vault backup: 2024-10-02 23:25:27

Autonomous Networking/notes/3 WSN.md

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+The main difference between an RFID network and a WSN is that nodes:
+- are battery powered
+- can sense the environment
+- can listen to the channel (carrier sense) and transmit spontaneously
+- can make more complex computation
+- can send packets to other nodes (e.g. for multi-hop communication)
+
+#### Roles of partecipants in WSN
+- Sources of data: measure data, report them somewhere
+- Sinks of data: interested in receiving data from WSN
+- Actors/actuators: control some devices based on data
+
+#### Deployiment options
+- Random deployiment
+	- dropped from an aircraft
+	- usually uniform random distribution for nodes over finite area is assumed
+- Regular deployment
+	- wel planned, fixed
+	- not necessarily geometric structure, but that is often a convenient assumption
+- Mobile sensor nodes
+	- Can move to compensate for deployment shortcomings
+	- Can be passively moved by some external force (wind, water)
+	- Can actively seek out "interesting" areas
+#### Characteristics of WSN
+- Scalability
+	- they need to support **large number of nodes**
+	- performance should not degrade with increasing number of nodes
+- Wide range of densities (very application dependent)
+- Limited resources for each device
+	- low amount of energy
+	- low cost, size and weight
+	- nodes may not have a global ID (e.g. an IP)
+- Mostly static topology
+
+- Service in WSN (not simply moving bits like traditional networks)
+	- in-network processing
+	- provide answers
+	- comunication is triggered by events
+	- asymmetric flow of information (from sensors to sink)
+- QoS
+	- traditional metrics do not apply
+- Fault tollerance
+	- be robust against node failure
+	- running out of energy, physical destruct
+- Lifetime
+	- the network should fulfill as long as possible
+	- lifetime of individual nodes relatively unimportant
+	- but if a critical node dies, the network dies
+- Programmability
+	- being able to re-program nodes on-field, to improve flexibility
+- Maintainability
+	- WSN has to adapt to changes
+
+#### Typical Adopted Mechanisms
+- Multi-hop wireless communication
+- Energy-efficient operation (both for computation, sensing, actuation)
+- Self-configuration
+- Collaboration & in-network processing
+	- the nodes in the network collaborate towards a joint goal
+	- pre-processing the data before sending it to the sink, to improve efficiency
+
+#### Mechanism to meet requirements
+- Data centric networking
+	- focussing network design on data, not on node identifiers
+- Locality
+	- do things locally as far as possible
+- Exploit tradeoffs
+	- e.g between invested energy and accuracy
+
+> [!PDF|yellow] [[3 WSN.pdf#page=29&color=yellow|3 WSN, p.29]]
+> > WSN: reasoning of existence
+> 
+> collect, couple, provide, establish
+#### Main sensor node components
+- antenna and RF transceiver
+- memory unit
+- CPU
+- sensor unit (i.e. thermostat)
+- power source (typ. battery)
+- operating system
+	- TinyOS
+
+sensing, processing and networking is all done by the sensor node.
+
+#### WSN vs conventional networks
+
+| **Conventional networks**                                           | **WSN**                                                   |
+| ------------------------------------------------------------------- | --------------------------------------------------------- |
+| general purpose design                                              | serving a single application or a bouquet of applications |
+| network performance and latency                                     | energy is the primary challenge                           |
+| devices and networks operate in controlled / mild environments      | unattended, harsh conditions & hostile environments       |
+| global knowledge is feasible and centralized management is possible | localized decisions - no support by central entity        |
+#### Wireless signal issues
+- **Attenuation**: the strength of the signal decreases rapidly over distance
+- **Multi-path propagation**:
+	- when a radio wave encounter an obstacle, all or part of the wave is reflected, with a loss of power
+	- a source signal can arrive, to successive reflections, to reach a station through multiple paths
+- **Interference:**
+	- from the same source (multi-path propagation): signal arrives multiple time
+	- from multiple sources: more stations transmit simultaneously
+
+We use **SNR** to measure the ratio of good to bad signal (signal to noise). Higher is better.
+
+> [!PDF|yellow] [[3 WSN.pdf#page=49&selection=77,0,77,15&color=yellow|3 WSN, p.49]]
+> > Synchronization
+> 
+> nodes have clocks but they may not be synchronized!
+
+To address these issues, we use MAC protocols. We need a protocol suitable for wireless networks, which emphasize energy-efficient operation.
+### CSMA/CA
+![[Pasted image 20241002114133.png]]
+
+IFS is random, so hopefully only a node starts transmitting at the same time.