4 5 6 7 How to decide this ? Time Frame

27. Z-MAC – Local Frame Exchange Time frame Rule: “ 𝐼𝑓 𝑓𝑜𝑟 𝑎 𝑛𝑜𝑑𝑒 𝑖, 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑆𝑙𝑜𝑡 𝑁𝑢𝑚𝑏𝑒𝑟 𝑖𝑛 𝑡𝑤𝑜 h𝑜𝑝 𝑛𝑒𝑖𝑔h𝑏𝑜𝑟 𝑖𝑠 𝐹𝑖, 𝑇𝑖𝑚𝑒 𝑓𝑟𝑎𝑚𝑒 𝑖𝑠 2𝑎 𝑤h𝑒𝑟𝑒 ′𝑎′ 𝑠𝑎𝑡𝑖𝑠𝑓𝑖𝑒𝑠 2𝑎−1 ≤ 𝐹𝑖< 2𝑎 ” Time frame decided on the basis of local information. (Advantages / Disadvantages ?) May lead to slot wastage on a global scale Allows for robustness towards local topology changes Only local synchronization is needed 1(5) E C 3(5) A F 2(5) 0(2) B D 1(2) 5(5) G 4(5) 5(5) H

28. Z-MAC - Transmission Control After slot and frame assignment each node sends these details to its two-hop neighbors. Two modes of operation : HCL (High Contention Load) LCL (Low Contention Load) Node is normally in LCL until it receives explicit contention notification message from two-hop neighbors within last 𝑡𝐸𝐶𝑁 period. Rules: Common rule : “Owner of the slot has highest priority” LCL Tx. rule : “Any node can compete for a transmission in a particular slot” HCL Tx. Rule : “Only owners and their 1-hop neighbors can compete for a slot” How these rules are imposed ? …. Answer is in next slide

29. Z-MAC - Transmission Control Busy Owner Accessing Channel Busy Owner Accessing Channel Random Backoff (Backoffs within fixed 𝑇𝑜) Busy Non-owner Accessing Channel 𝑇𝑜 Busy Non-owner Accessing Channel 𝑇𝑜 Random Backoff (Backoffs within 𝑇𝑜𝑎𝑛𝑑 𝑇𝑛𝑜) Owner Non Owner 𝒔𝒚𝒏𝒄 𝒆𝒓𝒓𝒐𝒓≤𝑻𝒐⇒𝒂𝒕 𝒎𝒐𝒔𝒕 𝟐 𝒕𝒐 𝟑 𝒐𝒘𝒏𝒆𝒓𝒔 𝒇𝒐𝒓 𝒂 𝒔𝒍𝒐𝒕 Trade off between slot size and network delay 𝑻𝒊𝒎𝒆 𝑺𝒍𝒐𝒕>𝒄𝒉𝒆𝒄𝒌 𝒑𝒆𝒓𝒊𝒐𝒅+ 𝑻𝒐+ 𝑻𝒏𝒐+𝑪𝑪𝑨 𝒑𝒆𝒓𝒊𝒐𝒅+𝒑𝒓𝒐𝒑𝒂𝒈𝒂𝒕𝒊𝒐𝒏 𝒅𝒆𝒍𝒂𝒚

30. ZMAC – Transmission Control Explicit Contention Notification (ECN) messages notify the two hop neighbors not to act as hidden terminal under high load Nodes make local decision of high contention : By keeping track of ACKs from a particular destination and see packet loss rate Since two hop collision are highly correlated to packet loss rate By checking the noise level of the channel by measuring the average number of noise backoffs before transmitting a packet Based on the fact that there is high correlation between the noise backoffs and traffic Flooding ECN is avoided by selective forwarding of ECNs : Wait for a random period before transmitting ECN messages.

31. Z-MAC - Synchronization Z-MAC performs like CSMA with or without synchronization at low traffic. At high traffic , Z-MAC behaves as TDMA , synchronization is necessary for improved performance. Synchronization is achieved by sending sync. control message limited to certain fraction of the data sending rate. ( How does this help? ) Sync. control messages from unsynchronized node should be given less priority. 𝐶𝑎𝑣𝑔=1−𝛽𝑡𝐶𝑎𝑣𝑔+ β𝑡𝐶𝑎𝑣𝑔 , 𝑤h𝑒𝑟𝑒 β𝑡 𝑖𝑠 𝑡𝑟𝑢𝑠𝑡 𝑓𝑎𝑐𝑡𝑜𝑟 Question: In Z-MAC do we need local synchronization or global synchronization ?

32. Z-MAC – Performance Evaluation Performance was measured for single-hop, two-hop, multiple hop configurations. It was compared mainly against BMAC (Default MAC in TinyOS) on the following metrics : Throughput Energy Efficiency Platform: ns2 Mica2 8-bit CPU at 4MHz 8KB flash, 256KB RAM 916MHz radio TinyOS event-driven

33. Z-MAC – Performance Evaluation Single hop configuration Nodes are kept equidistant from the receiver in a circle Each node transmits with full transmission power Two-hop configuration: Nodes are organized into two clusters of 7-8 node (Dumb-Bell shaped topology) Aim was to measure performance in presence of a Hidden Terminal Transmission Power was 1 dBm (1.3 mW) to control number of hidden terminals Multihop Configuration: 42 Mica2 nodes placed in different rooms of a building Routing paths were fixed after one run of Mint (default routing protocol of TinyOS)

34. Z-MAC – Experiment result multi-hop

35. Z-MAC- Experiment result multi-hop

36. Z-MAC – Limitations What are the limitations of Z-MAC ?

37. Hybrid Protocols : Various Approaches

38. Zigbee / IEEE 802.15.4 - Introduction 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. Around same time IEEE 802.15.4 (LR-WPAN MAC Protocol) committee started working on a low data rate standard. IEEE 802.15.4 and Zigbee Alliance joined hands to work on this technology and Zigbee is a commercial name of this technology.

39. Zigbee Application Interface Network Layer MAC Layer (IEEE 802.15.4) MAC Layer PHY Layer Application Customer 802.2 LLC ZigbeeAlliance SSCS IEEE Silicon Zigbee Stack Application

40. IEEE 802.15.4 based LR-WPAN Device Classification: Fully functional device (FFD) : Can talk to FFD as well as RFD Can function as PAN coordinator or as a device Can be used in any topology Reduced functional device (RFD): Can only talk to FFD Does extremely simple tasks. Limited to Star topology Network is made up many PANs each managed by a PAN Coordinator. PANs are identified by unique PAN ID. Network Topologies : Star Peer to peer Mesh Cluster Tree STAR Mesh Cluster Tree FFD PAN Coordinator RFD

41. IEEE 802.15.4 based LR-WPAN Two types of Communication modes : Beacon Enabled Beacon is transmitted by FFD periodically after each BI (Beacon Interval) Super frame structure is followed Suited for higher data rate kind of applications Non Beacon Enabled Simply reduces to CSMA/CA MAC Suited for very simple applications like periodic sensing Synchronization is achieved by : Beacon tracking mode Node simply synchronizes to first beacon and uses the information to switch on just before the next beacon Non-Tracking mode Sync done only when the data needs to be transmitted

42. IEEE 802.15.4 based LR-WPAN IEEE 802.15.4 MAC provides following services: MAC Data Service – responsible for transmission and reception of MPDU (MAC protocol data units) across the PHY data service MAC Management Services – interfacing to MLME-SAP (MAC Layer Management Entity – Service Access Points) Features : Beacon and GTS Management Channel Access Frame Validation and acknowledge frame delivery Association & Dissociation IEEE 802.15.4 PHY provides following services : PHY data service – responsible for transmission and reception of PPDU (PHY protocol data units) across the channel PHY management service – interfacing to PLME-SAP (PHY Layer Management Entity – Service Access Point) Features : ED (Energy Detection) LQI (Link quality Indication) CCA (Clear Channel Assessment)

43. IEEE 802.15.4 MAC – Super Frame Superframe consists of : Active period, whose length is defined by SD (Superframe Duration), is divided in 16 equal slots : Slot zero is reserved for Beacon CAP (Contention Access Period) starts in the next slot after beacon is transmitted and nodes compete using slotted CSMA/CA CFP (Contention Free period) starts in the slot after CAP. GTS (Guaranteed time slots ) are used for data transfer. Inactive period Node sleeps in inactive period

44. IEEE 802.15.4 MAC Layer – The Super Frame ( Transmitted by PAN Coordinator, is of variable length for GTS allocation ) ≥𝑎𝑀𝑖𝑛𝐶𝐴𝑃𝑙𝑒𝑛𝑔𝑡h Nodes Sleep here Slotted CSMA if Beacon enabled At MAX 7 GTS of one or more slots. GTS can uplink or downlink 𝟎≤𝑺𝑶 ≤𝑩𝑶 ≤𝟏𝟒 Questions: 1) Duty Cycle ? & 2) How should the radio be in sync in non tracking mode ?

45. IEEE 802.15.4 MAC Layer – Data Services Direct Data Transfer Message Sequence Diagram

46. IEEE 802.15.4 MAC Layer – Data Services Indirect Data Transfer Message Sequence Diagram

47. IEEE 802.15.4 MAC Layer – MLME

48. IEEE 802.15.4 - Association Message Sequence Diagram

49. IEEE 802.15.4 - Dissociation

50. IEEE 802.15.4 - Orphaning

51. Slotted CSMA/CA in IEEE 802.15.4 SlottedCSMA Delay for random(2𝐵𝐸 −1) unit backoff periods NB= 0 , CW= 0 Battery Life Extension ? BE = min(2, 𝑚𝑎𝑐𝑀𝑖𝑛𝐵𝐸) Perform CCA on backoff period boundary Y BE = 𝑚𝑎𝑐𝑀𝑖𝑛𝐵𝐸 Channel Idle ? Y N N Locate backoff period boundary 𝐶𝑊=2, 𝑁𝐵=𝑁𝑆+1, 𝐵𝐸=min⁡(𝐵𝐸+1, 𝑎𝑀𝑎𝑥𝐵𝐸) 𝐶𝑊=𝐶𝑊−1 N NB > macMacCSMABackoffs ? 𝐶𝑊=0 ? N Y Y Failure Success Used in Beacon Enabled mode

52. Unslotted CSMA/CA in IEEE 802.15.4 UnslottedCSMA NB= 0 , BE =macMinBE Y Delay for random(2𝐵𝐸 −1) unit backoff periods NB > macMacCSMABackoffs ? N Perform CCA Failure Channel Idle ? Y Success N Used Beacon Disabled Mode 𝑁𝐵=𝑁𝑆+1, 𝐵𝐸=min⁡(𝐵𝐸+1, 𝑎𝑀𝑎𝑥𝐵𝐸)

53. WSN MAC protocols summary

54. References [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) [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 [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, [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 [5] IlkerDemirkol, CemErsoy, and FatihAlagöz, “MAC Protocols for Wireless Sensor Networks: A Survey”, IEEE Communication Magazine, April 2006, Vol. 44 Issue 4 [6] Anurag Kumar, D. Manjunath and Joy Kuri, “Wireless Networking” , 2008 Edition [7] SinemColeriErgen, “ZigBee/IEEE 802.15.4 Summary”