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Z-MAC: a Hybrid MAC for Wireless Sensor Networks Injong Rhee, Ajit Warrier, Mahesh Aia and Jeongki Min Dept. of Computer Science, North Carolina State University SenSys’ 05 Presented by Seung-Min Jung Computer Architecture Laboratory
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2 Contents Design of Z-MAC 2 Conclusion 4 Introductions 31 Performance Evaluation 33
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3 What is Z-MAC? A Hybrid MAC Combine the strengths of CSMA and TDMA while offsetting their weakness CSMA (Carrier Sense Multiple Access) High channel utilization and low latency under low contention Hidden terminal problem (collisions) TDMA (Time Division Multiple Access) No hidden terminal problem and high channel utilization under high contention Not practical due to too many problems
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4 Four important sources of wasted energy in WSN: Idle Listening (required for all CSMA protocols) Overhearing (since RF is a broadcast medium) Collisions (Hidden Terminal Problem) Control Overhead (e.g. RTS/CTS or DATA/ACK) MAC Energy Usage Existing MAC Protocols (S- MAC, B-MAC) Z-MAC Approach
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5 Basic Idea of Z-MAC Each sensor node owns a time slot. A node may transmit at any time slot. The owner has the higher priority to transmit data than the non-owners. When a slot is not in use by its owner, non-owners can steal the slot. Z-MAC behaves like CSMA under low contention TDMA under high contention.
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6 # of Contenders Channel Utilization TDMA CSMA IDEAL Effective Throughput
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7 Existing Approaches Hybird (CSMA + TDMA) S-MAC by Ye, Heidemann and Estrin @ USC Duty cycled Synchronized over macro time scales for neighbor communication CSMA+Duty Cycle+LPL B-MAC by Polastre, Hill and Culler @ UC Berkeley Duty cycled, but Low power listen (LPL) –Clever way reducing energy consumption (similar to aloha preamble sampling)
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8 Design of Z-MAC Basic components Setup phase Transmission Control Explicit Contention Notification Receiving Schedule of Z-MAC Local Time Synchronization
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9 Basic components Baseline – CSMA Use Imprecise Topology and Timing Info in a robust way. Combining CSMA with TDMA Scalable and Efficient TDMA scheduling
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10 Setup Phase Including neighbor discovery slot assignment local frame exchange global time synchronization => Do many things in the setup phase! High overhead? It runs only once during the setup phase and does not run until a significant change in the network topology
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11 Setup Phase: Neighbor Discovery Steps Every node periodically broadcasts a ping to its one-hop neighbors. A ping message contains the current list of its one-hop neighbors. Through the process, each node gathers the information of its two-hop neighbors. Implementation Every node sends one ping at a random time in each second for 30 seconds.
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12 Setup Phase: Slot Assignment Using DRAND to assign time slots to every node. DRAND is a distributed implementation of RAND, used for TDMA scheduling or channel assignment for wireless networks. Ensuring no two nodes within a two-hop communication neighborhood are assigned to the same slot. The slot number assigned to a node does not exceed the size of its local two-hop neighborhood (δ). The running time and message complexity are also bounded by O(δ). 012 0 3
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13 Setup Phase: Local Framing Each node needs to decide on the period in which it can use the time slot for transmission. The period is called the time frame of the node. Time frame rule S i : the slot number assigned to node i F i : the maximum slot number within node i’s two-hop neighborhood Set node i’s time frame to be 2 a, where a satisfies 2 a-1 ≤ F i ≤ 2 a – 1. That is, node i uses the S i -th slot in every 2 a time slots. i uses the s i -th slot in every 2 a time frame (i's slots are L * 2 a + s i, for all L=1,2,3,...)
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14 Example 2 a-1 ≤ F i ≤ 2 a – 1 Node A a = 2 Node C a = 3 Network topology & the slot schedule of all nodes
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15 Transmission Control Two modes: low contention level (LCL) and high contention level (HCL). Under LCL, non-owners are allowed to compete in any slot with low priority. Under HCL, a node does not compete in a slot owned by its two-hop neighbors. To avoid being hidden terminal to the owners. A node is in HCL only when it receives an explicit contention notification (ECN) messages within the last t ECN period.
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16 Transmission Rule Node i acquires data to transmit Is node i the owner? Take a random backoff within period T o Is the channel clear? Wait until the channel is not busy Is node i in LCL? Is the current slot owned by its two-hop neighbor? Wait for T o and performs a random backoff within a contention window [T o, T no ] Transmit data!!! Postpone its transmission until the time slot is (1)not owned by a two-hop neighbor or (2) owned by itself NO YES NO
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17 Explicit Contention Notification ECN messages notify neighbors not to act as hidden terminals to the owner of each slot when contention is high. How to estimate two-hop contention? According to noise level of the channel High correlation between noise level and two-hop contention. Low noise indicates low contention.
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18 Explicit Contention Notification Steps: When a transmitting node detects high contention The node sends a unicast message, one-hop ECN, to the destination which is experiencing contention. If there are multiple destinations, it broadcasts a message with information about the multiple destinations. Assume node j receives one-hop ECN. If node j is the destination, it then broadcasts the ECN, two-hop ECN, to its one-hop neighbors. If not, it simply discards the one-hop ECN. When a node receives a two-hop ECN, it sets the HCL flags. ECN suppressing Take random backoff before the transmission of a one-hop ECN.
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19 Example S5, S2, S4 can compete as one-hop neighbors at slot 1
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20 Local Time Synchronization Time synchronization Among Neighboring senders Under high contention Z-MAC adopts a technique from RTP/RTCP (real-time transport protocol). The control message transmission rate is limited to a small fraction of session bandwidth. In Z-MAC, a node sends one synchronization packet per every 100 data packets.
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21 Performance Evaluations Implementing Z-MAC in both ns-2 and Mica2/TinyOS. Comparing the performance of Z-MAC with that of PTDMA(ns-2), Sift(ns-2) and B-MAC(ns-2 and TinyOS). Three benchmarks One-hop benchmark Two-hop benchmark Two clusters, 7 and 8 sending nodes. Multi-hop benchmark
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22 One-hop Benchmark
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23 Two-hop Benchmark Sources Sink
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24 Multi-hop Benchmark
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25 Default settings
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26 Throughput One Hop Utilization; Simulation Z-MAC B-MAC
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27 Throughput One Hop Throughput; Mica2 Experiment Z-MAC B-MAC
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28 Throughput Two Hop Utilization; Simulation Z-MAC B-MAC
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29 Throughput Multi-hop Throughput; Mica2 Experiment Z-MAC B-MAC MULTI-HOP
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30 Throughput Utilization Variation with Time Sync Error Z-MAC
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31 Energy Efficiency Z-MAC HCL B-MAC MULTI-HOP
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32 Conclusion Z-MAC combines the strength of TDMA and CSMA High throughput independent of contention. Robustness to timing and synchronization failures and radio interference from non-reachable neighbors. Always falls back to CSMA. Compared to existing MAC It outperforms B-MAC under medium to high contention. Achieves high data rate with high energy efficiency.
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Questions? Email : smjung@camars.kaist.ac.kr Office # : 5578
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34 Supplement: Low Power Listening(1) Receive data Carrier sense Receiver Long PreambleData Tx Sender Check Interval Similar to ALOHA preamble sampling Wake up every Check-Interval Sample Channel using CCA If no activity, go back to sleep for Check-Interval Else start receiving packet Preamble > Check-Interval
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35 Supplement: Low Power Listening(2) Receive data Carrier sense Receiver Long PreambleData Tx Sender Check Interval Longer Preamble => Longer Check Interval, nodes can sleep longer At the same time, message delays and chances of collision also increase Length of Check Interval configurable by higher layers
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36 Time period Time slice 12345 67 TDMA Scheduling : Using DRAND Two nodes in the interference range assigned to different time slots. Owners and non-owners C D A F B C D A E B E F Radio Interference Map Input Graph C D A E B F DRAND slot assignment 0 0 1 3 2 1
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37 DRAND Z-MAC requires a conflict-free transmission schedule or a TDMA schedule. DRAND is a distributed TDMA scheduling scheme. Let G = (V, E) be an input graph, where V is the set of nodes and E the set of edges. An edge e = (u, v) exists if and only if u and v are within interference range. Given G, DRAND calculates a TDMA schedule in time linear to the maximum node degree in G. DRAND is fully distributed, and is the first scalable implementation of RAND, a famous centralized channel scheduling scheme.
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38 Local Time Synchronization Trust factor(β t ): R drift : the max clock drift rate of each node ε clock : the max acceptable clock error I synch = ε clock / R drift : the min synchronization interval required to achieve the max clock error α synch : the max weight applying to the new clock value received S : the avg. rate at which a node receives or sends synchronization messages How to get new clock value? C avg : weighted moving avg. clock value C new : newly received clock value C avg = (1- β t )C avg + β t C new
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