Mixing multiple wireless technologies - Part 1

More and more we find ourselves living in a wireless world. From the consumer market perspective, most of us have a mobile phone — even our children. Most of us have Wi-Fi in our house, and when we do use a landline telephone, often enough it’s a wireless handset.

As a part of this, as consumers, our behaviour 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 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

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 usage outside of the consumer space.

Figure 1: Wireless technologies

W-WAN: Cellular and Wi-Max

We start with wireless wide area networks or W-WANs. Figure 2 illustrates a view of both the history and expected future of these technologies. For those purists — please don’t get too caught up in the dates as they are intended to represent a sort of worldwide average. The colours indicate the technology evolution and the throughput numbers are meant to illustrate best case. For comparison, Wi-Fi has also been added — even though it is really a LAN not WAN technology. It should also be added that CDMA cellular networks are no longer available in Australia.

Figure 2: Wireless WAN technology paths

While this article is not intended to be a W-WAN tutorial, it is difficult to discuss the trade-offs without providing some brief definitions of the technology at hand.

TDMA (time division multiple access)

This is the technology used in the original GSM and the original digital cellular networks. With TDMA, a single frequency band (or channel) is shared by splitting it into time slots. Effectively, it provides equal time for all, as long as there are time slots available. It is simple and easy to manage; however, it doesn’t tend to scale well for bursty, high throughput data applications. 2G and 2.5G cellular GSM systems use TDMA for GPRS and EDGE data services.

CDMA (code division multiple access)

By virtue of the same name, this is the technology used in the ‘CDMA’-style networks. With CDMA, the signal is spread across multiple frequencies using a pseudo random code. If you don’t know the code, the signal looks like noise. The amount of spread is related to the amount of data to be sent, so it scales to bandwidth demands. For this reason, high-speed data on GSM networks (UMTS) uses CDMA technology. 2.5G cellular CDMA systems use a scheme called 1XRTT. With the advent of 3G systems, GSM-based systems follow the UMTS/HSDPA track, while CDMA carriers follow the EV-DO track.

OFDMA (orthogonal frequency division multiple access)

This is the technology used for Wi-MAX and the other fourth generation cellular data systems. OFDMA’s cousin, OFDM, is used in higher speed Wi-Fi. With OFDMA, the signal is split to several narrow channels at different frequencies. Subcarriers are then assigned to individual users. It has the ability to offer bandwidth on demand and also offers immunity to multipath issues, enabling much higher data rates. OFDMA systems like Wi-MAX (802.16 family of standards) are completely optimised for data, which means they don’t carry any of the voice infrastructure overhead. In theory, this should make them more cost effective to deploy, more cost effective to certify and more open.

Key issues

Depending on the needs of the application, there are a number of key issues that must be looked at when choosing a W-WAN system. The first of which is to assess what kind of data throughput, latency and quality of service is needed. No matter what the technology, the higher the performance, the more you are going to pay. In addition, there are four other points worth remembering:

1. 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. Device-initiated connections are critical.
2. 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, it may be possible to get a 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 which allows pooling of data.
3. 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.
4. 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.

Wi-Fi

Wi-Fi most often refers to the 802.11 family of standards. For those of you that like letters, the range of standards stretches from 802.11 and 802.11a all the way past 802.11s. Most of us have become familiar with the letters b, g, a and n, with the more security savvy of you also knowing the letter i. Wi-Fi is used mostly as a WLAN (Wireless LAN), but also is used for some WAN-type access. Note that the ‘WAN’ environments are really LANs masquerading as a WAN, without any of the channel access quality of service attributes that you get from the aforementioned W-WAN technology. This is because, at its heart, Wi-Fi is a shared bandwidth system where access is handled using a collision avoidance system (CSMA-CA) akin to an intersection without traffic lights or stop signs — it works well when there isn’t much traffic.

Figure 3: Wi-Fi star topology

The typical architecture for a Wi-Fi system is a star topology where users associate with things called access points. Mobility is allowed between two access points — but often becomes problematic if the access points are on two different subnets. Probably the most important thing to note about Wi-Fi is manufacturers tend to cater to the mass of consumers carrying notebook computers and PDAs. 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. In addition, the other important points to note about Wi-Fi are that the system is not intended or well suited for low-power consumption and that you must be prepared to match the security policy of your environment.

Key issues

Given the items mentioned above, the following are key issues that should be considered when choosing Wi-Fi for commercial and industrial applications.

1. 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.
2. 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-Fi-related 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.
3. 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. Be wary of consumer radios that will trigger embedded driver upgrades. The cheap radio may not actually be low cost in the end.

In Part 2

In the second part of this article, we will look at personal area networks (PANs) such as ZigBee, and put the pieces together with some guiding principles on selecting the right technologies for your application.

*Joel K Young is senior vice-president and CTO of Digi International Inc.

T Data Pty Ltd
www.tdata.com.au