wireless sensor network
TRANSCRIPT
Wireless Sensor Network:MAC protocol
김대우
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INTRODUCTION TO WSN01
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Introduction to WSN
Definition:
• A sensor network is composed of a large number of sensor nodes that are densely deployed inside or very close to the phenomenon
– Multi-hop, self-organize
– Wireless communication
– Cooperative sensing, collection, process
– Send to observe.
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Introduction to WSN
WSN communication Architecture
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Wireless Communicationlink
Sensor fieldInaccessibleEnvironment
Sinkor
Base station (BS)
To Externalnetwork
Sensors
• Intel Research Laboratory at Berkeley initiated a collaboration with the College of the Atlantic in Bar Harbor and the University of California at Berkeleyto deploy wireless sensor networks on Great Duck Island, Maine (in 2002)
• Monitor the microclimates in and around nesting burrows used by the Leach's Storm Petrel
• Goal : habitat monitoring kit for researchers worldwide
Habitat Monitoring on Great Duck Island
Introduction to WSN
Introduction to WSN
• Wildfire Instrumentation System Using Networked Sensors
• A project by University of California, Berkeley CA.
FireBug
Introduction to WSN
Application
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Introduction to WSN
• Differences between WSN and ad-hoc network
– Battery powered nodes Energy efficiency
– Large quantity of densely deployed nodes
– This dense deployment brings high degree of interactions
– Resources constraint
– Auto configuration and auto organization
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Design Considerations
• Level 1 issues
– Energy efficiency • Often difficult recharge batteries or replace them
• Prolonging the life-time is important
– Collision avoidance - a basic task of MAC protocols
– Good scalability
– Hardware Constraints• Clock drift
• Memory Constraints
• Level 2 issues
– Latency, fairness, throughput, bandwidth
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The sensor network protocol stack
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• Power management• Mobility management
- Sensor node의 움직임
• Localization- Sensor node의 위치
• Synchronization
MEDIUM ACCESS CONTROL02
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MAC
• Link
– 통신 경로상의 인접한 노드들을 연결하는 통신 채널을 링크라고 한다.
• Link의 종류
– Point-to-point link
– Broadcast link• multiple access problem
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Challenges for MAC
• Energy Consumption
– Idle listening
– Collisions
– Protocol overhead
– Overhearing
– Transmit vs. receive power
• Event-based Networking
• Correlation
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WSN에서 MAC
• 3 Types of MAC techniques
– Contention Based
– Reservation Based
– Hybrid solution
• CSMA, TDMA가 사용된다.
– FDMA : BW의 한계 때문에 사용하기 힘들다• Narrow band
– OFDMA, CDMA의 경우 cost constraints
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CONTENTION-BASED MAC02-1
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Carrier Sense Multiple Access(CSMA)
• Listen before transmitting
• Stations sense the channel before transmittingdata packets.
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S R H
X Collision at R
Hidden Terminal Problem
CSMA with Collision Avoidance
• Stations carry out a handshake to determine which one can send a data packet
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S R H
RTSCTS
Data
ACK Backoff due to CTS
Contention-Based Medium Access
• 장점
– Flexible
– Robustness
– Scalability
– With out Message exchanges, infrastructure
• 단점
– Density 높아지면 collision probability 높아진다
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Contention-Based Medium Access
• CSMA/CA
– Energy efficiency가 낮다• Idle listening
• Sleeping mode
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AsynchronousSynchronous
Contention-Based Medium Access
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Contention-
based MAC
S-MAC B-MAC CC-MAC
Synchronous MAC
• Nodes define common active/sleep periods
Active period
– Used for communication
Sleep period
– Saving energy
• Nodes maintain a certain level of synchronization
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S-MAC
• Design
– Goal• Reduce energy consumption
• Support good scalability and collision avoidance
– Solutions to energy inefficiency issues• Collision avoidance - using RTS and CTS
• Overhearing avoidance - switching the radio off when the transmission is not meant for that node
• Control overhead - Message Passing
• Idle listening - Periodic listen and sleep
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S-MAC (Sleep MAC)
• Goal: Reduce idle listening -> power save
• Trade off : throughput, latency
– Virtual Cluster 구성
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Network Assumption
• Composed of many small nodes
– Short-range, multi-hop communication
• Most communication will be between nodes as peers, rather than to a single base station
• In-network processing is critical to network life time
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• Periodic listen and Sleep
– Duty Cycle
– MAC scheme
Periodic Listen and Sleep
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Choosing Schedule (Virtual cluster)
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Listen first
Set its own
schedule
(Synchronizer)
Broadcast SYNC
Listen SYNC
w/o neighbor
discard schedule
Follow the
schedule
(Follower)
Announce its
schedule
Both schedules
(Border Node)
No signal SYNC
Border Node
• Not all neighboring nodes can synchronize together
• Border node follow both schedule
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Schedule 2
Schedule 1
Maintaining Schedule
• Synchronization error
– Clock Drift
• Solution
– Relative timestamp
– Listen period is significantly longer than clock drift rates
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Collision Avoidance
• Problem: Multiple senders want to talk
• Solution: Following IEEE 802.11 ad hoc procedures
– Carrier sense
– Randomized backoff time
– RTS/CTS for hidden terminal problem
– RTS/CTS/DATA/ACK sequence
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Overhearing Avoidance
• Problem: Receive packets destined to others
• Solution: Letting other nodes sleep after they hear an RTS or CTS packet
– Which nodes should sleep?• All neighbors except sender and receiver
– How long?• The duration field in each packet informs other nodes the
sleep interval
• Sleep until the NAV becomes zero
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Adaptive Listening
• Problem : There is potential Delay on each hop
• Solution : Transmit two hop in one duty cycle
– RTS, CTS -> both the neighbors of the sender and receiver will learn about transmission
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Message passing• Problem: Sensor net in-network processing
requires entire message
• Solution: Don’t interleave different messages
– Advantages• Reduces latency of the message
• Reduces control overhead
– Disadvantage• Node-to-node fairness is reduced, as nodes with small
packets to send has to wait till the message burst is transmitted
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S-MAC 평가
• 장점
– 기존 CSMA/CA에 비해 Energy를 적게 사용
– Lightly loaded networks에 강하다
– Cluster-based protocol에 적용하기 쉽다
• 단점
– Heavy loaded에선 Energy 이득이 적다
– High-density, Heavy loaded의 경우Short listen interval 동안의 contention으로 충돌 가능성이 높아질 수 있다.
– Sync를 위한 Energy 소비
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T-MAC
• Static schedules is the waste of energy
• When traffic is low, reduce the Listen Interval
– 𝑇𝐴 > 𝐶 𝑐𝑜𝑛𝑡𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙 + 𝑅(𝑅𝑇𝑆) + 𝐶(𝐶𝑇𝑆)
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Asynchronous MAC
• B-MAC
– Adaptive preamble sampling scheme
• CC-MAC
– Using spatial correlation information
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B-MAC
• Design goal
– Low power operation
– Effective collision
– Simple implementation, small code & RAM size
– Flexible Interface
– Efficient channel utilization at low & high data rates
– Reconfigurable by network protocols
– Tolerant to changing RF/Networking conditions
– Scalable to large numbers of nodes
B-MAC Design
• Simple
– Low power listening via Preamble
– CSMA via CCA (Clear Channel Assessment) & back-off
– Acknowledgment
• Export control to higher services to support wide variety of WSN workloads
– WSNs are supposed to support various applications
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B-MAC: motivation
• Drawback of Synchronous Operation
1. Periodic messages; SYNC packet
2. All nodes are active during the listen period
3. S-MAC의 경우 virtual cluster 구성해야 한다.
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B-MAC: motivation
• S-MAC is not only a link protocol, but also network and organization protocol
• Application must rely on S-MAC’s internal policies
• B-MAC
– Particular lower policy
– Easy to use
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• Sender Sends a preamble before each packet to wake up the intended receiver
Preamble period = 𝑇𝑝
• Sender node broadcast Preamble
– Intended node : Start listening
– Other node : Sleeping mode
B-MAC: Low power Listening
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B-MAC: Low power Listening
• Sleep and wake schedule w/o synchronization
• Each node determines its own schedule
• Only Transmitter and receiver synchronize each
other
• Longer Preamble
• Length of Check Interval configurable by
higher layers
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B-MAC: Clear Channel Assessment
• Ambient noise change by environment
• Sensing the activity channel 중요
• Failure of detecting Preamble
– Detection failure
– Collision
• CCA
– Noise floor estimation
– Signal detection
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Noise floor estimation
• Each node calculate noise floor
• Signal strength samples are taken while channel is idle
– Right after transmission packet
– No valid data is received
• Weighted moving average with decay𝐴𝑡 = 𝑎𝑆𝑡 + 1 − 𝑎 𝑆𝑡−1
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Signal Detection
• Traditional approach: threshold approach
• Detecting failure
– Channel noise depends on the environment
– Fluctuation
• Outlier detection
– Valid packet could never have an outlier significantly below the noise floor
– If outlier exists, channel is clear
– If 5 samples are taken and no outlier is found, the channel is busy
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Signal Detection
• Outlier detection
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B-MAC: 평가
• 장점
– Efficiency carrier sensing(CCA)
– Noise floor estimation -> 주위 환경 변화에 강하다
– Simple light weight -> memory 적게 사용
– Can be controlled by higher layer easily• High layer가 turn on/off 가능 -> 다양한 적용 가능
• 단점
– Simple CSMA – hidden terminal problem 해결X• High traffic 에서 나쁘다
– LPL mechanism• High-traffic 에서 preamble 계속 쏴줘야 함 비효율적
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STEM
B-MAC의 단점
• Sender가 Preamble를 긴 시간 동안 보낸다.
• 목적 노드를 발견해도 Preamble 주기가 끝난 후data 전송 시작한다.
– Energy 낭비
– Latency 증가
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STEM
• Preamble 보내는 중간에 목적노드 발견하면 바로data 전송 시작
• 각 노드에서는 listen interval 이 조금 늘어나는 단점
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Wise MAC
• Sender node에서 목적 노드의 sleep schedule를알고 있다면?
– Preamble을 많이 줄일 수 있다.
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Wise MAC
• ACK에서 자신의 schedule을 알려준다
– 그 schedule에 맞춰서 preamble을 broadcast한다.
• 단점
– Memory 사용
– Clock drift 오차 생각해서 preamble 시간 정해줘야함
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CC-MAC
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CC-MAC
• Spatial Correlation-Based Collaborative MAC
• Spatially dense sensor deployment
• Sensor records may be spatially correlated subject to an event
• Iterative Node Selection– At Sink Node
• CC-MAC protocol– E-MAC
– N-MAC• At Sensor Nodes
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Architecture & Spatial Correlation Model
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• Correlation model and architecture
Spatial Correlation Model
• Distortion𝐷𝐸 = 𝐸 𝑑𝐸 𝑆, 𝑆
• Observations of each sensor node can be modeled as joint Gaussian random variables
𝐸 𝑆𝑖 = 0, 𝑖 = 1, … , 𝑁
𝑣𝑎𝑟 𝑆𝑖 = 𝜎𝑆2, 𝑖 = 1, … , 𝑁
𝑐𝑜𝑣 𝑆𝑖 = 𝜎𝑆2𝑐𝑜𝑟𝑟 𝑆𝑖 , 𝑆𝑗
𝑐𝑜𝑟𝑟 𝑆𝑖 , 𝑆𝑗 = 𝜌𝑖,𝑗 = 𝐾𝜗 𝑑𝑖,𝑗 =𝐸[𝑆𝑖 , 𝑆𝑗]
𝜎𝑆2
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Spatial Correlation Model 𝑋𝑖 = 𝑆𝑖+ 𝑁𝑖
𝑌𝑖 =𝑃𝐸
𝜎𝑆2+𝜎𝑁
2 𝑋𝑖 , (𝑃𝐸:encoding power constraint)
𝑍𝑖 =𝐸[𝑆𝑖𝑌𝑖]
𝐸[𝑌𝑖2]
𝑌𝑖
𝑆 𝑀 =1
𝑀
𝑖=1
𝑀
𝑍𝑖
𝐷 𝑀 = 𝐸 𝑆 − 𝑆 𝑀2
= 𝜎𝑆2 −
𝜎𝑆4
𝑀 𝜎𝑆2 + 𝜎𝑁
2 2
𝑖=1
𝑀
𝜌 𝑠,𝑖 − 1 +𝜎𝑆
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𝑀2 𝜎𝑆2 + 𝜎𝑁
2 2
𝑖=1
𝑀
𝑖≠𝑗
𝑀
𝜌 𝑖,𝑗
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Spatial Correlation Model 결론
• Located close to the event source S
• Located as far apart from each other as possible
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Iterative Node Selection Algorithm
• Goal– Find the ideal locations of representative nodes
such that the required distortion can be maintained at the sink
• Input– Statistical properties of the node distribution
• Output– Correlation radius value for distributed operation
Assumption• Statistical properties of the WSN topology is known by the
INS algorithm; density and node distribution
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Iterative Node Selection Algorithm
Vector Quantization method in coding theory
• Form a sample topology
• Start with selecting all the nodes in the event region as representative node
• Iteratively, decreases the number of representative nodes until the distortion constraint 𝐷𝑚𝑎𝑥 is met
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Iterative Node Selection Algorithm
• 결과
– Correlation Radius
– Correlation Neighbor
• correlation radius,𝑟𝑐𝑜𝑟𝑟,is informed to the individual nodes
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CC-MAC Protocol at node
• 𝑟𝑐𝑜𝑟𝑟 is broadcast to each sensor node during the network setup
• E-MAC
– To prevent the transmission of redundant information
• N-MAC
– To prioritize the forwarding of filtered data to sink
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Packet Structure
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• First Hop(FH)
–Differentiate the type of packet
• Newly generated packet
• Route-thru packet
–처음에는 set 한번 거치는 순간clear
Event MAC
• Forming correlation regions based on the correlation radius
• In each correlation region, single representative sensor node transmits data for a specific duration
• All other node stop transmission attempts
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Event MAC
• First Contention phase(FCP)
– All node with event contend for the medium
– Each node sets FH field of RTS and tries to capture the medium
– Some of sensor nodes access the channel: Representative Sensor Node
• 𝑛𝑗 listen to RTS packet of 𝑛𝑖 with set FH𝒅(𝒊,𝒋) < 𝒓𝒄𝒐𝒐𝒓
– Stop its transmission𝒅(𝒊,𝒋) > 𝒓𝒄𝒐𝒐𝒓
– Contend for the medium if it has a packet to send
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Network MAC
• Route-thru packet must be given priority
– correlation has already been filtered out using E-MAC
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RESERVATION-BASED MAC
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02-2
Reservation-Based Medium Access
• Collision-free communication
• TDMA가 사용된다.
– FDMA : BW의 한계 때문에 사용하기 힘들다• Narrow band
– OFDMA, CDMA의 경우 cost constraints
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TRAMA
• Traffic-adaptive Medium Access Protocol
• Energy Efficiency
– No collision
– Sleeping node
• Based on time-slot structure
• Distributed election scheme
– No central entity
– Using priority information, allocate transmission slot
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TRAMA
4 Main Phase
• Neighborhood discovery
• Traffic information exchange
• Schedule establishment
• Data transmission
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TRAMA
3 Mechanism
• Neighbor Protocol
• Schedule Exchange Protocol
• Adaptive Election Algorithm
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Neighbor Protocol
• Main Function: – Gather two-hop neighborhood information by
using signaling packets.
• TRAMA start in random access mode
• Random access period– Periodically operates
– New node join the network
– Time synchronization
– Send out their neighborhood updates and receive update from neighbors
– Collision 발생 가능• 충분한 시간 할당
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Neighbor Protocol
• Signal packet 교환
– Neighborhood update
– “Keep alive” beacon when no update
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• Because a node knows the one-hop neighbors of its one-hop neighbors, eventually consistent two-hop neighborhood information makes its way across the network
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Schedule Exchange Protocol (SEP)
• Schedule consists of list of intended receivers for future transmission slots.
• Schedules are established based on the current traffic information at the node.
• Propagated to the neighbors periodically.
• SEP maintains consistent schedules for the one-hop neighbors.
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Adaptive Election Algorithm (AEA)
• Decides the node state as either Transmit, Receive or Sleep.
• Uses the schedule information obtained by SEP.
• Nodes without any data to send are removed from the election process, thereby improving the channel utilization.
Adaptive Election Algorithm (AEA)
• Adaptive election algorithm
– Priority정보를 이용하여 time slot을 할당
– Globally known hash function𝑝 𝑢, 𝑡 = ℎ 𝑢 + 𝑡
– After each node determines the slot to transmit, informs its intended receivers
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TRAMA: 평가
• 장점
– Energy efficiency: increasing sleeping
– Decrease the collision rate
• 단점
– Delay
– Frequent message exchange
– High density network -> overhead
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PMAC (Pattern MAC)
Pattern 교환을 통해 서로의 스케쥴 정한다.
• Each node makes own patern
• A sleep-wakeup pattern is a stream of bits
• Pattern is tentative plan, it can change
• The schedule for a node is derived from its own pattern
• The schedules are decided based on a node’s own traffic and neighbors
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PMAC (Pattern MAC)
Pattern Generation
• 𝑃𝑗: binary string representing the pattern of node j
• 𝑃𝑗 is restricted a pattern to be 0𝑚1(𝑤ℎ𝑒𝑟𝑒 𝑚 = 0,1, … , 𝑁 − 1)
• Example
1, 01, 001, 0001, … , 0𝑁−11
– Bit 1: the node intends to stay awake
– Bit 0: the node intends to sleep
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PMAC (Pattern MAC)
Pattern Exchange
• Pattern repeat time frame
– Each node repeats its current pattern
• Pattern exchange time frame
– New patterns are exchanged between neighbors
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PMAC (Pattern MAC)
• Schedule Generation
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Pattern bit at node j
Packet to sendPattern bit at receiving node
Schedule at node j
1 1 1 1
1 1 0 1-
1 0 * 1-
0 1 1 1
0 1 0 0
0 0 * 0
HYBRID MAC
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02-3
Hybrid Medium Access
장점 단점
Contention-based
Overhead 낮다Low contention
High traffic
Reservation-based
Schedule-> collision 낮다
CapacityEnergy efficiency
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• Combine best of both
• Eliminate worst of both
Z-MAC (Zebra-MAC)
• Use a base TDMA schedule – Rely on time slot
• Each slot can be stolen– Provided owners are not transmitting
– Stealing done through competition (CSMA)
• CSMA in low contention
• TDMA in high contention
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Z-MAC
• 4 main components
– Neighbor discovery• 2-hop의 정보를 알고 있음
– Local frame exchange• Neighbor 수에 따라서 Frame 크기 조절
– Transmit control• Steal time slot
– Time synchronization
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Z-MAC: Local frame exchange
Time Frame Rule (TF Rule)
• Let node i be assigned to slot 𝑆𝑖, and let number of nodes within two hop neighborhood be 𝐹𝑖
• then node i's time frame is set to be 2𝑎, where positive integer a is chosen to satisfy condition
2𝑎−1 ≤ 𝐹𝑖 < 2𝑎 − 1
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Z-MAC: Transmission Control
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Time Slots
A(0)
B(1)
0 021
Ready to Send, Start Random(To) Backoff
Ready to Send, Start To + Random(Tno) Backoff
After Backoff, CCA Idle
After Backoff, CCA Busy
Non-Owner Backoffs
Owner Backoffs
Z-MAC: Time synchronization
• Local clock synchronization among senders
• Each data sender transmits a synchronization message containing its current clock value periodically
• Update clock value by taking a weighted moving average
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결론
• Energy efficiency
• Purpose of Application
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WSN communication Architecture
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Physical Layer
• Use available Wireless Protocols:
• Radio Frequency : ISM Band 433MHz to 2.4GHz
• Modulation : Phase-Shift Keying
• BPSK or MPSK
• Data Rate : 0.25 Mbps to 54 Mbps
Source [2]
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Data Link Layer
Medium Access Control (MAC) Protocols:
- Sensor-MAC (SMAC):
- periodic listen & sleep
- collision avoidance
- Timeout-MAC (TMAC):
- enhance the energy savings in SMAC
Source [1]
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Network Layer
Two Types of Routing Algorithms:
1) Hirarchical Routing Protocols:Low-Energy Adaptive Clustering Hierarchy (LEACH)
2) Location-Based Routing Protocols:Geographical Adaptive Fidelity (GAF)
Source [2]
CSMA protocol
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CSMA/CA
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NAV mechanism in IEEE 802.11.
• Network Allocation Vector(NAV)
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