How to Compare
Given that we now understand the basics of networks, in particular mesh networks, the next question that should be addressed is how do we compare them? For this, we look at criteria of security, reliability, power management, scalability, data movement, and cost.

Security. This is as much about the perception of threat as actual threat. Nonetheless, security is easily evaluated by the traditional factors that are well understood in the industry.

The first is encryption " protecting the information itself. Modern encryption wants at least AES128 as an algorithm (128 bit key). The next is authentication, which is validating that the users (or nodes) are who they say they are. This is typically handled by key exchange or authenticated certificate.

Last is authorization, which should be thought of as granting permission associated with having the right key or certificate. Beyond these, there are other factors which are associated with the ease of distributing and configuring the authorization and authentication mechanisms.

Reliability. The best way to think of reliability is the ability of a message to be delivered to the desired destination on time. If the message always arrives at the destination when expected, the network is very reliable.

Secondly, we want the message to arrive at the destination, even if it is a bit late. The components for evaluating reliability for wireless mesh networks have to do with the following:

1) Frequency agility: this is detecting and adapting the network around potential interference.

2) Message loss potential:  this is the question as to whether messages get lost in the shuffle. With all the re-routing and different paths, the network must be very careful to ensure messages don't get lost and that duplicate messages following different routes get discarded.

3) Adaptability: This is best described as the network's ability for changing the routing to accommodate for nodes disappearing while still preventing lost messages. This is most effective if done quickly.

4) Single points of failure: Are there any single points of failure, what is the risk of them failing and how is recovery handled?

Power Management. The most frequent question asked when discussing wireless sensor networks is how long will my batteries last? As soon as the cord is cut, everyone wants to still keep maintenance low.

Viewed in the context of the network architecture, power management is analyzed in terms of end nodes, router nodes and network coordinators. It is most important to have low powered, power efficient end nodes because they are most likely to be far from traditional wired power sources.

The routers are second. Battery powered routers, or routers that sleep, extend the flexibility of the architecture are necessary. Finally, the coordinator is usually powered. Now, in the context of nodes that can sleep, we then look at their average power consumption.

his is best assessed by looking at the combination of how they wake up, how frequently they wake up, total transmitting time and total listening time. Since the most power is consumed when radios transmit, it is important to keep this to a minimum.

Scalability. How big can the network get before it fails, at least on a practical level? All the networks have large physical limits in the 10s of thousands, but the practical design of the network is always much smaller.

This is because scalability is related both to reliability mechanisms and nature of the application. If a network never has any problems which cause rerouting, then network routing tables will never change, meaning cached routes will always work and there will be few retransmissions or reroutes because of failures. The end result is a very stable network that can be very large.

The other aspect concerns the type and volume of data. This data flow can be placed into three categories: Dribble Data, Bursty Data and Streaming Data " and they mean just like they sound.

Dribble data is periodic, infrequent and slow, while streaming data is constant, etc. A network can be very large if the traffic is dribble data because the flow follows consistent patterns, with plenty of bandwidth. Sleeping networks do well with dribble data, but scale poorly with streaming data.

Data Movement. Now we look in more detail at data flow, not for network scalability purposes, but for raw capacity. There is a classic trade-off in needs: Does the application require lots of data with low latency or does it require dribble data with long, non-deterministic latency?

As such, in evaluating networks for data movement, a combination of the following five variables needs to be considered: data rate, latency, packet size, fragmentation and range.

 Cost. Cost is measured by the individual unit cost as well as the cost to maintain the network. In this context, maintenance is often difficult to quantify and deployment cost is often forgotten.

It is easiest to quantify those variables that are most perceptible, namely the actual purchase cost of a transceiver system per node. This becomes a bit more complicated when trading off the number of battery powered or sleeping nodes.

For example, assume all end points need to be sleeping end points and a point to multipoint system is not practical due to range. Then a network that does not have sleeping routers will need to deploy powered routers in addition to the end points as compared to a network that has sleeping routers.

Hence, even if all radios are the same cost, more radios are needed in the powered router system. However, if power is available, then it becomes a non issue. So, the cheapest radio may not be the best for the application. The other point is that the cost of the radio tends to be looked at related to the cost of the device to which it is connected.

Next in Part 2: Comparing Zigbee and alternative wirlesss mesh technologies.

Joel K. Young is senior vice president and Chief Technical Officer at Digi International.