master-degree-notes/Autonomous Networking/notes/3 WSN.md

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, 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, 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

!

IFS is random, so hopefully only a node starts transmitting at the same time.