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.

Wireless HART
Wireless HART uses the Time Synchronized Mesh Protocol (TSMP) created by Dust Networks. Unlike other networks, the time based system uses TDMA (Time Slots) for an access method.

Key Characteristics. The network is optimized for low power and all nodes can be sleeping routers and every node is a router. A gateway is required to keep the network synchronized due to the critical time synchronization of sleeping and waking functions. Like ZigBee, it is built on top of 802.15.4 DSSS, but it adds a more deliberate frequency hopping algorithm. Security includes encryption and authentication.

Network Architecture. Figure 6 below illustrates a typical network topology. Note that all the nodes are routers. The illustrated routes change dynamically based on visibility within specific time slots as it hops through the different DSSS channels.

The relationship between any two nodes is negotiated to be in a specific time slot, thereby minimizing the probability of any collisions. When sleeping, nodes awaken during their time slot and listen to see if there is any traffic. Clocks are kept synchronized by the gateway.

Figure 6. A Wireless Hart Time synchronized mesh network configuration.

Strengths. Every node is a router at very low power consumption. Most of the time is spent listening. Since transmissions occur only within the allocated time slot, retransmissions are minimized.

Communications are very reliable with every message acknowledged. Networks are able to scale to moderate level or around 1000 nodes. Frequency hopping minimizes the probability of interference. Security includes encryption and appropriate authentication.

Limitations. Because of the time slot approach, latency is long and non-deterministic. It takes a network a while to form and all of the nodes to negotiate their individual time slots. Because communications is slotted, the available 802.15.4 bandwidth is split up, meaning that throughput is minimized for bursty traffic.

A powered gateway (coordinator) is required for the network to stay functioning opening up a single point of failure if the gateway is unavailable for an extended period of time. Finally, the radios are very expensive compared to the other available solutions.

6LoWPAN
6LoWPAN is a distorted acronym for IPv6 over low power wireless personal area networks. Presently it is a proposed standard based on the IETF RCF 4944. It is designed to be used over 802.15.4 chips and radios.

Key Characteristics. Unlike traditional IPv6, 6LoWPAN deals with packet size incompatibilities in message transport (128 bytes vs. MTU of 1280 bytes in IPv6) and it is designed for a small memory footprint system. Today it is a point-to-multipoint architecture and it is proposed to be augmented with a mesh routing scheme.

Network Architecture. Figure 7 below illustrates an example network topology. Note that for now it is only point-to-multipoint. Unlike the other networks discussed, the figure shows an end to end IP based link from a host computer to an end device.

In this case it is illustrated by a meter. The end device is directly addressable by the host computer on the far end of the network. The interworking function provided in the pictured box provides a transport change and re-packetization at the data-link level.

Figure 7. A 6LoWPAN IPv6 over wireless network configuration.

Strengths. The most powerful strength is that 6LoWPAN is able to take advantage of the existing TCP/IP suite of internet protocols, all of which are well understood due to the proliferation of the internet. Hence it is able to capitalize on existing protocols, existing quality of service and security framework supported by the IETF. Hence, it enables seamless routing of message payloads.

Limitations. This system is still very new and is only a proposed standard. Because it is officially in the public review stage, it will most likely undergo a number of changes.

In fact, the mesh routing working groups are still being formed meaning that wide scale adoption is still a few years away. As such, interoperability is a nice concept that has not been proven yet. Finally, because it is still new, it has not yet been ported to a large group of chipsets.

Ad hoc On-Demand distance vector routing (AODV)
Like its sibling Wireless HART, AODV (including the variant used in the Digi Mesh) is designed to meet the very low power sensor networks where battery powered routers are required.

Key Characteristics. Our AODV variant is available in multiple frequency bands, 2.4 GHz DSSS and 900 MHz FHSS and does not rely on a full 802.15.4 implementation as it has some of these functions internal.

For both message routing and discovery, it uses a variant of AODV, meaning that routing tables are built only for needed destinations, leaving it to be referred to as a peer-to-peer mesh instead of a cluster-tree.

All nodes are viewed as equal participants meaning that they are all routers and they can all sleep. Channel access is a sort of time synchronized CSMA method, enabling bursty traffic, but the benefits of few collisions. It has a full security suite.

Network Architecture. Figure 8 below illustrates a typical ad hoc network topology. Unlike the Cluster-Tree method described in ZigBee, routes are only determined on an as needed basis. This means that routes that are never used never get routing table entries and routes that are used frequently are continuously updated, optimizing their efficiency.

Figure 8. The Digi Mesh AODV wireless network topology.

One of the other keys to note about the topology is that there is no coordinator or gateway function. Time synchronization is accomplished through a nomination and election process, enabling the network to operate autonomously.

Routing Methodology. Figure 9 below shows the process of how routing failures are handled. The first shows the initial network configuration where a route has been established from one point to another.

The second illustrates a failure where one of the nodes has been removed for an unknown reason, removing relationships in the center of the route. Finally, the last figure shows how this route is reconstituted using a path that didn't previously exist. The relationships were there, but they had never been used, but were newly discovered using AODV after the failure.

Figure 9. Here's how routing failures are handled with an AODV wireless mesh network configuration.

Strengths. Every node is a router at very low power consumption. Further, because every message is acknowledged and routes are determined on an as needed basis, the network is not overwhelmed with unnecessary discovery traffic " very important if the routers are battery powered and sleeping.

Efficient route discovery and routing means that the network only learns routes that actually get used (AODV). Frequency agility is supported and security meets the requirements of both encryption and authentication. Reliability is projected at 99.99%. Finally, the system supports larger Payloads with support for message fragmentation.

Limitations. Unfortunately, efficient power management means latency is long and non-deterministic. Even though throughput is not limited by time slots, it is still limited depending on loading and discoveries. The network can scale to a moderate size of around 500+ nodes and can be very large if traffic is light and message flow doesn't change much.

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.