Star and mesh networks are the two fundamental architectures for IoT wireless networking.
With Bluetooth mesh recently gaining traction, mesh networking might become even more widespread in the years to come, especially in applications where thousands of IoT nodes must communicate across a geographically large area. Mesh networks can facilitate applications such as building automation, energy management, industrial automation, and asset management.
To highlight the advantages and limitations of mesh networking, this article delves into a discussion on the basic features of both star and mesh topologies. It then looks at a typical Zigbee mesh network as an example. In a future article, we’ll examine Bluetooth mesh networking separately.
Choosing the Right IoT Wireless Network Solution
There are several different wireless connectivity solutions for IoT applications. Considering the wide range of use cases, it can be quite challenging to choose the right wireless connectivity solution to meet the requirements of a given IoT application.
When choosing a connectivity solution, various factors such as range, data rate, security, power consumption, and scalability, should be considered.
Network topology—how the sensor, actuator, and gateway nodes are arranged or connected to each other—is another important factor that can impact network performance.
The two fundamental architectures for IoT wireless networking are the star and mesh connections.
Star Topology Advantages and Disadvantages
A star topology is depicted in Figure 1.
Figure 1. A high-level representation of a start topography. Image used courtesy of Texas Instruments
A star network consists of a central hub to which all other nodes connect. Nodes communicate with each other through the central hub, which is also the gateway to the internet in most cases.
A home Wi-Fi network is a familiar star topology where phones, tablets, and printers all connect to a central hub (the wireless access point). This central hub acts both as a router in the local network as well as a gateway to the internet.
With the hub being responsible for distributing data packets along a star network, a message can reach its destination through either a single “hop” (data transmission between a node and the hub) or two “hops” (data transmission between two nodes through the hub). This feature leads to a fast network with consistent and predictable performance.
Another advantage is that an IoT network based on star topology can easily identify and isolate a faulty node since each node has its own separate connection with the hub.
However, with the data packets having to pass through the central node, the network has a single point of failure. If the central node fails, the whole network will cease to exist.
Another major limitation of a wireless star connection is that all nodes should be within direct radio reach of the central node. This limits the physical size of the network.
Besides, a star network doesn’t have the flexibility to route around RF obstacles or environments with high RF interference.
This is not the case for mesh networks, which typically incorporate multiple routing paths between every two nodes as we will discuss shortly. Mesh networks have a more flexible layout and are more likely to be able to route around RF obstacles.
Mesh Networks: Full and Partial Mesh Topology
In a mesh network, one node can directly communicate with multiple other nodes.
There are two types of mesh networks: full mesh and partial mesh.
In a full mesh topology, every node can directly communicate with every other node in the network.
In a partial mesh network, shown in Figure 2, each node can directly connect to one or several other nodes in the network but not necessarily to every other node in the network.
Figure 2. A high-level representation of a partial network mesh. Image used courtesy of Texas Instruments
IoT applications commonly use a partial mesh topology to extend the network range as we’ll discuss below.
In a mesh network, the nodes can act as a repeater, routing the data through the network. As a result, there are several different paths between every two nodes. This redundancy improves the network resilience; if one path fails, an alternative path can be used to propagate the data through the network.
Since the nodes are capable of acting as a repeater, nodes that are not within the direct radio range of each other can still communicate through the router nodes. This is a major advantage of mesh networking in IoT applications because it allows a user to extend the range of the network beyond that of a single radio.
The downside is that the multi-hop nature of the communications can increase the delay of propagating a data packet across the network.
The hop counts, and consequently, the network latency is a function of the number of routers the data packet passes through. This makes evaluating network performance more complicated than a simple structure such as the star topology discussed above.
In this case, one might use a quality of service (QoS) metric: the ratio of transmitted packets that reach the end destination within a specified time duration (say, 300ms).
The routing nodes of a mesh network should implement some routing algorithms to efficiently deliver the packets to destinations. To implement these routing functions, the routing nodes should have more processing power and memory, which increases the complexity and cost of these nodes.
Zigbee Protocol for Star, Tree, and Mesh Topologies
Zigbee is an open global standard designed to meet the needs of low-cost, low-power wireless IoT networks.
Zigbee is based on the IEEE 802.15.4 link layer and operates in unlicensed bands including 2.4 GHz, 900 MHz, and 868 MHz. ZigBee supports star, tree, and mesh topologies.
A typical Zigbee mesh network is shown in Figure 3.
Figure 3. An example of a Zigbee mesh network. Image used courtesy of S.M. Song W.J. Yao
The radios in a Zigbee mesh network play different roles. A node can be a coordinator, router, or end device. The coordinator sets up the network and allows routers and end devices to join the network. In addition to creating the network, the coordinator is also responsible for managing the security of the network.
The router nodes are always listening to route the information they receive through the network. These nodes are commonly mains powered.
Finally, end devices are nodes that don’t route the information. These devices remain in a sleep mode to save power and wake up only briefly to poll their parents and pick up the messages that have been sent to them.
End devices are commonly battery-powered nodes.
A Zigbee mesh network can configure itself automatically (self-forming).
Also, when a node leaves the network or fails, the network can reconfigure routing paths based on the new combination of the nodes. This self-healing feature increases the stability of the network in changing conditions.
In the next article in this series, we’ll examine different features of Bluetooth mesh networks that have recently attracted a lot of attention from the IoT industry.
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