What a mesh!
The ins and outs of mesh networking — Part 2

In the first part of this article we discussed the basics of wireless mesh networking and what criteria you should use when assessing a mesh networking technology. This month we compare four available technologies.

The four network topologies we will examine are:

• Point-to-multipoint,
• ZigBee,
• Wireless HART,
• Digi Mesh.

Point-to-multipoint

Key characteristics

This is also known as a simple star and it isn't really a mesh network, although it is often confused with one. Point-to-multipoint networks tend to use the modern air interfaces of either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS, 802.15.4). They need to be statically configured for PAN ID, routes and security. Note that all of the nodes can see all other nodes and that they need to be told which nodes to talk to. Security tends to be pair-wise for both the encryption and the key. End points may go to sleep or stay awake, but the central router is always awake.

Network architecture Figure 1 illustrates a typical topology. All nodes are on the same channel (or hop to the same channel) and bandwidth/throughput is limited by simultaneous data at the concentration point. Collisions happen with lots of traffic or lots of nodes. Strengths The beauty of the basic non-mesh point-to-multipoint network is simplicity. Communication, unless traffic is very heavy, is relatively deterministic because 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 this simplicity, it also tends to be the lowest cost for its specific size and function.

Figure 1: A typical point-to- multipoint topology.

Limitations

Unfortunately, the simplicity also causes a number of limitations. The networks will tend to be small — large networks only work if polled from a central point and 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 always work. Therefore, you must be sure of good RF conditions.

ZigBee PRO (ZigBee 2007)

Key characteristics

ZigBee is built on top of 802.15.4 using DSSS in the 2.4 GHz band. ZigBee has had three different versions of the standard — 2004, 2006 and ZigBee PRO (ZigBee 2007). ZigBee 2004 is no longer used and ZigBee 2006 had significant limitations. ZigBee PRO 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 (Figure 2) consists of three specific types of points. A ZigBee coordinator is required for each network and initiates network formation; the coordinator may act as a router once the network is formed. The ZigBee router is actually an optional network component, although a network without routers becomes a point-to-multipoint network; the router participates in multihop routing of messages. Finally, the ZigBee end device 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 2: ZigBee network architecture.

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 PRO, frequency agility switches away from problem channels automatically in a sort of on-demand frequency hopping. Message fragmentation support allows for long messages and security is flexible with support for 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, cluster-tree routing means that network changes require a lot of route discovery traffic, and 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

Key characteristics

Gaining in popularity, Wireless HART uses the time synchronised mesh protocol (TSMP) created by Dust Networks. Unlike other networks, the time-based system uses TDMA (time slots) for an access method. The network is optimised for low power, all nodes can be sleeping routers and every node is a router. A gateway is required to keep the network synchronised due to the critical time synchronisation of sleeping and waking functions. Like ZigBee, Wireless HART 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 3 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 the message hops through the different DSSS channels. The relationship between any two nodes is negotiated to be in a specific time slot, thereby minimising 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 synchronised by the gateway.

Figure 3: Wireless HART network architecture.

Strengths

Every node is a router at very low power consumption and most of its time is spent listening. Since transmissions occur only within the allocated time slot, retransmissions are minimised. Communications are very reliable, with every message acknowledged. Networks are able to scale to a moderate level or around 1000 nodes. Frequency hopping minimises the probability of interference and security includes encryption and appropriate authentication.

Limitations

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

Digi Mesh

Key characteristics

Like its sibling Wireless HART, Digi Mesh is designed to meet the need for very low power sensor networks where battery-powered routers are required. It is available in 2.4 GHz DSSS and 900 MHz FHSS. It does not rely on a full 802.15.4 implementation, as some of these functions are internal. For both message routing and discovery, it uses a variant of AODV, so routing tables are built only for needed destinations. Because of this it is referred to as a peer-to-peer mesh instead of a cluster-tree.

All nodes are viewed as equal participants; they are all routers and they can all sleep. Channel access is a sort of time-synchronised CSMA method, enabling bursty traffic but offering few collisions. It has a full security suite.

Network architecture

Figure 4 illustrates a typical ad hoc network topology. Unlike the cluster-tree method described for ZigBee, routes are determined on an as needed basis, therefore routes that are never used never get routing table entries and routes that are used frequently are continuously updated, optimising their efficiency.

Figure 4: Digi Mesh network architecture.

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

Strengths

Every node is a router at very low power consumption. Because every message is acknowledged and routes are determined on an as needed basis, the network is not overwhelmed with unnecessary discovery traffic, which is very important if the routers are battery powered and sleeping. Efficient AODV route discovery and routing means that the network learns only routes that actually get used. 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

Table 1 illustrates the author's best attempt at evaluating the different network approaches. Note that they all do very well in security because they have well-defined encryption, authentication and authorisation schemes. ZigBee gets a slight nod here because their key systems should be easier to implement and a bit more flexible.

With respect to reliability, point-to-multipoint takes the biggest hit because, inherently, it has a single point of failure. Prior to the 2007 standard, Zigbee had a weakness in the frequency agility area; this is fixed in the 2007 standard along with adding support for message fragmentation. Wireless HART is similar — it rates highly because it is designed to never lose a message. While Digi Mesh has a similar approach to Wireless HART, it is still somewhat unproved in large deployments.

Table 1: A comparison of the four networking architectures.

Power management will no doubt be hotly debated. The nod was given to Wireless HART and Digi Mesh 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 consumption, the fact that routers can't sleep caused a lower rating.

The scalability rating follows directly from the question of how big can the network get and still function. This is where the Zigbee PRO stack shines. The cluster-tree architecture creates a hierarchy that enables scalability. Digi Mesh and Wireless HART scale well, particularly if most communication is kept local; however, the networks 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 the focus can be 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 minimising power and maximising reliability and this naturally leads to less deterministic latency and lower throughput. I recognise, 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 being the most hotly debated issue. The ratings here were based primarily on the cost of available chipset solutions, assuming that the right architecture is chosen for the right job. If this is not the case 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. Given this caveat, point-to-multipoint, ZigBee and Digi Mesh have common costs because they all use similar chipsets. Wireless HART has a low rating predominantly because the limited number of suppliers has kept chipset prices five to ten times higher than other solutions. Customers have not demanded lower costs due to nodes' primary use on expensive assets in process control environments. This will most likely change as more competitors enter the market.

Conclusion

Each of the wireless mesh architectures examined has benefits as they optimise on different functional characteristics. There is not a one-size-fits-all approach as throughput is traded off against reliability and power consumption. Therefore, it is important to match the needs of the application to the capabilities of the network. It is also important not to settle for the wrong network because of fad or hype in the marketplace. No doubt many of the conclusions here will be hotly contested by different network architectural advocates, something that is always true where there are shades of grey in evaluating different criteria. Finally, this is a view of the state of the topic at this point in time. Had this article been written a year ago, the results would have looked very different, as they will look different a year from now.

OEM Technology Solutions
www.OEM.net.au
Digi International
www.digi.com