Clearing up the mesh about wireless networking topologies: Part 1

Joel K. Young

August 25, 2008

Joel K. Young

Network Types
Now that we have a feel for the terminology, we first look at different network topologies commonly used. Figure 1 below shows three such topologies: Star, Bus and Ring.

Figure 1. Three typical wireless network topologoes

In the Ring, nodes are connected from one to the next. Communications messages are forwarded around the ring in either a clockwise and/or counter-clockwise fashion.

As a message is forwarded, the node checks to see if the message is meant for itself, if so, it keeps the message, if not, it forwards it on. It is most common in cabled networks (wire or fiber), but could conceivably be used in a wireless fashion as well but is not practical unless being used over long distances.

In the Bus, all nodes share a common communications medium and contend for using it. Typically this means some kind of CSMA type approach. Since a common medium is used, collisions and retransmissions increase with traffic loading.

In the wired case, these types of systems are referred to as hubs - which are generally no longer used. In the wireless case, it is more complicated because open space is often a shared medium, so even if routing is handled in a star, ring or other topology, open space often appears as a bus. More on this later as it is one of the fundamental hurdles in wireless networking.

In the Star, nodes are connected through a master, central node. This central node is responsible for looking at each message and forwarding it out on the proper communications link.

While various star architectures have been used over time, the most commonly known in the wired space is the Ethernet Switch. In the wireless space, the WiFi access point is also a familiar example since all messages are routed through the access point; however, even though messages are routed through the access point, open space is accessed via CSMA, a bus type protocol.

Mesh networking is more complete
A mesh network employs some level of more complete interconnection among nodes. This means that paths are not defined by a specific architectural pattern, but rather by the connections themselves.

In the full mesh topology, each node (workstation or other device) is connected directly to each of the others. In the partial mesh topology, some nodes are connected to all the others, but some of the nodes are connected only to those other nodes with which they exchange the most data. Figure 2 below illustrates a full mesh where each of the five nodes is connected to all the others.

Figure 2. A fully connected wireless mesh network configuration.

Other important thing to note about a mesh is that some or all nodes may be routers and some or all nodes may be end points. Typically, full interconnection is not achieved unless the network is very small.

Full interconnection gets very complex very quickly. In addition, wired mesh networks tend not be practical due to the complexities of connecting all the wires.

Figure 3 below illustrates three different instantiations of mesh networks. The green nodes are end devices, the yellow are routers (which may also be end devices) and the purple is the network coordinator responsible for allowing joining and departing from the mesh (more on this later).

Note that one instantiation of a mesh can be a star - a mesh with one router and the rest end points. The Cluster-Tree network is a combination of near full connectivity among routers and end points hanging off individual routers. The Peer-to-peer mesh generally gives equal rights to all nodes, including routing and end point functionality.

Figure 3. Three mesh networking instantiations

What makes wireless networks so different.
While we have discussed that mesh networks aren't really practical for wired networks, it is also important to look at the other differences between wired and wireless domains.

So what else makes wireless different? On the positive side, wireless makes it possible to have more connections since it is not practical or cost effective to create a full mesh with wires. However, on the negative side, wires are predictable, reliable and well understood.

Wireless forces the sharing of an already noisy, uncontrollable medium called open space. Hence, while wireless gives us more flexibility, the uncertainty of wireless drives the need for more connection pathsand more complexity.

When evaluating wireless, particularly mesh networks, there are a number of hard problems that need to be solved, including:

Accessing the medium. Since we all share open space, listening is more important than talking. If everyone talks at once, listening is difficult. So radios must be good listeners if they are going to have a chance to get a word in edge wise, so to speak.

Discovering routes. Determining paths in a wireless mesh network is difficult because the environment is dynamic. In this case, there are two choices: Planning the trip in advance, or taking it one step at a time. Often times doing both is best " this usually involves retracing one's steps and repeating well traveled routes.

Adapting to a changing environment. In a wireless world, paths to nodes can disappear and re-appear. This is due to changing signal conditions or traffic conditions.

Sleeping and Waking. Once we go wireless, the next step is often to find a way to do away with the traditional power cable. This means batteries and the need for effective power management.

The most common way of handling power management is putting the nodes to sleep when they are not being used. This sounds well and good until it is time to wake up.