The sensor network Protocol stack consists of the physical layer, data link layer, network layer, transport layer, application layer and the power management, mobility management, task management planes.
1. Physical layer:
The physical layer is responsible for frequency selection, carrier frequency generation, signal detection, modulation and data encryption.
The 915 MHz ISM band has been widely suggested. ‘Ultra wideband (UWB)’ is an attractive candidate because of low transmission power, simple circuitry, use of pulse position modulation, resilience to multipath.
Modulation depends on transceiver and hardware design constraints, which aim for simplicity, low power consumption, and low cost per unit. Binary modulation schemes are simpler to implement and deemed to be more energy efficient.
The physical layer addresses the needs of simple but robust modulation, transmission and receiving techniques
2. Data Link Layer:
- The data link layer is responsible for multiplexing data streams, data frame detection, medium access and error control. It ensures reliable point-to-point and point-to-multipoint connections in a communication.
- The MAC must establish communication links for data transfer in wireless multihop self organizing sensor network. This forms basic infrastructure.
- MAC protocols must fairly and efficiently share communication resources between sensor nodes. Since there is noise and sensor nodes are mobile, MAC must be power-aware and able to minimize collision with neighbors’ broad cast.
- The main features of MAC are periodic listen and sleep, collision and over hearing avoidance, and message passing.
- The number of modes of operation of sensor node depending on states of processor, memory, transceiver, A/D converter must be power efficient.
- ‘Forward Error Correction (FEC)’is more feasible than ‘Automatic Repeat Request (ARQ)’.
3. Network Layer:
- The network layer takes care of routing the data supplied by the transport layer.
- The network layer is designed according to some principles.
- Power efficiency is always an important consideration: Energy efficient routes are found like the route with maximum available power (PA); route with minimum energy consumption (ME); route with minimum hops (MH); etc.
- Sensor networks are mostly data centric: The data is given only to the node which will perform task. This is called interest dissemination the sink broadcasts the data availability and sensor nodes broadcast advertisement. Sink waits for a request by interested nodes.
- Data aggregation is useful only when it does not hinder the collaborative effort of the sensor nodes: data aggregation combines data from many nodes into a more compact form before forwarding it for processing.
- An ideal sensor network has attribute-based addressing and location awareness: the user is interested in querying an attribute of the phenomena; rather than querying an individual node.
- Other feature include clock synchronization, fault tolerance etc.
4. Transport layer :
- The transport layer helps to maintain the flow of data if sensor network application requires it.
- It is needed when a system is planned to be accessed through internet or other external networks.
- The ‘Transmission control protocol (TCP)’ needs to be split into two parts. One connects sensor network to the other network like internet and the other connects the sink node to sensor nodes.
- The addressing scheme here is not based on global addressing. It is based on attribute-based naming. Factors such as power consumption, scalability, etc. are taken care of.
5. Application Layer :
- Depending on sensing tasks, different types of application software can be built and used on application layer.
- These protocols make hardware and software of lower layers transparent so that system administrators can edit settings.
- Three such protocols are ‘Sensor management protocol (SMP)’ ‘Task assignment and data advertisement protocol (TADAP)’, ‘Sensor query and data dissemination protocol (SQDDP).
- SMP provides software operations needed to perform tasks such as introducing rules related to data aggregation, attribute-based naming, clustering sensor nodes; time synchronization of sensor nodes; moving sensor nodes, etc.
6. Power, Mobility and Task Management Planes :
- When the power level of a sensor node is low, it broadcasts to its neighbors that it is low in power and cannot participate in routing messages. This is managed by power management plane.
- The mobility management plane detects and registers movement of sensor nodes. The sensor nodes can keep track of who their neighbors are.
- The task management plane balances and schedules sensing tasks given to a specific region. Not all sensor nodes perform sensing at same time.
Thus, the protocol stack combines power and routing awareness, integrates data with networking protocols, communicates power efficiently through wireless medium and promotes co-operative efforts of sensor nodes.