Sensor Taxonomy

Omnidirectional or narrow-beam

The second division is about direction, and it applies to passive sensors.

• Omnidirectional sensors respond equally from every direction: a thermometer, an ordinary microphone.
• Narrow-beam sensors see only in one direction: a camera, a directional microphone, a passive infrared detector behind a lens.

So Karl and Willig's three kinds are passive omnidirectional, passive narrow-beam and active. For the network the difference matters: a narrow-beam sensor's reading depends on which way it points, so its orientation has to be known, and covering an area takes more of them.

By output, by contact and by principle

• Analogue or digital output. An analogue sensor gives a voltage or current that the node's ADC converts; a digital sensor has its own converter and sends a number over a serial bus.
• Contact or non-contact. A soil probe or a strain gauge must touch what it measures; an infrared thermometer or a camera measures from a distance.
• Operating principle. How the quantity becomes an electrical signal: a change in resistance (thermistors, strain gauges), in capacitance (humidity and soil-moisture sensors), in inductance (proximity and position sensors, where a moving core changes a coil's inductance), a piezoelectric voltage from stress (vibration sensors), a photoelectric current from light, a thermoelectric voltage from a temperature difference, or a micro-electro-mechanical (MEMS) structure on a chip, as in most accelerometers.

Part 2: classifying sensor networks

The three parts of every sensing application

Tilak, Abu-Ghazaleh and Heinzelman begin by naming three participants, and the rest of their taxonomy is about how the three relate.

• Sensor: the device that implements the physical sensing and reports its measurements; in their words it typically consists of sensing hardware, memory, battery, embedded processor, and trans-receiver.
• Observer: the end user who wants information about the phenomenon, who may indicate interests (or queries) to the network and receive responses.
• Phenomenon: "The entity of interest to the observer", the thing being sensed and potentially analysed; several may be observed at once.

By data delivery model

How and when readings are sent to the observer. This is the classification most often asked.

1. Continuous. The sensors send their data continuously at a set rate: the vineyard, every 15 minutes. The paper notes that clustering is most efficient for static networks with continuous data.
2. Event-driven. The sensors report only when an event of interest occurs: an intruder, a fire. Most of the time the network is silent; when the event happens, many nearby sensors detect it together and contend for the channel at once, which raises both the risk of losing the critical report and its delay.
3. Observer-initiated, or request-reply. The sensors report only in response to an explicit request from the observer, sent to them directly or through other sensors: what is the temperature in zone 3 now?
4. Hybrid. The three can coexist in one network: continuous background readings, event alarms and answers to queries all at once.

By network dynamics

What moves. The paper divides networks into static, where nothing moves (sensors, observer and phenomenon are all fixed, as in a group of temperature sensors), and dynamic, where something does. Three kinds of motion:

• Mobile observer. The observer moves while the sensors stay put: a vehicle or a person carrying the collecting device drives past the field and gathers readings as it goes.
• Mobile sensors. The sensors themselves move: sensors on animals, vehicles or drifting buoys. Paths to the observer break as sensors move and must be rebuilt.
• Mobile phenomena. What is being watched moves: a vehicle, an animal, a fire front. The set of sensors that should report changes as the phenomenon moves; in the paper's words, sensors can hand the "responsibility of monitoring to a closer node" as the target drifts.

Motion is the commonest source of change, but the paper notes others: sensors failing and observers' interests changing.

By communication

The paper divides the network's traffic into two kinds.

• Application communication carries the sensed data towards the observer. It is cooperative when sensors work together to meet the observer's interest (for example a cluster head combining its members' readings) and non-cooperative when each sensor reports on its own.
• Infrastructure communication is the traffic needed to set up, maintain and optimise the network: discovering neighbours, building paths, rebuilding them when a sensor moves or dies. It is overhead, so a good protocol keeps it small, though spending some on it can reduce application traffic overall.

By structure, composition and deployment

• Flat or hierarchical. In a flat network every node has the same role; in a hierarchical one some nodes are cluster heads. Sohraby, Minoli and Znati make the related division into two categories: networks that are mesh-based with multi-hop radio links and dynamic routing, and networks that are point-to-point or star-based with single-hop radio links and static routing to a node on the fixed network.
• Homogeneous or heterogeneous. All nodes the same, or some with more energy, better radios or extra sensors, such as mains-powered cluster heads.
• Planned or random deployment. Nodes placed at chosen points, or scattered, for example from the air.

The text book's own categorisation

Sohraby, Minoli and Znati, in their chapter 1 (Table 1.1, which they adapt from an earlier source), sort the issues of a sensor network under four headings. It is a compact taxonomy in its own right, and it overlaps with everything above.

The same text book notes that passive sensors "tend to be low-energy devices" while active sensors such as radar and sonar "tend to be high-energy systems", which is the energy point made above in its own words.

Worked example: classifying four networks

Each column needs a different protocol. The vineyard wants a stable schedule and long sleeps; the volcano wants to move bursts of data reliably and time them exactly; the collars must store data until two animals, or an animal and a base station, come within range; and the roadside boxes must wake up and hand over their data in the few seconds a vehicle is in range.

Distinctions

What it does not mean

"Sensor taxonomy" is not only a list of sensor types. An answer that stops at temperature, pressure and humidity has done half the job; the classification of networks is the half with the design consequences.

Passive does not mean unpowered here. In this subject a passive sensor is one that does not emit; it may still need a supply.

Event-driven does not mean light traffic. Most of the time it is silent, but at an event many sensors report at once, which is the hardest moment for the MAC protocol.

A mobile phenomenon does not make the network mobile. The sensors can all be fixed while the target moves; what changes is which of them report.

Quick revision

• Two classifications: of sensors and of networks.
• Sensors: what they measure; passive (only observe) or active (emit and measure the return); omnidirectional or narrow-beam; analogue or digital; contact or non-contact; operating principle. Karl and Willig: passive omnidirectional, passive narrow-beam, active.
• Networks (Tilak and colleagues, 2002): participants sensor, observer, phenomenon; delivery continuous, event-driven, observer-initiated (request-reply), hybrid; dynamics static, or mobile observer, mobile sensors, mobile phenomena; communication application (cooperative or not) and infrastructure.
• Also: flat or hierarchical; single or multi-hop (Sohraby and colleagues: mesh with dynamic routing, or star with static routing); homogeneous or heterogeneous; planned or random.

Test yourself

1. Explain the data delivery models of sensor networks. Continuous: sensors send at a set rate. Event-driven: sensors report only when an event of interest occurs. Observer-initiated (request-reply): sensors report only when the observer asks. Hybrid: the three coexist in one network.

2. Distinguish passive and active sensors, with examples. A passive sensor only observes and emits nothing (thermometer, microphone, passive infrared detector); an active sensor emits energy and measures what returns (ultrasonic range finder, radar) and so uses more energy.

3. What are the three kinds of mobility in the Tilak taxonomy? Mobile observer (the collector moves), mobile sensors (the nodes move, breaking paths) and mobile phenomena (the target moves, changing which nodes should report).

4. What is infrastructure communication, and why should it be kept small? The traffic needed to configure, maintain and optimise the network, such as discovering neighbours and building or repairing paths. It carries no sensed data, so it is overhead that costs energy, though a little of it can reduce application traffic overall.

5. Classify a network of GPS collars on elephants. Passive sensors (GPS receivers, accelerometers); continuous logging with delivery when in range; dynamic, with mobile sensors; homogeneous nodes; deployment carried by the animals; paths formed opportunistically when two collars or a collar and a base station meet.