Network Topology

The communication network can be configured into several different topologies, as seen in Figure 2.1. We describe these topologies below.

Single-hop star
The simplest WSN topology is the single-hop star shown in Figure 2.1(a). Every node in this topology communicates its measurements directly to the gateway. Wherever feasible, this approach can significantly simplify design, as the networking concerns are reduced to a minimum. However, the limitation of this topology is its poor scalability and robustness properties. For instance, in larger areas, nodes that are distant from the gateway will have poor-quality wireless links.

Multi-hop mesh and grid
For larger areas and networks, multi-hop routing is necessary. Depending on how they are placed, the nodes could form an arbitrary mesh graph as in Figure 2.1(b) or they could form a more structured communication graph such as the 2D grid structure shown in Figure 2.1(c).

Two-tier hierarchical cluster
Perhaps the most compelling architecture for WSN is a deployment architecture where multiple nodes within each local region report to different cluster-heads. There are a number of ways in which such a hierarchical architecture

Figure 2.1 Different deployment topologies: (a) a star-connected single-hop topology, (b) flat multi-hop mesh, (c) structured grid, and (d) two-tier hierarchical cluster topology

may be implemented. This approach becomes particularly attractive in heterogeneous settings when the cluster-head nodes are more powerful in terms of computation/communication. The advantage of the hierarchical cluster-based approach is that it naturally decomposes a large network into separate zones within which data processing and aggregation can be performed locally. Within each cluster there could be either single-hop or multi-hop communication. Once data reach a cluster-head they would then be routed through the second-tier network formed by cluster-heads to another cluster-head or a gateway. The second-tier network may utilize a higher bandwidth radio or it could even be a wired network if the second-tier nodes can all be connected to the wired infrastructure. Having a wired network for the second tier is relatively easy in building-like environments, but not for random deployments in remote locations. In random deployments there may be no designated cluster-heads; these may have to be determined by some process of self-election.

Connectivity using power control:

Regardless of whether randomized or structured deployment is performed, once the nodes are in place there is an additional tunable parameter that can be used to adjust the connectivity properties of the deployed network. This parameter is the radio transmission power setting for all nodes in the network.

Power control is quite a complex and challenging cross-layer issue. Increasing radio transmission power has a number of interrelated consequences – some of these are positive, others negative:

• It can extend the communication range, increasing the number of communicating neighboring nodes and improving connectivity in the form of availability of end-to-end paths.
• For existing neighbors, it can improve link quality (in the absence of other interfering traffic).
• It can induce additional interference that reduces capacity and introduces congestion.
• It can cause an increase in the energy expended.

Most of the literature on power-based topology control has been developed for general ad hoc wireless networks, but these results are very much central to the configuration of WSN. We shall discuss some key results and proposed techniques here. Some of these distributed algorithms aim to develop topologies that minimize total power consumption over routing paths, while others aim to minimize transmission power settings of each node (or to minimize the maximum transmission power setting) while ensuring connectivity. These goals are not necessarily complementary; for instance, providing minimum energy paths may require some nodes in the network to have high transmission powers, potentially limiting network lifetime due to partitions caused by rapid battery depletion of these nodes.