## 4 Classification  
Routing techniques are required for sending data between sensor nodes and the base stations for communication. Different routing protocols are proposed for wireless sensor network. These protocols are classified according to different parameters. Protocols can be classified as proactive, reactive and hybrid, based on their mode of functioning and type of target applications. In a proactive protocol the nodes switch on their sensors and transmitters, sense the environment and transmit the data to a BS through the predefined route. The Low Energy Adaptive Clustering hierarchy protocol (LEACH) utilizes this type of protocol [7]. In case of a reactive protocol if there are sudden changes in the sensed attribute beyond some pre-determined threshold value, the nodes immediately react. This type of protocol is used in time critical applications. The Threshold sensitive Energy Efficient sensor Network (TEEN) [8] is an example of a reactive protocol. Hybrid protocols like Adaptive Periodic TEEN (APTEEN) incorporate both proactive and reactive concepts [9]. They first compute all routes and then improve the routes at the time of routing. Further, routing protocols can be classified as direct communication, flat and clustering protocols, according to the participation style of the nodes. In direct communication protocols, any node can send information to the BS directly. When this is applied in a very large network, the energy of sensor nodes may be drained quickly. Its scalability is very small. SPIN is an example of this type of protocol. In the case of flat protocols, for example Rumor Routing, if any node needs to transmit data, it first searches for a valid route to the BS and then transmits the data. Nodes around the base station may drain their energy quickly. Its scalability is average. According to the clustering protocol, the total area is divided into numbers of clusters. Each and every cluster has a cluster head (CH) and this cluster head directly communicates with the BS. All nodes in a cluster send their data to their corresponding CH (example: TEEN). Furthermore, depending on the network structure, protocols can be classified as hierarchical, data centric and location based. Hierarchical routing (examples: LEACH, TEEN, APTEEN) is used to perform energy efficient routing, _i.e._, higher energy nodes can be used to process and send the information; low energy nodes are used to perform the sensing in the area of interest. Data centric protocols are query based and they depend on the naming of the desired data, thus it eliminates much redundant transmissions. The BS sends queries to a certain area for information and waits for reply from the nodes of that particular region. Since data is requested through queries, attribute based naming is required to specify the properties of the data. Depending on the query, sensors collect a particular data from the area of interest and this particular information is only required to transmit to the BS and thus reducing the number of transmissions. SPIN [10] was the first data centric protocol. Location based routing protocols [11] need some location information of the sensor nodes. Location information can be obtained from GPS (Global Positioning System) signals, received radio signal strength, _etc._ Using location information, an optimal path can be formed without using flooding techniques. GEAR is an example of a location based routing protocol. The present review discusses the intricate details of the roles of different routing protocols. Furthermore it provides a comparative analysis between these.