Wireless machine-to-machine networks
IEEE 802.11 and ZigBee local area networks each provide the embedded designer with unique benefits for wireless machine-to-machine communications.
By Bob Blumenscheid, Digi International -- EDN, November 6, 2007
When designing monitoring or control devices that will communicate over wireless networks, there are currently two alternatives to consider: IEEE 802.11 a/b/g WLAN (Wireless Local Area Network) and ZigBee. Although they were originally designed for very different purposes, they each offer distinct advantages for wireless machine-to-machine (M2M) communications.
A Summary of Technologies
The initial focus of WLAN 802.11 was on computer to computer connectivity, and a wireless replacement for wired Ethernet networking. The technology is widely adapted to M2M applications, especially in industrial environments. It offers the full Ethernet networking environment, with layered networking levels and extensive protocol and networking services support.
802.11b and g-based devices communicate on the 2.4 GHz frequency. An alternate technology, 802.11a, communicates on the 5 GHz frequency, and is deployed in environments that may already have interference at 2.4 GHz, or for networks where there are many wireless devices connected, or where reliability is an absolute necessity.
It is possible to connect WLAN-enabled devices in a peer-to-peer mode, but it is much more common for them to communicate via a wireless access point, which facilitates communications among the devices as well as a connection to other networks, humans monitoring the machines, or connection to the Internet.
ZigBee, which also operates at 2.4 GHz, is a more recently-developed protocol and is targeted specifically at device connectivity. It is a simple networking solution for small devices, providing wireless communication to simple devices such as sensors or even light switches. It is used to connect to devices with very low bandwidth and power requirements. It does not require a central access point, because a unique feature of ZigBee is mesh networking. ZigBee-enabled devices connect and communicate with other ZigBee devices detected in range, forming a mesh network. Data is passed through devices in the mesh network to reach its intended device. A ZigBee to Ethernet protocol converter is typically deployed to connect the ZigBee network to Ethernet-based networks, and transfer data from and to devices on the ZigBee mesh network.
The specific differences between WLAN and ZigBee technologies are data transmission rates, power consumption and network topologies. 802.11 provides fast networking connections (11-54 Mbps), suited to its original purpose of computer networking, while ZigBee has a defined rate of 250 Kbps. ZigBee-enabled devices have lower power requirements than 802.11, and ZigBee is more suited to low-power mobile or battery operated applications.
While 802.11 is capable of streaming large amounts of data and supports web-based applications, ZigBee is best suited to periodic or intermittent data, or a single signal transmission from a sensor of input device. It is useful to think of WLAN as wireless Ethernet networking, and ZigBee as a wireless analogue of serial communications. Typical performance characteristics of ZigBee and WLAN networking devices are summarised in Table 1.
| ZigBee | 802.11b | 802.11g | 802.11a | |
| Max Data Rate | 250 Kbps | 11 Mbps | 54 Mbps | 54 Mbps |
| Max Throughput | >200 Kbps | 7 Mbps | 25 Mbps | 25 Mbps |
| Frequency | 2.4 GHz | 2.4 GHz | 2.4 GHz | 5 GHz |
| Typical Indoor Range | 30 m | 30m at 11 Mbps 90m at 1 Mbps |
30m at 54 Mbps 90m at 1 Mbps |
30m at 54 Mbps 90m at 6 Mbps |
| Typical Outdoor Range | 100m | 30m at 11 Mbps 300m at 1 Mbps |
120m at 54 Mbps 460m at 1Mbps |
120m at 54 Mbps 460m at 1Mbps |
Advantages of WLAN-based M2M networks
WLAN-based devices can be easily deployed in an existing WLAN network infrastructure, and provide higher data transfer rates than is available with ZigBee. In addition, these devices will include a wide variety of networking protocols and web services, including many important for M2M deployment: SNMP for device configuration, standards-based protocols such as Ethernet/IP, and proprietary protocols such as ProfiNET or Modbus. WLAN also provides extensive security features to protect data being transferred. This is a key benefit if personal or financial data is being transferred, for example—by medical devices or point-of-sale terminals.
Advantages of ZigBee-based M2M networks
ZigBee's lower data rates provide lower power consumption. ZigBee-based battery-operated devices can often "sleep" for much of their lives and wake on activation or for periodic timed status updates. With this low activity, it is possible to deploy a device with several years of battery life.
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Mesh networking allows ZigBee-based M2M networks to grow hundreds and thousands of devices, which will automatically transfer data across the best available path for reliable message delivery. ZigBee networks are self-forming and self-healing: if one device is offline, a new data path is automatically discovered and network performance is not affected. Mesh networking formation is ideal for an M2M network with many devices deployed, such as a sensor network.
Designing embedded devices for wireless M2M networks
While it is possible to implement a radio design using networking chipsets, this approach requires RF design expertise and a great deal of time and expense. Chip-based radio components are not sold for the relatively smaller quantities needed for embedded products, and wireless designs must be certified to emissions standards for worldwide deployment.
A better approach is the integration of a wireless module into an embedded product design. ZigBee modules work very well for M2M applications with low-bandwidth requirements, such as collecting data or checking a status periodically. 802.11 modules provide the advantage of integrating easily into an existing WLAN network and offer a wide variety of standard networking services. ZigBee modules provide a lower power requirement and a lower cost, making it more economical where a large number of networked devices are needed in a typical application.
Pre-certification
Engineers should look for modules that have been tested and received certification for deployment in major global markets. By using certified modules in an embedded device, the certification will apply to the embedded device as well. The product can be labelled with certification and shipped worldwide, avoiding the major time and expense of product certification.
An example of ZigBee-based M2M application
"Drop-in networking" is a new way to deploy wireless networks in environments where it would not be appropriate to use wired networking. An example of this is a tank-monitoring application (Figure 3). ZigBee was chosen for Drop-in Networking in this application because there were no wireless access points available, and the small amount of data being transferred. In this application, M2M communication is required between the devices because the system requires measurement of liquid in the tanks and the transfer of liquid between tanks to maintain even levels. In this example, the data on the ZigBee network is transferred to an Ethernet network via a cellular transmission using a gateway product.
Any ZigBee-enabled embedded device can be deployed this way to provide effective M2M communication.
WLAN and ZigBee networking are both effective protocols for deploying embedded devices on M2M networks. They meet different needs, and can be used separately or combined within an environment to meet the exact requirements of an M2M wireless network.
| Author Information |
 Bob Blumenscheid is product marketing manager for Digi International and has more than 20 years embedded/electronics industry experience.
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