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