Protocols

Autoconfiguration Neighborhood discovery Topology discovery Medium access control schedule construction Hidden terminal problem Exposed terminal problem Security protocol configuration

1. Autoconfiguration The initial deployment of sensors in an area could be rather large. Allowing these sensors to configure themselves into a functioning network has a number of advantages. First, human intervention may be impractical or impossible. For example, sensors may be deployed in an environment that is unsafe for humans, whether at a military location, a remote geographic location, or an industrial setting that is not suitable for humans. Second, large sensor networks require significant labor to configure each sensor individually.

Preconfiguration is also challenging if sensors that have exhausted their battery supply or have failed are replaced periodically with new sensors. Automatically configuring nodes in an ad hoc network achieves good results if the overhead of configuring nodes is modest and the algorithms for automatically configuring the nodes.

Neighborhood discovery Neighborhood information in an ad hoc network informs each node about the surrounding set of nodes in the network. however, nodes require some knowledge of their neighbors for collaboration and coordination of efforts. For example, if a sensor observes an event, the node may wish to check with neighboring nodes to determine whether or not other sensor nodes also have observed the same event. Neighborhood discovery may choose to allow only bidirectional links or also include unidirectional links.

Topology discovery Topology information in an ad hoc network consists of the locations of nodes relative to each other, including the density of nodes in a particular region. The topology of the network also could include the number of nodes in the network and the size of the network—in other words, the physical dimensions of the sensor network and the number of nodes in this area.

Medium access control schedule construction Wireless bandwidth is a relatively scarce resource in wireless sensor networks, so this bandwidth must be used efficiently by making sure that as many packets as possible are delivered correctly.

Security protocol configuration Security protocols for wireless ad hoc networks are required for many applications. Military applications have an obvious need for security, but other types of sensors, such as those carrying medical or personal information, need adequate security as well.

Besides the need to encrypt or authenticate data produced by sensors, control information such as routing updates also may require authentication to prevent a malicious node from keeping data from being routed properly in the network

Energy-Efficient Communication Multihop routing Communication scheduling Duplicate message suppression Message aggregation Dual-radio scheduling Sleep-mode scheduling Clustering

2. Energy-Efficient Communication Controlling the communication cost is fundamental to achieving a long operating lifetime for the wireless node. Hardware designs that lead to more efficient radio transceiver designs help to some extent. The need to transmit with sufficient power, as well as to receive the signal.

2.1 Multihop routing Energy consumed to transmit a packet on a wireless channel increases substantially with the distance a packet is sent (Shih et al., 2001). Ensuring that a packet is transmitted with a sufficient signal-to-noise (SNR) ratio requires that the energy used for communication increases at a rate of at least the square of the distance between the sender and the receiver.

2.2 Communication scheduling scheduling wireless communication among sensor nodes offers a number of opportunities for saving power. Scheduling nodes to operate on different frequencies or at different times reduces the number of collisions. Collisions consume additional energy because transmitted packets are not received correctly and hence need retransmissions.

2.3 Duplicate message suppression To further reduce power consumption for communication, transmitters can send fewer packets. One way of accomplishing this is for sensor nodes to transmit less frequently and thereby compress multiple sensor readings into a single packet. When sensor readings do not change, there may be no need to report the same information repeatedly.

2.4 Message aggregation Duplicate message suppression is one method for reducing the volume of information that is transmitted through the sensor network. This is essentially loss-free data compression, especially if a counter is included that indicates how many sensors generated matching packets. Message aggregation is often a lossy data-compression technique.

2.5 Sleep-mode scheduling The density of a network depends on the number of nodes in the region occupied by the sensor nodes, as well as the transmission range of the sensors. As the transmission range of a sensor increases, the number of direct neighbors increases.

2.6 Clustering Clustering is a hierarchical approach to support routing and data aggregation in an ad hoc wireless network.The wireless network is divided into regions, in which each region forms a cluster with a single node, the cluster head, designated as the leader of the cluster.

` Mobility Requirements Movement detection Patterns of movement Changing group dynamics Resynchronization

3. Mobility Requirements 3.1 Movement detection In order for a protocol to address mobility, there must be some mechanism for detecting mobility. Besides determining if a node is mobile, relative mobility is also important. For instance, if all the nodes in the ad hoc network move together, the mobility is not an issue. Consider a number of ad hoc device users seated on a train. All these devices may be moving at a high rate of speed, but as long as the passengers remain seated and the devices stay with these passengers.

Use of the Global Positioning System (GPS) is sufficient to determine that a node is moving, but the relative movement of nodes is required to make mobility a factor in the protocols.

3.2 Patterns of movement In an ad hoc mobile network, nodes move in an arbitrary pattern depending on the movement of the user or object that carries the node. For this reason, it is difficult to determine what mobility patterns arise in practice. Determination of this pattern is valuable so that reasonable simulation studies of wireless protocols can be performed. It is reasonable to expect that the performance of various algorithms for ad hoc networks will differ depending on the mobility pattern.

3.3 Resynchronization For group communication, all members of the group may need to maintain consistent information. In other words, group members must receive information generated by each member in order to have the same view of the system state. A database application with data distributed among the various mobile ad hoc nodes must ensure that each node receives all transactions.