Giving structure to a wireless network
To give structure to selecting a wireless protocol when deciding between ZigBee, 802.15.4, or a proprietary network, the following criteria are helpful to keep in mind.

1. Application considerations
2. Robustness and reliability
3. Ease of use
4. Hardware and RF considerations

Application considerations
The initial steps in network design, much like any other system design, are to define the high-level requirements for the application. The following list captures some of the most important network parameters that should be defined before considering any wireless protocol as the solution of choice.

The possible implementations addressing these application considerations can then be traced throughout the paper, as the detailed explanation of the selection criteria and the protocols themselves require further elaboration.

1. Network topology
    a. How many nodes and in what general organization within the network does the application require?
2. Reliability of communications
    a. How critical is the reception of each data packet within the network?
3. Network security
    a. Does the data need to be secure? If so, how critical is the absolute security of the network data transmissions?
4. Customization and design freedom
    a. How customized does the networking protocol solution need to be to fit the application, and does the protocol offer the design freedom to do so?
5. Development time and protocol complexity
    a. Along the same lines of #4, what complexity exists within the protocol in question?
    b. How much time will be spent learning and integrating the protocol in question to the existing application versus the time that would be spent developing a more customized solution?
6. Interoperability
    a. Would the end product benefit from interoperability with other vendors, or will it be a completely proprietary solution?

Figure 4 below presents four of the most common wireless network topologies available for implementation.

Figure 4--low power network topologies

A peer-to-peer network topology is one that supports either uni- or bi-directional links between individual nodes on the network. Nodes will only communicate if they are within range of each other, as they must maintain direct physical links for communication; the only exception being a broadcast message, which could be re-broadcast and propagated through the network.

A tree network topology is one each node in the network associates itself to a parent node, and the network addressing is reflected as such, much like an IP internet address. This allows for more efficient routing algorithms to be implemented, as the more significant digits of a node's network address can identify the node's location in relation to its peers.

A star network identifies a single node as the network coordinator, responsible for a variety of possible network management controls, such as node associations, network join and linking permissions, message forwarding, and security exchanges. A star network depends upon the coordinator to keep the network communicating and is susceptible to disruption if the coordinator node goes down.

A mesh network, in its most general sense, is defined as a network in which there are at least two pathways to each node and a fully meshed network means that every node has a direct connection to every other node in the network.

The latter case somewhat unreasonable in many cases, as this would quickly limit the size of a network to the minimum range of the weakest device, and the former is perhaps too strict a requirement.

Instead, one tends to see interpretations that exist somewhere in between the two cases, where a central node is responsible for starting the network, and a tree addressing technique is used to locate nodes and manage the associations between them.

Range extenders, or routing nodes, exist to route messages throughout the network and if one node or the coordinator goes down, the network can still continue to function, sacrificing some degree of operability. Additional features such as self-healing route discovery and route expiration can make such routing algorithms increasingly reliable and efficient.

Another important factor to consider may be the financial cost of using a certain protocol. It is not uncommon that a membership or royalty fee must be paid to the organization representing a proprietary solution.

ZigBee, for example, does not have royalties but does require membership in the ZigBee Alliance for a nominal, yearly fee. There can also be a cost, both in time and money, resulting from a certification process. Silicon vendor proprietary protocols typically require that their devices be used in lieu of a royalty.