The processing unit  is basically associated with a small storage unit and manages the collaboration of sensor nodes with each other to carry out a specific task. This is the main unit to process the data and contains a memory and a processor to complete the task.
A transceiver unit allows the transmission and reception of data to other devices connecting a wireless sensor node to a network.
Wireless sensor nodes typically communicate using an RF (radio frequency) transceiver and a wireless personal area network technology such as Bluetooth or the 802.15.4 compliant protocols ZigBee and MiWi.
Wireless sensor nodes are supported by a power unit which is typically some form of storage (that is, a battery) but may be supported by power scavenging components (for example, solar cells). Energy from power scavenging techniques may only be stored in rechargeable (secondary) batteries and this can be a useful combination in wireless sensor node environments where maintenance operations like battery changing are impractical. To conserve energy a power unit may additionally support power conservation techniques such as dynamic voltage scaling.
1.5 Topology used in wireless sensor networks 
As the sensor networks consists of large number (several hundred to thousands nodes) of nodes which are deployed in a small area. So, the dense deployment of these sensor nodes requires careful topology maintenance . The types of topologies that can be used in sensor networks can be explained as follows:
Star Network (Single Point-to-Multipoint) :
In a star network topology, a single base-station communicates with other remote nodes in the network. This single base station can send or receive messages from other nodes. The remote nodes are not allowed to communicate with each other directly. So, they send or receive messages through the base station. The advantage of star network is that it is simple and reduces the power consumption by remote nodes. Star network also allow low latency time communication between the base station and the remote node. The disadvantage of this type of network is that the base station must be within the radio transmission range of all the remote nodes and is not as much robust as the other networks are because here a single node manages the whole network .
Figure 1.5: star Topology 
Mesh Network :
In the mesh network each node can communicate with every other node which is in radio transmission range of each other. It allow multi-hop communication i.e. if a node wants to send a data packet to any other node which is out of its radio transmission range  then, it can use an intermediate node to forward this packet to the destination node. This network topology has an advantage of scalability and redundancy. If the node fails to work, a remote node can still communicate with any other sensor available in its range, which can further forward the packets to the destination node. The range of the whole network is not limited by the range in between individual nodes. This range of network can be simply extended by adding more nodes in the system. The disadvantage of mesh network is that the power consumption of the nodes that have the capability of multi-hop communications are generally higher than for the nodes that donââ‚¬â„¢t have this capability of multi hop communication. Thus, the battery life becomes limited. Also, when the number of hop count for communication to a destination node increases, then the time to deliver the message also increases especially if low power operation of the nodes is a requirement .
Figure 1.6: Mesh topology 
A hybrid of the star and mesh network gives a versatile and robust communications network, while maintaining the ability to make the power consumption by the nodes of network at a minimum point. In this network topology, sensor nodes with lowest power are not able to forward the messages. Hence, maintenance of minimum power consumption is must. However, other nodes in the network have multi-hop capability that allows them to forward messages from the low power nodes to other nodes in the network. Basically, the nodes with multi-hop capability have higher power and they can be plugged into the electrical mains line if possible. This is the topology implemented by the upcoming mesh networking standard called as Zig-Bee .
Figure 1.7 Hybrid Topology