Comparison of the alternatives
Using the criteria defined earlier in Part 1 in this series, Table 1 below illustrates my best attempt at evaluating the different network approaches.

It is important to note they all do very well in security in that they have well defined encryption, authentication and authorization schemes. ZigBee and 6LoWPAN get a slight nod here only in that their key systems should be easier to implement and a bit more flexible.

Table 1. Comparing the wireless mesh alternatives

With respect to reliability, Point-to-Multipoint takes the biggest hit because it inherently has a single point of failure. Some schemes may have frequency agility options while others do not.

Prior to the 2007 standard, Zigbee has a weakness in the frequency agility area; this is fixed in the 2007 standard along with adding support for message fragmentation.

The others are similar - Wireless HART is designed to never lose a message so it gets the nod here while 6LoWPAN does well on the assumption that the existing TCP/IP protocol suite has class of service built in. While the AODV-based Digi Mesh has a similar approach to Wireless HART, it is still somewhat unproven in large deployments.

Power management will no doubt be hotly debated. The nod was given to Wireless HART and our AODV variant because they both define systems where all nodes in the network, including routers, can sleep.

Even though sleeping Zigbee end devices are most efficient when it comes to power, the fact that routers can't sleep bumped the rating down. Until 6LoWPAN settles on a mesh and power management strategy, the rating will remain unknown.

The scalability rating follows directly from the question of how big can the network get and still function. This is where the Zigbee 2007 Pro stack shines. The Cluster-Tree architecture creates a hierarchy which enables scalability.

Digi's AODV variant and Wireless HART scale well; particularly if most communication is kept local " however, the networks can tend to get very slow when they get too big. Finally, point-to-multipoint has an obvious limitation in the number of nodes that can be attached to one central point.

The best data mover is no doubt the simplest system - namely point-to-multipoint. The simple network design means that focus can be made on short, deterministic latency and high data throughput.

There is a direct trade-off here with power. Wireless HART and Digi Mesh rate lower here because they are focused on minimizing power and maximizing reliability " this naturally leads to less deterministic latency and lower throughput.

I recognize of course, that as a network gets bigger, these two networks will actually do better; however, this is represented in the high scalability ratings for these networks. Zigbee fits in the middle here because the backbone of powered routers can move data very efficiently " but can get stuck if too many route discoveries are needed.

<>Cost may end up with the most debate. The ratings here were based primarily on the view of the cost of available chip set solutions under the assumption that the right architecture is chosen for the right job. If not, then the cost ratings go out the window. For example, trying to deploy a Zigbee solution where battery powered routers are desired means infrastructure costs will skyrocket. So given this caveat, point-to-multipoint, Zigbee and Digi Mesh have common costs because they all use similar chipsets. 6LoWPAN is somewhat unknown " depending on resource requirements.

The assumption is that similar to current chipsets can be used without substantial feature degradation. Wireless HART has a low rating predominantly because the limited number of suppliers has kept chipset prices 5X to 10X other solutions and customers have not demanded lower costs due to primary use on expensive assets in process control environments. This will most likely change as more competitors enter the market.

Conclusion
We have traced the architectures of wireless mesh networks and the respective architectural trade-offs. Each of the Wireless Mesh Architectures has respective benefits as they optimize on different components. There is not a one size fits all approach as throughput is traded off against reliability and power consumption.

Hence, it is important to match the needs of the application to the capabilities of the network. Further, it is important not to settle for the wrong network because of fad or hype in the market place.

No doubt many of the conclusions here will be hotly contested by different network architectural advocates. This is always true where there are shades of gray in evaluation of different criteria. For example, had this article been done a year ago, the results will have looked very different " as they will look different a year from now.

To read Part 1, go to The basics of wireless mesh networking topologies.

Joel Young has more than 15 years of experience in developing and managing data and voice communications. Mr. Young joined Digi International as Vice President of Engineering in June 2000 and is currently the Vice President of Research and Development and Chief Technical Officer of Digi. In his current role, Mr. Young is responsible for research and development of all of Digi's core products.