Having covered the basics of wireless networking in Part 1, as well as the criteria for evaluating the various topologies, it is now time to look closely at five different network architectures - point-to-multipoint, Zigbee 2007, Wireless Hart, 6LoWPAN, and on-demand distance vector routing - and assess their strengths and limitations.

Point-to-Multipoint
The point-to-multipoint architecture is also known as a simple star and it is not really a mesh network, but it is also often confused with one.

Key characteristics. Such networks tend to use the modern air Interfaces of either Frequency Hopping Spread Spectrum or Direct Sequence Spread Spectrum (802.15.4). They need to be statically configured for PAN ID, routes, and security.

It is important to note that all of the nodes can see other nodes and that they need to be told who to talk to. Security tends to be pair-wise for both the encryption and key. End points may go to sleep or stay awake, but the central router is always awake.

Network Architecture. Figure 4 below illustrates a typical topology. All nodes are on the same channel (or hop to the same channel). Bandwidth / throughput is limited by simultaneous data at concentration point. Collisions happen with lots of traffic or lots of nodes.

Figure 4. A typical point to multipoint wireless configuration. Green end points see other end points but are told only to talk to the orange central point.

Strengths. The beauty of the basic non-mesh point-to-multipoint network is simplicity. Communication, unless traffic is very heavy, is relatively deterministic since there are no hops and minimal or managed collisions.

It also allows for maximum throughput because there is no added routing and no added route discovery. Finally, it is easy to understand and easy to manage. Because of the simplicity, it also tends to be the lowest cost for its specific size and function.

Limitations. Unfortunately, the simplicity described in the strengths also drives a number of limitations. The networks will tend to be small. Large networks only work if polled from a central point.

This requires very specific message management. There are also single points of failure and no ways to route around changing conditions. The network follows the belief that if it worked the first time, it will work forever so you must be sure of good RF conditions.

ZigBee 2007
ZigBee is built on top of 802.15.4 using DSSS in 2.4 GHz. End points sleep, routers don't sleep and a coordinator is needed to start the network and to allow points to join the network.

Key characteristics. ZigBee has had three different versions of the standard " 2004, 2006 and 2007. ZigBee 2004 is no longer used and ZigBee 2006 had significant limitations. ZigBee 2007 includes key features for frequency agility, message fragmentation and enhanced security associated with key management.

The routing of messages follows the previously described Cluster-Tree methodology where routes to all points are maintained at each cluster. This allows a very short routing time, but requires lots of routes. Discovery of routes uses the AODV algorithm where paths are explored between clusters.

Network Architecture. The network consists of three specific types of points. The ZigBee Coordinator (ZC) has one required for each network and it initiates network formation. The coordinator may act as a router once the network is formed.

The ZigBee Router (ZR) is actually an optional network component, although a network without routers becomes a point-to-multipoint network described earlier.

The router participates in multi-hop routing of messages. Finally, the ZigBee End Device (ZED) does not allow association and does not participate in routing. As such it is often referred to as a child because it doesn't really have any responsibilities. Figure 5 below illustrates a Zigbee 2007 network.

Figure 5. A typical Zigbee 2007 wireless network configuration.

Strengths. End devices are very low power because they are subservient to parental routers. Cluster-Tree routing provides quick knowledge of routes and thereby efficient routing.

With ZigBee 2007, frequency agility switches from problem channels automatically in a sort of on demand frequency hopping. Long messages are allowed with message fragmentation support and security is flexible with support of separated keys. Finally, the network can scale to be very large.

Limitations. The biggest limitation tends to be in terms of power in the routers. Routers must be powered " they can never go to sleep. In addition, the benefit of Cluster-Tree routing also means that network changes require a lot of route discovery traffic. Heavy traffic volume means lots of collisions and potential message loss. Finally, a coordinator is needed to start and manage the network, so if the coordinator goes down, no one can join and the network can't start.