Choosing Wireless Connectivity: Part 2

This is the second article in a series. The previous article gives an overview of the factors you should consider when choosing connectivity for your IoT device. This article talks about how network technologies differ in four of those factors: bandwidth, latency, power, and range.


HSPA, LTE Cat 1, LTE Cat M, NB-IoT, Wi-Fi, Zigbee 802.15.4, Zwave, Sigfox, LoRa, RPMA, Weightless, and BT-LE are just a few wireless technologies vying for your IoT device, and this doesn't even include satellite networks. Are all these networks really going to survive? Why can't IoT adhere to the "Highlander Principle" (there can be only one)? To answer these questions, we need to learn about a theory developed during World War II.


Way back in the 1940's, Claude Shannon developed "Information Theory" that describes (among other things) how bandwidth, latency, power, and range are interrelated. Essentially, portions of wireless spectrum an be visualized as pies, where different size chunks of spectrum correlate to different size pies. Wi-Fi, Bluetooth, and Zigbee all work with one pie (2.4GHz ISM band). Sigfox and LoRa work with another pie (900 MHz ISM band). Cellular networks work with still other pies (licensed bands in different parts of the spectrum).


A pie can be sliced into four pieces that represent bandwidth (the amount of data you can send over a period of time), latency (the time it takes for a bit of data to go from origin to destination), power (how slowly the communication link drains the battery), and range (the maximum transmit distance). If you cut yourself a very big bandwidth slice to enable high-speed data, your other three slices need to be smaller because the pie size cannot change. Wi-Fi has very big bandwidth and latency slices (it's fast and responsive), but very small power and range slices (it uses a ton of power and you have to be within 10 meters of a Wi-Fi base station). Sigfox has very small bandwidth and latency slices (you can only send 1700 bytes of data a day and each message takes multiple seconds to reach the destination), but very large power and range slices (it can transmit up to 20 miles with very low battery drain).  


This is an oversimplification of Information Theory, but the underlying message is this- when choosing a wireless technology (or designing one) you cannot have it all. You must make choices depending on what your application needs most. One disclaimer before getting into the details- all these comparison tables are very rough so use them for comparison purposes only. For example, speed is affected by network load and interference in unlicensed spectrum. Average transmit (tx) power is often affected by distance to the base station, and you have to factor in transmit time as well when calculating battery life. Range is affected by walls, trees, hills, etc. Latency is affected by interference and network load. In other words, your mileage will absolutely vary. Let's take a closer look at the options.


Cellular


The first widespread IoT connectivity solution (with apologies to ARDIS and RAM) was GPRS, or "2G" to most people. It's still used widely in the world today, although in the US its time is over. 3G and 4G LTE are also cellular technologies. There are different speed options (called "categories") in LTE. Increasing the LTE category (from Cat 1 to Cat 5 and up) gives you faster speeds and higher power usage. 5G is coming sooner than you think, and it too includes high-speed versions as well as a low-speed/low-power version called NB-IoT.


Cellular connectivity is extremely reliable where it's available (coverage is a more important factor than range for public networks) because it operates in licensed frequency bands where there is predictable interference. The downside is it costs money to send data and it's fairly power hungry when compared to Zigbee and LPWAN.













Local Area Networks


Wi-Fi, Bluetooth, and Zigbee are well-known consumer connectivity brands for building local area networks (LANs), but can be used in industrial settings as well. Wi-Fi is usually the first-timer's choice for wireless connectivity because it's familiar and it's free (from a data plan perspective). It is also very fast with extremely low latencies. As a result, it also uses a lot of power (almost a watt) and it is short range (<100m). Bluetooth and Zigbee (802.15.4) both trade off data rate for lower power when compared to Wi-Fi. Bluetooth Low Energy (BLE) and Zigbee both consume < 100 mW when transmitting but have data rates < 1 Mbps. All these LAN standards require a gateway for access to the Internet. In the home the gateway is your cable modem/router. In the field you may need a cellular or satellite gateway, which means the data is no longer free. All these LAN standards operate in unlicensed bands such as 2.4 GHz so are highly subject to interference, which will degrade performance and quality when there are too many radios in the same area (e.g.- Wi-Fi at CES).















LPWAN


Loosely speaking, cellular is a "public network" technology, meaning your devices connect directly to a carrier like Verizon, whereas LAN standards are mostly used in "private" networks, meaning you own the network and control which devices access the network. LANs are short-range private networks. For industrial applications that require longer distances between devices, such as oil fields, communication engineers usually resort to using proprietary protocols in a private network to achieve both low power and long range. As with LANs, these private networks need at least one gateway for the backhaul (Ethernet, cellular, or satellite) to the Internet. The exhibition floors of conferences such as Mobile World Congress and Distributech are littered with companies that market a variety of solutions, from Zigbee modifications to completely custom proprietary protocols for licensed bands, that can be used to construct long-range private networks. The downside to proprietary private networks is you're tied to a single-vendor solution and you need to design and maintain your own network (significant resources).


Low Power Wide Area Networks (LPWAN) such as Sigfox and LoRa are essentially long-range versions of Zigbee and Bluetooth. They take the low-power/long-range benefits of private networks and improve them in two ways. First, they "standardize" the wireless link between the devices so there are multiple vendors to choose from. LoRa is the most available choice for a standards-based long-range private network. Second, the public LPWAN networks take care of the network design for you. Instead of rolling out a private network, you connect your devices to a public LPWAN service provider and just pay for the data. If there's no coverage in your area, you can still fall back to installing your own private network or you can work with a company like Sigfox to extend coverage (Sigfox cannot be used in a private network). LPWANs have asymmetric channels, meaning there is much more bandwidth on the uplink than the downlink. For example, Sigfox gives you 140 uplink messages a day, but only 4 downlink messages. Public LPWAN networks are quickly rolling out coverage worldwide and the traditional cellular carriers are taking notice.  


Some notes on the below chart. Pure LoRa is just a physical transmission technology and you need a protocol stack to run on top of it. LoRa public networks like Senet use LoRaWAN as the stack. Other companies have taken the LoRa physical layer and written their own proprietary stack. Our comparison below is for LoRaWAN. These are only three of the LPWAN options- there are many more.















Satellite


There a huge amount of variability between satellite networks. We'll go into more detail about satellite in a future article, but you should be aware that IoT satellite networks such as Iridium SBD have long latencies, high transmit power, low speed, and expensive data plans. However, if you're installing in the middle of nowhere it could be the only option you have.


Summary


There is no free lunch with wireless technologies. You cannot have high data rate, long range, low latency, and low power all at once. Hopefully you've designed your application so that you have a wide range of options at your disposal.  


By Alfred Tom on April 16, 2017