Protocols for Secure Communication in Wireless Sensor Networks

2.3. Related Network Types 23
• Physical characteristics Nodes in ad hoc networks are usually more resourceful
than sensor nodes. While sensor nodes are designed to be as
small as possible, devices in ad hoc networking are usually handled by a
human user. Therefore they cannot be arbitrarily small (which allows their
microprocessors to be more powerful), and they must be equipped with a
minimal user interface. Besides, their human users take care of recharging
their batteries. In contrast, sensor nodes are often not accessible after
deployment. They have to operate autonomously and manage with their
limited energy supply.
• Failure modes A device in an ad hoc network that stops working is likely
to be replaced (or recharged) by its user, while a failed sensor node will
hardly be noticed. A user will keep her devices under guard most of the
time, and protect them against external threats to her best abilities. Sensor
nodes are susceptible to external influences and may often be disabled by
environmental conditions.
User devices such as PDAs or mobile phones are often vulnerable to
network-level attacks such as malicious code [102]. Although sensor
nodes are not immune per se against such threats, code injection attacks
require special equipment and are only possible in physical proximity to
the device. Code injection attacks are further impeded by the Harvard
computer architecture that is often used in embedded microcontrollers.
This architecture physically separates memory sections for code and data.
It is thus possible to prevent unintended code updates.
• User vs location As devices in ad hoc networks are associated with human
users, an attack on a device usually means that its owner, or rather
personal information she possesses, is the actual target of the attack. In a
sensor network, nodes are associated with locations rather than users. An
attack on a sensor node may provide the adversary with some control over
information associated with the node’s location. This may allow the adversary
to observe an object (such as a human), but will not directly reveal
sensitive information about it.
• Network scale In a sensor network, the number of nodes is usually very
high, and so is the density, leading to some inherent redundancy. Taking
control over a single node will not even allow the adversary to completely
control the data flow in some area. Also, only the small fraction of the
total network traffic passing through that one node will be revealed to the
adversary. Ad hoc networks are usually smaller, and the significance of