i. Fault tolerance:

• Some sensor may fail or be blocked because of lack of power, physical damage, or environmental interference.
• This should not affect the overall task of sensor network.
• Reliability of sensor node: $R_{k}(t)=e^{-\lambda_{k} t}$

ii. Scalability:

• The number of nodes in a sensor network ranges from a few hundred to the order of thousands and even an extreme value of millions.
• The applications should be able to work with this high density.
• The density ‘μ’ can be calculated as $\mu(R)=\frac{N \pi R^{2}}{A}$

iii. Production cost:

• The cost of a single sensor node should be less so that the overall cost of sensor network feasible.
• Hardware constraints:
• The required size of a sensor network, consisting large number of sensor nodes, may be a match-box size.
• Apart from size, the nodes must be able to consume very less power, operate unattended (no manual assistance in working), adapt to environment, be dispensable and autonomous.

iv. Sensor network topology:

• Any topology will require handling because the node density is high, sometimes as high as 20 nodes/$m^3$.

v. Operating environment:

• Sensor nodes are densely deployed either very close to or directly inside the phenomenon to be observed.
• They need to work in large machinery at the bottom of an ocean, in a biologically or chemically contaminated field, in a battlefield beyond enemy lines or in home or large building.

vi. Transmission media:

• Infrared and optical media are used if line of sight is required. Radio link is also used.
• To enable global operation of sensor network, chosen transmission media must be available worldwide.

vii. Power consumption:

• Power consumption is divided into three parts; sensing power, communication power, processing power.
• Process like event detection is complex, so power needed for sensing increases.
• Communicating power refers to both power needed for transmission as well as reception.
• The sensor nodes should have enough processing capability.
• However, power needed for processing is usually less than power needed for sensing and communicating.

All these factors influence the design of wireless sensor networks.