Scalability - The number of sensor nodes deployed in the sensing area may be in the order of hundreds, thousands, or more. Any routing scheme must be scalable enough to respond to events and capable of operating with such large number of sensor nodes. Most of the sensors can remain in the sleep state until an event occurs, with data from only a few remaining sensors providing a coarse quality.
•Communication range - The bandwidth of the wireless links connecting sensor nodes is often limited, hence constraining inter sensor communication. Moreover, limitations on energy forces sensor nodes to have short transmission ranges. Therefore, it is likely that a path from a source to a destination consists of multiple wireless hops
Fault tolerance - Some sensor nodes may fail or be blocked due to lack of power, physical damage, or environmental interference. If many nodes fail, MAC and routing protocols must accommodate formation of new links and routes to the data collection BSs. This may require actively adjusting transmit powers and signaling rates on the existing links to reduce energy consumption, or rerouting packets through regions of the network where more energy is available. Therefore, multiple levels of redundancy may be needed in a fault tolerant WSN.
•Connectivity - High node density in sensor networks precludes them from being completely isolated from each other. Therefore, sensor nodes are expected to be highly connected. This, however, may not prevent the network topology from varying and the network size from shrinking due to sensor nodes failures. In addition, connectivity depends on the, possibly random, distribution of nodes.
Transmission media - In a multi-hop sensor network, communicating nodes are linked by a wireless medium. Therefore, the traditional problems associated with a wireless channel (e.g., fading, high error rate) also affect the operation of the sensor network. In general, bandwidth requirements of sensor applications will be low, in the order of 1-100 kb/s. As we have seen in Chapters 4 and 5 and in the previous section, the design of the MAC protocol is also critical in terms of conserving energy in WSNs.
•QoS- In some applications (e.g., some military applications), the data should be delivered within a certain period of time from the moment it is sensed, otherwise the data will be useless. Therefore, bounded latency for data delivery is another condition for time constrained applications.
Structure of a sensor node
1Kbps- 1 Mbps
3m-300 m
Transceiver Lossy Transmission 128Kb-1 Mb
Limited StorageMemory
Embedded 8 bit, 10 MHz
Processor Slow Computation
Requires
SupervisionSensor
Multiple sensors Limited Lifetime Battery
The sensor node consist of a
1.sensing unit
2.processing unit
3.memory unit
4.self power unit
5.wireless transreceiver
•Sensing unit: it consists of a sensor and analog to digital converter [ADC]. the analog signal produced by sensor is converted to digital and is fed into processing unit. The sensing unit is responsible for collecting the data externally and interacts with central processor
•Processing and memory unit: the processing unit is responsible for performing some computations it executes some instructions which is responsible for setting up the connection with another node. The memory unit is used for storing the data.
Self power unit: it is responsible for powering the node and keeping it alive. The main task of the sensor node is to identify events , to process data , and then to transmit the data. The power of a node is consumed mainly in the transmitter and receiver unit. The sensor node can be supplied by a self-power unit, self-power unit battery, or solar cells