Challenges

Constrained Resources The most obvious limitation of a wireless sensor is the fact that the resources available to the sensor are severely constrained relative to a desktop computer or even a personal digital assistant (PDA). 10/26/2019

No centralized authority To demonstrate the similarities and differences in centralized control between traditional networks and ad hoc networks, consider the problem of routing. Routing on the Internet consists primarily of packet transmissions from one host in the network, the source, to another host, the destination, on a hop-by-hop basis. In contrast, routing in a wireless network consists mainly of transmissions on a hop-by-hop basis from a sensor node, the source, to the sink or base station 10/26/2019

Likewise, ad hoc networks operate without a central authority. In this case, however, the network is even more decentralized. Although routing on the Internet is decentralized, there are nodes in the network that function as routers and provide this service for other hosts, whether routing packets from these nodes or to these nodes. By contrast, typically no designated routers exist in an ad hoc network. Routing is accomplished either by source routing protocols such as Dynamic Source Routing (DSR) in which each source knows the complete route, perhaps by first querying the network Distributed routing protocols such as Destination-Sequenced Distance-Vector (DSDV) , in which each node along the path provides routing services for each packet. 10/26/2019

Limited power Power available to an ad hoc node generally is limited because the node uses wireless communication for networking and often is placed in an environment where there is no readily available power supply. If an external power source were required to operate the node, the advantages of wireless communication would be reduced. 10/26/2019

If power lines needed to be run to a large number of remotely deployed sensor nodes, the cost of deployment would escalate rapidly, and the advantages of wireless networking would be reduced greatly. There are essentially two options for providing power to a sensor node. The first option is to connect a battery to the device. The power density of batteries is increasing at a very slow rate relative to computing power 10/26/2019

However, there are times when this power source is unavailable. The second option is for sensor nodes to rely on passive power sources, such as solar and vibration energy. However, there are times when this power source is unavailable. For example, solar collectors cannot obtain significant power during overcast or rainy days or at night. Even under optimal environmental conditions, passive power sources typically provide a very modest amount of energy. 10/26/2019

A third possibility is a combination of the two preceding options. A passive power supply, such as a solar collector, could be used as the primary power source, with a rechargeable battery attached to the sensor node as a secondary power supply 10/26/2019

Wireless communication The main advantages of wireless communication are the reduced cost of not providing cabling for wired connections, the flexibility of mobile connections, and the freedom to deploy individual sensors anywhere. Along with these advantages, certain disadvantages must be accepted. Wireless connections provide lower bandwidth, require more power from the nodes, and are less reliable than traditional wired connections. 10/26/2019

From a sensor-node perspective, the power drain of wireless communication is the most significant factor. Because sensor nodes tend to generate small amounts of data and require infrequent communication, the limited bandwidth is not a significant factor. 10/26/2019

Limited computation and storage Sensor nodes have very limited computational resources. For example, a Mica 2 mote, which is third-generation sensor technology, has an Atmel ATMEGA processor that runs at 4 MHz with 512 kB of programmable memory and 128 kB of SDRAM. along with several sensors, a battery, and radio . This represents a processor speed that is approximately three orders of magnitude less than a high-end workstation. memory that is at least six orders of magnitude less than this same workstation. 10/26/2019

The computational capabilities of ad hoc nodes are limited for a number of reasons. The most significant reasons are the power limitations, the cost, and often the size of the device. These limitations are fundamental to the design and operation of wireless sensor nodes used in many applications. Therefore, it is reasonable to expect that sensor nodes will continue to lag behind the computational and storage capabilities of more traditional computing platforms. 10/26/2019

Typical sensors have no permanent storage devices such as hard disks. Storage constraints In addition to the limited computing power, sensor nodes also have very limited storage. Typical sensors have no permanent storage devices such as hard disks. A few kilobytes of nonvolatile memory may be available, and additional kilobytes of volatile memory usually are available. The operating system and application code must run in this extremely limited area. An example of such an operating system is TinyOS, which runs on a Mica sensor. The application running on the sensor nodes is compiled with the OS into a single program. All variables, values, and temporary workspace must fit within the remaining space. 10/26/2019

Limited input and output options Even for devices as large as PDAs, a standard keyboard is not feasible. Sensor nodes are significantly smaller. Input and output peripherals will be almost nonexistent on a sensor node. There is no output display and no keyboard, mouse, or stylus for input. The output peripherals consist of at most a few light- emitting diodes (LEDs) and a speaker. 10/26/2019

LEDs could be programmed in novel ways to flash patterns, although at the cost of some power consumption and program code overhead. Consequently, LEDs are limited to conveying simple messages, such as indicating that the sensor is turned on, that it is functioning correctly, or that it is currently transmitting or receiving data. Speaker output is limited to at most frequency, duration, and volume level, which constrains the amount of information transmittable to the user. 10/26/2019

Because direct human-to-sensor-node interaction is extremely limited, simulators and emulators are the primary mechanisms for debugging. These tools are effective only to the extent that they correctly model the operating environment of the sensors. 10/26/2019

Sensor configuration also becomes a more difficult task because downloading even simple instructions likely is performed via radio. Acknowledgments from the sensor may be required to determine that the sensor has been configured properly. 10/26/2019

Security Sensor networks are deployed for a wide range of purposes. Some of these sensors require absolutely no security. A noncritical application, such as a sensor that simply recognizes that someone has picked up an object and responds, may not need any security. On the other hand, sensors deployed for military applications require stringent security mechanisms. 10/26/2019

the sensor data need to be available, but only to those with a legitimate need for this information. Arbitrary access could lead to unintended problems. For many applications it is difficult to determine in advance the potential misuses of sensor data, implying that either security mechanisms exist as a precautionary measure. 10/26/2019

Small keys wireless sensor nodes have significantly less storage space than other wireless computers. Since a significant amount of this modest space is dedicated to the program code and the data processing, storing large keys is not practical. For example, a 1024-bit RSA1key would take up a significant fraction of the memory on a sensor. Since the level of security increases with larger keys, smaller keys reduce the security dramatically. 10/26/2019

Limited computation Security protocols perform additional computations to increase the level of security. In other words, more complicated algorithms offer improved security, but this requires that sufficient computing power is available. Security protocols exist that can operate on more constrained devices, but not at the same level of performance. The power available to the sensor nodes is also limited, and extensive computations for security purposes limit the lifetime of the sensor node or reduce the energy available for other sensor tasks 10/26/2019

Changing network membership Over the lifetime of a sensor network, the active membership of the sensor network varies. This variation may arise because of sensor nodes powering their radios off to conserve energy. Sensor-node failures and sensors that die owing to depletion of their energy also change the membership of the network. Security protocols that rely on sharing keys between neighboring nodes need to continue to operate even though the neighbors of a sensor node may change frequently during the lifetime of the sensor network. 10/26/2019

Sharing keys between individual pairs of neighboring sensors increases security but leads to problems with key distribution and key management. Arbitrary Topology: Sensor networks are deployed in different ways. For example, sensors in biomedical applications are likely to be implanted in specific locate the neighbors of each sensor are predetermined. For applications such as transportation monitoring and management, sensors are deployed with less precision but still with a well-defined distribution 10/26/2019

For these applications, sensors are strewn from lowflying aircraft or vehicles or by hand. Not only is the exact position of each sensor be somewhat arbitrary, but also the density of nodes varies over the region, and which nodes are neighbors of each sensor is unpredictable before network deployment. Care must be taken with this approach to prevent malicious nodes from adding themselves to the network. In other words, an adversary should not be able to deploy a malicious node that can establish secure communication with existing sensor nodes. Sensor nodes must not authenticate nodes that do not belong to the sensor network. 10/26/2019