Network Layer in Wireless Sensor Networks

Most of the data in the sensor network will be directed towards the sink. Special multi hop routing protocols are needed between sink and sensor nodes for wireless sensor networks. Most of the data conveyed to the sink form a sensor node will pass through many intermediate nodes before reaching the sink. Communicating data directly from the node to sink will be very energy expensive, so multi hop communication is preferred in wireless sensor network. The figure below makes the concept clearer.



Data form node N1 reaches sink through multiple intermediate nodes, N2, N3, and N4.

The network layer for sensor network is designed considering the following
1. Every protocol designed should be power efficient.
2. The protocol should support dynamic nature of sensor networks.
3. The sensor network protocols should make sensor network self configurable.
4. Data aggregation should be done, if it is advantageous.
5. Data centric design is more preferred, rather than address centric or location centric architecture.
There are several protocols and papers written regarding different issues in network layer for sensor networks, we shall understand a few in the proceeding sections.
A route form source to destination can be found based on many different criteria.



A scenario to explain different criteria that can be used to select a route form source (S) to Sink

One may choose a route which has maximum available power; in that case the route form source (S) to sink in the above diagram will be S-C-B-A- Sink. This is so because the power available in this route is 9, which is the highest compared to other routes.
One may go for route which consumes least power, and then the route selected can be S-B-A-Sink or S-D-Sink. S-D-Sink is preferred because less number of hops is present.
If the criterion to select is minimum number of hops then, the route S-D-Sink is selected.
There may other criteria that can be used, like a route with less probability of failure, a route less congested.
The routing may also be data centric routing. In this type of routing sink advertises the interest, and all nodes which have the data reply back to sink. Or it may be that all nodes advertise what kind of data they have, and other nodes which require that data request the node to give the data. In this type of routing, there is no importance given to a particular node but importance is given to the data or attribute. This type routing differs from the conventional routing schemes. Many papers and authors prefer data centric routing than conventional routing for sensor networks.
Data aggregation is done in sensor network to reduce the redundant data and hence save energy. For example in the figure shown before data from S and C is aggregated at node B, if the data form both C and S are almost similar. This reduces the amount data sent form B to A, hence conserves energy at B. Similarly data form A, D, E is aggregated at sink and sent further by sink, if the data has to be sent further. Data aggregation introduces delay in forwarding the data, as the node which aggregates the data has to wait for data from all nodes, to which the node aggregates the data. Even the process of aggregation consumes some amount of time.