Mixing Multiple Wireless Technologies
A practical industrial wireless system may involve wireless components at the WAN, LAN or PAN level, and components must be selected based on the flow of data and the type of service required.
We are living in a wireless world. From the consumer market perspective, most of us (even our children) have cell phones; most of us have Wi-Fi in our houses; and when we do use a landline telephone, often enough it’s a wireless handset. As a part of this, as consumers, our behavior has adapted as well. We know to look at our phone and notebook computer to see “how many bars we have.” We have learned to deal with moving around to improve the signal, how long our battery will last and the pitfalls of not putting our device back on the charger. Fortunately or unfortunately for consumers, mission-criticality in the instant tends not to be as important, and necessary human intervention is an accepted outcome. Finally, as consumers, we have also begun to expect that the cost of wireless will continue to approach the cost of the corresponding wired counterpart.
So what does this all mean for us in the world of industrial and commercial things? As advancements in wireless technology have caused the wired paradigm to fade and as pervasive wireless in consumer markets has driven our belief in easy, low-cost connected solutions, we need to remember that commercial needs are different. We can’t always rely on the human presence to adjust for the “number of bars” or to initiate a “new call.” We also need to account for the relatively small number of devices in a consumer application environment compared to the thousands of units in a commercial or industrial environment. Sure, there are millions of consumer devices, but each of us still only has a handful to manage. Finally, we need to acknowledge that reliability and mission-criticality needs have an impact and a cost. As such, we are forced to ask if a wireless solution in the commercial and industrial world can be cost-effective.
Wireless Technologies and Key Issues
Given both these consumer perceptions and commercial realities, we will explore how to mix different wireless technologies to form a successful commercial deployment. Figure 1 illustrates the different wireless technologies found in deployments today. For this article, we will be focusing on the interactions between all of them except Bluetooth, as we haven’t seen a preponderance of its usage outside of the consumer space.
When considering wireless for a project, it is helpful to consider the key issues of each technology, as summarized below.
W-WAN: Cellular & Wi-Max
- IP addresses and routing are a big deal. Depending on the plan, IP addresses are often private and dynamic and destination routing usually doesn’t work. Hence make sure that you use device-initiated connections or have a way of simulating an extended network using something like a VPN. Note: device-initiated connections are critical.
- Data plans are complicated. This remains one of the immutable laws for telecom. There are very few unlimited data plans for the non-human tethered device. However, you can get a very low-telemetry data plan as long as you know how much you are going to use. Hence it is important to know your data needs and select a plan that allows pooling of data. And remember that the higher the performance in terms of data throughput, latency and quality of service, the more you are going to pay.
- Carrier cooperation is critical. Whoever your carrier might be, make sure that the device you are using is supported and certified by their network. If it isn’t, you may find yourself without service. Just because a SIM card works doesn’t mean that you are allowed to connect your device into the network.
- There is no ubiquitous coverage. Even the carriers with the best coverage can’t cover everywhere and the networks most often have capacity where humans are present. So if you are deploying to a remote site, you may need to employ extraordinary measures to get coverage.
- Plan to fit into the existing infrastructure—whatever it might be. One of the key benefits of Wi-Fi networks is that there are so many already deployed. This is one of the most common reasons why Wi-Fi is chosen as a technology. Nonetheless, remember that the network was probably deployed with humans in mind, so coverage might not exist where your device needs it. Hence, it is a good idea to perform a site survey to understand the range of the system.
- Interoperability starts and ends with SECURITY. Since the network you are using is most likely already deployed, it also already has a security policy. The family of Wi-Firelated standards has a myriad of different encryption and authentication methods—and as the new device on the network, it is your responsibility to conform to that policy. This means choosing an embedded Wi-Fi supplier that has implemented the full range of security options.
- Choose embedded partners wisely. While this relates to security, it also means that you want to make sure that the radio design will be available for the long term. Wi-Fi manufacturers tend to cater to the mass of consumers carrying notebook computers and smart devices. Hence chipsets vendors turn over quickly looking for the lowest cost solution—which may not be appropriate for a long-term commercial or industrial deployment. Be wary of consumer radios that will trigger embedded driver upgrades. The cheap radio may not actually be low-cost in the end.
ZigBee & Related PANs
- Assess the environment. Generally you want to use a PAN when no other infrastructure is present or when you have low-power consumption requirements. Then ask what does the spectrum look like? If it is noisy, a frequency-hopping or frequency-agile solution is probably necessary. Do you have power?
- Test before you deploy. Even though you might be deploying an ad hoc network on the fly, you still need to identify the RF weak spots and single points of failure. This includes assessing the environment at different times of day.
- Practical interoperability over the air. What level of interoperability among different devices is required? Interoperability means more than just conforming to the standard. For example, two 802.15.4 or two ZigBee-based applications may not be able to talk to each other, even if they comply with the standard. A standard only goes so far and almost never guarantees application compatibility—rather, they are meant to define the level of interoperability that can be expected. Application context and provisioning are really important for a practical sensor system
Putting the Pieces Together
A practical wireless system may involve wireless components at the WAN, LAN or PAN level. The critical components in architecting a well-designed system must be selected with understanding and awareness regarding the flow of data and the type of service required.
Hence, in closing out this article, I leave you with both a selection of critical questions and decisions as well as a set of guiding principles that should help ensure success.
Looking at four areas can best help define the ideal solution that mixes multiple environments.
- Define how you want the application to work. In this context, it is important to assess the end-to-end functionality, level of service, location of intelligence and level of security. The question of functionality usually relates to whether the application is for logging, control, alarming or potentially all of them. Level of service involves whether the data is mission-critical or best-attempt. This then helps drive where intelligence should be placed. As a general rule, intelligence can be placed at the device, at an intermediate point or at the enterprise—and it is generally unwise to put intelligence everywhere and mission-critical communication almost always requires intelligence at the end device. Security then should be overlaid by evaluating what happens if the system is compromised from both an access and eavesdropping perspective.
- Infrastructure—what’s already in place? Evaluate the environment for what already exists. This includes whether there are any opportunities to use existing cabled communications and local power as well as the availability of wireless infrastructure like Wi-Fi and cellular signal strength.
- Fill in the gaps. With the application needs determined and the available infrastructure evaluated, it is time to complete the puzzle. This now involves doing an environmental assessment, site survey and cost trade-offs for different deployment options.
- Look beyond the pilot. Deploying a mixed wireless system can be complex on the environment, but also on the deployment. As such, it is important to answer the final questions of how will the system be deployed over many sites? Is it scalable? Is it maintainable? Often times it is easy to get an initial system in place through brute force, but fall down when duplicating it or maintaining it.
With this in mind, the following are guiding principles that should help with the system design and deployment, giving you the highest probability of success.
- Focus energy on high-payback things first. Don’t try to solve world hunger from the start or you will have an unwieldy system to trouble-shoot. Nonetheless, you must do this with an eye for potential future expansion.
- Focus intelligence at a common level. Sometimes there is a natural tendency to build intelligence into the system at every level under the belief that it makes a more robust system. Unfortunately, custom logic and filtering at too many places makes troubleshooting difficult—especially when using multiple wireless technologies. Place decisionmaking where it is most efficient.
- Minimize the number of vendors and/or different technologies. We all dream of a beautiful, efficient, multi-vendor environment. This is often quoted as the true benefit of standards. It is important to remember that standards give you multiple sources, but standards don’t mean you must mix and match. Too much mixing and matching obviates your suppliers from responsibility because they can always point to the other supplier.
- Match the network to the criticality of communications. Don’t try to over-engineer the system. If your system communications aren’t mission-critical, don’t try to make it that way. It will add cost and complexity in the end.
- If you have a cable, use it! Wireless technologies are wonderful things, but a short cable always works better.
Notice that I said these guiding principals will give you the “highest probability” of success, because with wireless, there always seems to be some magic involved.
Joel Young, vice president of research and development, CTO at Digi International, has more than 22 years of experience in developing and managing data and voice communications. He joined Digi International as vice president of engineering in June 2000.