The Sensor Network Protocol Stack and Its Three Planes
Chapter Three
Syllabus topic Module 1, "Introduction and Overview of WSNs: Basic sensor network architectural elements"
Pages 15 to 19 of 862
In one line
The protocol stack of a sensor network is the usual five layers, from physical to application, crossed by three planes that manage power, mobility and tasks across all of them.
In the wording a student can write in an examination: the protocol stack used by the sink and the sensor nodes consists of the physical layer, data link layer, network layer, transport layer and application layer, together with the power management plane, mobility management plane and task management plane. The stack "combines power and routing awareness, integrates data with networking protocols, communicates power efficiently through the wireless medium, and promotes cooperative efforts of sensor nodes". The planes help the nodes coordinate the sensing task and lower the overall power consumption.
Why a sensor network needs its own stack
A student who has learnt the TCP/IP model may ask why the Internet's layers are not simply reused. Two reasons, and they are the reasons for the planes.
The Internet's layers do not care about energy. An Internet router forwards a packet the same way whether its battery is full or nearly empty, because it has no battery. A sensor node must take its remaining energy into account in almost every decision: whether to listen, whether to relay, whether to take a turn at sensing.
The Internet's layers keep to themselves. Each layer is meant to know nothing about the others. In a sensor network, energy, movement and the sharing of the sensing task are concerns of every layer at once. A layer cannot solve them alone, so the survey adds planes: management functions that run alongside all the layers and coordinate them.
Figure 3.1 The sensor network protocol stack: five layers and three management planes
The five layers, bottom to top
Physical layer
The survey gives its job as "frequency selection, carrier frequency generation, signal detection, modulation, and data encryption". It turns bits into a radio signal and back.
What is different in a sensor network: the energy cost of distance. The power needed to send a signal over a distance d grows as d to the power n, where n lies between 2 and 4, and the survey notes that n is "closer to four for low-lying" antennas and near-ground channels, which is exactly where sensor nodes sit, on the ground or on a post. So doubling the distance can cost up to sixteen times the power. This one fact is why sensor networks prefer short hops and simple, robust modulation. At the time the survey was written, the 915 MHz industrial, scientific and medical band had been "widely suggested"; today most nodes use the 2.4 GHz band of IEEE 802.15.4, taught in [The 802.15.4 Physical Layer].
The Sensor Network Protocol Stack and Its Three Planes
Data link layer
Its job is "the multiplexing of data streams, data frame detection, medium access and error control". It gets a frame reliably from one node to its neighbour.
Two parts, both different in a sensor network:
- Medium access control (MAC) decides when a node may use the shared radio channel. In a sensor network it "must be power-aware and able to minimize collision with neighbors' broadcasts", and above all it must let the radio sleep, because a radio left listening wastes most of a node's energy. The chapters from [MAC Protocols for Sensor Networks: The Job and Where the Energy Goes] onwards are this sublayer.
- Error control repairs or detects damaged frames. The survey names the two modes: forward error correction (FEC), which adds redundant bits so the receiver can correct errors itself, and automatic repeat request (ARQ), which resends a frame that did not arrive intact. A sensor node needs simple codes, because decoding a complex one costs energy too.
Network layer
It routes the data from the node that produced it to the sink, across many hops. The survey lists four design principles for this layer in a sensor network:
- Power efficiency is always an important consideration.
- Sensor networks are mostly data-centric: what matters is the data, not which node sent it.
- Data aggregation is useful only when it does not hinder the collaborative effort of the nodes.
- An ideal sensor network has attribute-based addressing and location awareness: a packet is sent to "the nodes in region A" or to the nodes reading above 70°F, not to node number 145.
The whole of [Routing Strategies in WSNs: A Map] and the chapters after it are this layer.
Transport layer
It "helps to maintain the flow of data if the sensor networks application requires it". Note the condition: many sensor applications tolerate lost readings (another reading comes in a minute), so they need little from this layer.
The survey's key point is where TCP stops. It suggests splitting the connection at the sink: the user talks to the sink over ordinary TCP or UDP through the Internet or satellite, and the sink talks to the nodes with a light UDP-type protocol, "because each sensor node has limited memory" and "acknowledgments are too costly for sensor networks". The chapter [Why a Sensor Network Cannot Simply Run TCP] works out why.
Application layer
It carries the software of the particular application: the monitoring, tracking or alarm program. The survey also proposed three application-layer protocols, which it called open research issues:
- Sensor Management Protocol (SMP), through which administrators manage the network: introducing rules for aggregation, naming and clustering; time synchronisation; turning nodes on and off; querying and reconfiguring the network; and authentication and key distribution.
- Task Assignment and Data Advertisement Protocol (TADAP), for spreading a user's interest (what the user wants to know) into the network, or for nodes to advertise what data they have so users can ask for it.
- Sensor Query and Data Dissemination Protocol (SQDDP), the interface through which an application issues queries and collects replies. Queries are attribute-based ("the locations of the nodes that sense temperature higher than 70°F") or location-based ("temperatures read by the nodes in region A"), never addressed to a named node.
The Sensor Network Protocol Stack and Its Three Planes
The three planes
The planes are the part of the stack an examiner is really asking about, because they are what the TCP/IP model does not have.
Power management plane
It "manages how a sensor node uses its power". The survey gives two examples, and both are worth writing in an answer:
- A node turns off its receiver after receiving a message from one of its neighbours, so as not to receive the same message again from another neighbour.
- When its power is low, a node broadcasts to its neighbours that it is low in power and cannot take part in routing, and keeps what is left for its own sensing.
Mobility management plane
It "detects and registers the movement of sensor nodes, so a route back to the user is always maintained". Knowing which nodes are its neighbours also lets a node balance its power and its tasks with theirs. In a network where nothing moves this plane does little; on a network of nodes carried by animals, vehicles or water currents, it is essential.
Task management plane
It "balances and schedules the sensing tasks given to a specific region". Not every node in a region needs to sense at the same time, so some nodes take on more of the task than others, depending on their remaining power. Ten nodes watching one field can take turns, and the field is watched just as well at a fraction of the energy.
Worked example: one reading, all the way down
Back to the vineyard of [What a Wireless Sensor Network Is]. Node 23, in zone 7, has a moisture reading of 18 per cent and it is time to report. Follow the reading down its stack and across the planes.
| Step | Layer or plane | What happens |
|---|---|---|
| 1 | Task management plane | Of the three nodes in zone 7, node 23 is the one scheduled to sense this hour; the other two stay asleep |
| 2 | Application layer | The monitoring program forms the message zone 7, moisture 18 per cent, 14:15 |
| 3 | Transport layer | No connection is set up. The reading goes as a single UDP-type datagram; if it is lost, the next reading in 15 minutes replaces it |
| 4 | Network layer | The routing table says node 23's next hop towards the sink is node 17, the neighbour on the best path |
| 5 | Data link layer, MAC | The MAC waits until the channel is free and node 17 is awake, then sends the frame |
| 6 | Data link layer, error control | The frame carries a checksum; node 17 acknowledges it, and without the acknowledgement node 23 would resend (ARQ) |
| 7 | Physical layer | The frame is modulated onto the 2.4 GHz radio and transmitted at the lowest power that reaches node 17 |
| 8 | Power management plane | The frame sent, node 23 switches its radio off until its next scheduled slot |
| 9 | Mobility management plane | Nothing to do here: vines do not move. A node on the tractor would report its new position here |
The Sensor Network Protocol Stack and Its Three Planes
Node 17 receives the frame, passes it up only to its network layer (it is a relay, so the application layer never sees it), and sends it down its own stack towards the next hop. At the sink the reading climbs all five layers and is handed to the gateway.
Distinctions
| A layer | A plane | |
|---|---|---|
| Runs | Above one layer and below another | Across all five layers |
| Deals with | One step of communication | A concern every step shares: energy, movement, sharing the task |
| Example | The network layer chooses the next hop | The power plane stops a low-battery node from routing |
| In TCP/IP | Yes | No equivalent |
| Sensor network stack | TCP/IP stack | |
|---|---|---|
| Designed around | Energy and cooperation | Throughput and generality |
| Addressing | Attribute-based and location-based | Global IP addresses |
| Transport | Light, UDP-type inside the field; TCP split at the sink | TCP end to end |
| Cross-layer management | Three planes | None |
| Intermediate nodes | May process and combine data | Only forward it |
What it does not mean
The planes are not extra layers. They sit beside the stack, not in it. Drawing them as three more boxes on top of the application layer is the commonest mistake in this answer.
"Transport layer" does not mean TCP. The survey says the opposite: TCP stops at the sink, and inside the field the transport layer is light, UDP-type, or absent.
The stack is a reference model, not an implementation. Real systems merge and split the layers freely. TinyOS has no layers at all in this sense, only components, and IEEE 802.15.4 defines the physical and MAC layers together. The model is for understanding and for answers; [Design Principles: Data Centricity, Location, Activity and Heterogeneity] explains why real designs deliberately cut across layers.
Encryption at the physical layer is the survey's wording, not a rule. Most real sensor networks encrypt at the data link layer (802.15.4 does), which the chapter [Keys and Link Security: Key Predistribution, SPINS and 802.15.4] covers.
The Sensor Network Protocol Stack and Its Three Planes
Quick revision
- Stack (Akyildiz and colleagues, 2002): physical, data link, network, transport, application, plus power, mobility and task management planes.
- Physical: frequency selection, carrier generation, signal detection, modulation, encryption; power to send over distance d goes as d to the n, n from 2 to 4, near 4 close to the ground.
- Data link: multiplexing, frame detection, MAC (power-aware, few collisions) and error control (FEC and ARQ).
- Network: power efficiency, data-centric, aggregation where it helps, attribute-based addressing and location awareness.
- Transport: only if needed; TCP split at the sink, UDP-type inside.
- Application: SMP, TADAP, SQDDP.
- Power plane: receiver off after a message; announce low power, stop routing. Mobility plane: track movement and neighbours, keep a route to the user. Task plane: schedule and balance sensing so not every node senses at once.
Test yourself
1. Draw and explain the WSN protocol stack. Five layers (physical, data link, network, transport, application) crossed by three planes (power, mobility and task management). Physical: frequency selection, carrier generation, signal detection, modulation, encryption. Data link: multiplexing, frame detection, medium access, error control. Network: data-centric, energy-efficient routing to the sink. Transport: maintains flow where needed. Application: the application software and management protocols. Planes: coordinate energy, movement and the sensing task across all layers.
2. What are the three management planes, and give an example of each. Power management: a node turns off its receiver after receiving a message to avoid duplicates, and announces low power so it is not used for routing. Mobility management: records node movement so a route to the user is always kept and neighbours are known. Task management: schedules sensing in a region so that nodes take turns according to their power.
3. Why does a sensor network's stack need planes at all? Because energy, movement and the sharing of the sensing task concern every layer at once. No single layer can manage them, so they are handled by functions that run across all the layers.
4. What does the survey recommend for the transport layer, and why? Splitting the connection at the sink: TCP or UDP between the user and the sink over the Internet or satellite, and a light UDP-type protocol between the sink and the nodes, because nodes have little memory and acknowledgements cost too much energy.
5. State the four design principles of the network layer. Power efficiency; data-centric operation; aggregation only where it does not hinder collaboration; and attribute-based addressing with location awareness.
6. Name the three application-layer protocols the survey proposes. Sensor Management Protocol (SMP), Task Assignment and Data Advertisement Protocol (TADAP), and Sensor Query and Data Dissemination Protocol (SQDDP).
The rest of this subject
These notes are cut from the University's printed syllabus. Open the syllabus itself for the same subject.