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Building networks for wireless M2M apps

Posted: 24 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Multi-hop? mesh protocols? machine-to-machine?

Multi-hop mesh protocols such as Zigbee has the capability to link together low data-rate machine-to-machine (M2M) applications. These applications operate through a point-to-multipoint network, otherwise known as a star network. Touching on many of the requirements of star networks, like low data rate, low power, enhanced range through the mesh, and automated on-demand routing, Zigbee, in particular, has been positioning itself as the standard bearer for wireless, low-power meshing protocols.

Requirements for star networks
It is important to consider the actual data flow through the network when implementing a Zigbee-type mesh. While all nodes may be capable of communicating with each other, in reality, most networks are point-to-multipoint (or multipoint-to-point, depending on the perspective), and form a star topology. Data flows from a central server to specific end points, which in turn collect data or provide some sort of action. Data from the end points is also able to flow back to the central point. For a star network, a multihop mesh is not a requirement; but rather a feature to ensure connectivity from all nodes. In fact, for star networks, the amount of overhead required for a Zigbee network may be restrictive to an optimal solution.

Figure 1 shows an example of a star network with multiple end nodes, in which all nodes communicate to a single central sever.

Figure 1: Star network.

Zigbee for star apps
Zigbee has a dual-layer addressing scheme with a lower layer Institute of Electrical and Electronics Engineers (IEEE) address being hard coded on the nodes, and a dynamically assigned network address being used for transport. Because only the network address is used to route data, an end user must translate between the IEEE address and the network address in order to properly address the packets. This is analogous to how the Address Resolution Protocol (ARP) operates in traditional Ethernet networks. This dual-layer addressing is common for routed networks and provides a layer of abstraction from the hardware (IEEE) layer. For star networks, it only serves to provide another layer of complexity to a simple issue of connectivity (figure 2).

Figure 2: Star network implemented over a multihop mesh.

The Zigbee mesh is based on an underlying RF protocol defined by the IEEE 802.15.4 standard. The 802.15.4 standard is a direct-sequence spread spectrum (DSSS) modulation system designed to operate in the 868MHz, 900MHz, and 2.4GHz industrial, scientific and medical (ISM) bands. In practice, most transceivers operate at 2.4GHz, as it provides worldwide acceptance and the higher 250 kbps RF data rates. In many parts of the world though, including Europe, 2.4GHz DSSS transceivers are limited to 10 mW of radiated output power. Compare this to frequency-hopping systems such as Bluetooth and proprietary RF, which can radiate up to 100 mW per Conformit Europenne (CE) regulations (10x the output power). This limitation reduces the overall power consumption of the module, but it also limits the range of the module as well. Zigbee addresses this range issue through multihop mesh routing.

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