無線網路CH9
學校 : 中華大學 系所 ： 通訊工程學系 老師 ： 余誌民教授
1
Sensor Network
1.Introduction Goal
Wireless Sensor Network Ubiquitous Computing Ubiquitous Network Society Human-centric
1.Introduction Ubiquitous
Ubiquitous 7A Anytime Anyone Anywhere Any Device Affordable All Security Any Information/Service
1.Introduction General Purpose
A wireless sensor network (WSN) is a wireless network using sensors to cooperatively monitor physical or environmental conditions The development of wireless sensor networks was originally motivated by military applications. Wireless sensor networks are now used in many wide-range application areas.
1.Introduction Sensors
Ultrasonic Magnetic Sensor (22.5× ×22.5× ×39mm)
WII sensor (240× ×35× ×15mm)
Image Sensor Modules (8× ×8× ×5.7mm)
1.Introduction Typical Sensor Network sensor
Data gathering sensor Data transmitting
sensor
Relay node
Center
Relay node
processing sensor
sensor
sensor
1.Introduction sensor characteristics
Wireless sensors are small devices that gather information. Pressure, Humidity, Temperature Speed, Location
Wireless sensors have some characteristics: Low power Small size Low cost
1.Introduction sensor network characteristics
Primary Function Sample the environment for sensory information Propagate data back to the infrastructure
Traffic pattern in sensor network Low activity in a long period Bursting data in short time Highly correlated traffic
1.Introduction sensors categories
Sensors can be classified into two categories: Ordinary Sensors Data gathering Ordinary Sensors require external circuitry to perform some dedicated tasks like data analyzing.
Smart Sensors Data gathering and processing Smart Sensors have internal circuitry to perform dedicated tasks.
1.Introduction Related Work
Related work CSMA To improve the energy consumption by avoiding overhearing among neighboring nodes
TDMA No contention-introduced overhead and collisions Not easy to manage the inter-cluster communication and interference Not easy to dynamically change its frame length and time slot assignment
1.Introduction Related Work PAMAS Power off radio when not actively transmitting and receiving packet.
Zigbee Combined with IEEE 802.15.4 (Low-Rate
Wireless Personal Area Network, LR-WPAN) Low rate: 250kbps Short distance: 50-300m Low power consumption frequency band:
Global: 2.4GHz ,16 channels America: 915MHz, 10 channels Europe: 868MHz, 1 channel.
1.Introduction Zigbee stack Zigbee Platform Stack and IEEE802.15.4 ZigBee or OEM
Application/Profiles
Application Framework Network / Security Layers
ZigBee Alliance Platform
MAC Layer
IEEE 802.15.4
PHY Layer
1.Introduction Zigbee Application
Reference: NTP無線感測網路與ZigBee 協定簡介
2.MAC for Sensor Network Sensor Network MAC Protocol
Carrier Sensing Only during low traffic load.
Backoff Backoff in application layer is desired other than in MAC layer.
Contention RTS-CTS only during high traffic load.
2.MAC for Sensor Network Sources of Energy Wastage
The major sources of energy wastage are:
Collisions Overhearing Control packet overhead Idle listening
Achieving good scalability and collision avoidance capability is necessary.
2.MAC for Sensor Network S-MAC Sensor-MAC (S-MAC): Medium Access Control for Wireless Sensor Networks
S-MAC is a medium-access control (MAC) protocol designed for wireless sensor networks.
Sensor networks are deployed in an ad hoc fashion, with individual nodes remaining largely inactive for long periods of time, but then becoming suddenly active when something is detected.
2.MAC for Sensor Network S-MAC
These characteristics of sensor networks and applications motivate a MAC that is different from traditional wireless MACs such as IEEE 802.11 in almost every way Energy conservation and self-configuration are primary goals. Per-node fairness and latency are less important.
2.MAC for Sensor Network Three techniques in S-MAC
S-MAC uses three techniques to reduce energy consumption. Nodes go to sleep periodically. Nearby nodes form virtual clusters to synchronize their wake-up and sleep periods to keep the control packet overhead of the network low. Message passing is used to reduce the contention latency and control overhead.
2.MAC for Sensor Network Three techniques in S-MAC
Periodic Listen and Sleep: Nodes do not waste energy by listening to an empty channel or when a neighboring node is transmitting to another node. Nodes use RTS and CTS to talk to each other and contend for the medium.
2.MAC for Sensor Network Three techniques in S-MAC
Collision and Overhearing Avoidance: S-MAC adopts a contention-based scheme to avoid collisions. A duration field is introduced in each transmitted packet which indicates how much longer the transmission will last. When a node receives a packet, it will not transmit any packets for at least the time that is specified in the duration field.
2.MAC for Sensor Network Three techniques in S-MAC
Collision and Overhearing Avoidance: Overhearing is avoided by letting the nodes, which get RTS and CTS packets which are not meant for them, go to sleep. All immediate neighbors also go to sleep till the current transmission is completed after a sender or receiver receives the RTS or CTS packet.
2.MAC for Sensor Network Three techniques in S-MAC
Message Passing: Long messages are fragmented into smaller messages and transmitted in a burst. To avoid the high overhead and delay encountered for retransmitting when message is lost.
ACK messages are used to indicate if a fragment is lost at any time. The sender can resend the fragment again. The ACK message also have the duration field to reduce overhearing and collisions.
3. Challenges Challenges: 1. Energy Efficiency: Power consumptions are crucial to wireless sensor network applications because sensor nodes are not connected to any energy source. Energy efficiency is a dominant consideration no matter what the problem is. Sensor nodes only have a small and finite source of energy. Many solutions, both hardware and software related, have been proposed to optimize energy usage.
3. Challenges 2. Ad hoc deployment: Most sensor nodes are deployed in regions which have no infrastructure. We must cope with the changes of connectivity and distribution.
3. Unattended operation: Generally, once sensors are deployed, there is no human intervention for a long time. Sensor network must reconfigure by itself when certain errors occur.
3. Challenges 4. Dynamic changes: As changes of connectivity due to addition of more nodes or failure of nodes, Sensor network must be able to adapt itself to changing connectivity.
4.Coverage Coverage can be classified into three types: Area coverage deployment of sensors to cover a given area Point coverage deployment of sensors to cover a set of points Barrier coverage The goal is to minimize the probability of undetected penetration through the barrier. To find a path in a region For any point on the path, the distance to the closest sensor is minimized.
4.Coverage Area coverage
Area coverage deployment of sensors to cover a given area
4.Coverage Point coverage
Point coverage deployment of sensors to cover a set of points
4.Coverage Point coverage A
Barrier coverage To find a path from A to B For any point on the path, the distance to the closest sensor is minimized.
B
5.Localization In sensor networks, nodes are deployed without priori knowledge about their locations.
Estimating spatial-coordinates of the node is referred to as localization.
5.Localization GPS
Global Positioning System (GPS) is an immediate solution. There some factors against the usage of GPS: GPS can work only outdoors. GPS receivers are too expensive to unsuitable for wide-range deployment. It cannot work in the presence of obstructions.
5.Localization Categories
Localization can be classified into two categories: Fine-grained Based on timing / signal strength Coarse-grained Based on proximity
5.Localization Proximity base localization
Trilateration / Multilateration technique Proximity based localization: Some nodes which can know their position through some technique (ex. GPS) broadcast their position information. Other nodes listen to these broadcast messages and calculate their own position. A simple method would be to calculate its position as the centroid of all the positions it has obtained. This method leads to accumulation of localization error.
5.Localization Trilateration Example
Trilateration A is 5m from B A is 10m from C A is 8m from D
C B
A
D
5.Localization Trilateration
Trilateration is a geometric principle which allows us to find a location if its distance from other nodes are known. The same principle can be extended to three-dimensional space. Four spheres would be needed to locate certain point in 3D space.
5.Localization Fine-grained method
Signal strength method Attenuation happens when signals are propagated. We can use the degree of attenuation to calculate the distance.
Timing method The distance between two nodes is determined by the time of flight of the signal.
6.Routing Categories
Routing protocols can be divided into two types. Proactive routing protocol Proactive routing protocol maintain consistent updated routing information between all nodes. To update routing table periodically.
Reactive routing protocol Routes are created only when they are needed.
6.Routing Three types in sensor network
Because of the energy constrained nature of sensor networks, conventional routing protocols have many limitations when being applied to sensor networks. Three types of routing protocol in sensor network: Data-centric Hierarchical Location-based
6.Routing Data-centric
Data-centric: Managers broadcast a Query message to the network. If a sensor observes some events related to the Query message, it sends the data to the data center. Data aggregation: sensor1 Data A Data A Relay node1
sensor2
Data A
Data Center
6.Routing Data-centric
Data centric: Flooding Flooding is one of basic data transmitting methods. If any sensor receives or generates some packets, it will broadcast these packets to all its neighbors. Nodes may receive duplicate data. More power consumption.
6.Routing Data-centric
Data centric: Sensor Protocols for Information via Negotiation (SPIN) There are three messages in SPIN: Advertisement (ADV): When a node has some data to send, it sends an ADV message to its neighbors containing data descriptor (meta-data). Request (REQ): When a node wants to receive some data. It sends an REQ message first. DATA: Actual data message with a meta-data header.
6.Routing Data-centric SPIN:
ADV (meta data A)
Node1 ADV (meta data A)
Node2
Node3
ADV ADV DATA REQ (meta data A) (meta data A)
Node4 ADV (meta data A)
Node5
Node6 DATA REQ ADV (meta data A)
Node7
6.Routing Data centric Data centric: Directed Diffusion This is a destination-initiated reactive routing technique. Routes are established when requested.
A interest is propagated throughout the network for named data by a node and data which matches this interest is then sent toward this nodes. Interests are described by a list of attribute-value pairs. Example: type=birds & response=20 ms
6.Routing Data centric
Directed Diffusion The propagation of data and its aggregation at intermediate nodes on the way to the request originating node are determined by the messages which are exchanged between neighboring nodes within some distance.
6.Routing Data centric Directed Diffusion: Node1
Node1 interest Gradient
interest interest
Node2
interest
Node4
Gradient
Node2
interest
Gradient
Gradient
interest
Node3
Gradient
Node5 interest
Node3
Node6 interest
Forward interest
Gradient
Node5
interest
interest
Node4
Gradient
Gradient
Gradient
Node7 Node6
Gradient
return path (Gradient)
Node7
6.Routing Data centric Directed Diffusion: Sender can choose the best return path. EX: minimum response time, least hops Node1 Node4 Node2 Node5 Node3
Node7 Node6
6.Routing Hierarchical
Hierarchical: Low Energy Adaptive Clustering Hierarchy (LEACH) LEACH is a two-tier protocol. Cluster head Cluster member Every node runs a random algorithm periodically to decide its identity. (cluster head or not)
6.Routing Hierarchical
LEACH All cluster heads broadcast Advertisement (ADV) message and other nodes decide which cluster they belong to according the strength of ADV message. Cluster members only send data to their cluster head. Then, cluster heads reply data to Sinks.
6.Routing Hierarchical Cluster 2 Node6
LEACH Example
Node5
Node7 Cluster Head2
Node8
Node4
Node9
Node3
Node10
Sink Cluster Head1
Cluster Head3
Node2
Node11
Node1
Node12
Node13
Cluster 3 Cluster 1
6.Routing Location-based
Location-based: Geographic Adaptive Fidelity (GAF) GAF divides the network into several virtual grids.
For adjacent virtual grids A and B, every node in A can directly connect with every node in B.
In GAF, every node has three types of status:
Active Discovery Sleep
6.Routing Location-based
GAF: Initially, every node is in discovery status and tries to find out nodes belong to the same grid with itself. Every node in discovery status sets a timer “Td�. Once the Td timer ends, Nodes broadcast discovery message and get into active status.
6.Routing Location-based
GAF: When a node is in active status, it will start a timer “Ta�. Data transmission is allowed until Ta timer ends. In active status, nodes will periodically broadcast discovery message at Td intervals. Once Ta timer ends, nodes return to discovery status.
6.Routing Location-based
GAF: If a node in discovery status receives a discovery message sent from the node which is in the same grid and has higher ranking, it will get into sleep status. After a Ts timer, it will return to discovery status. Ranking can be done by remaining power or ID sequence.