Over the last few weeks I have seen numerous announcements of companies that plan to deliver gigabit wireless speeds using unlicensed spectrum. For example, RST announced plans to deliver gigabit wireless all over the state of North Carolina. Vivant announced plans to do the same in Utah. And I just scratch my head at these claims.
These networks plan to use the 5 GHz portion of the unlicensed spectrum that we have all come to collectively call WiFi. And these firms will be using equipment that meets the new WiFi standard of 802.11ac. That technology has the very unfortunate common name of gigabit WiFi, surely coined by some marketing guru. I say unfortunate, because in real life it isn’t going to deliver speeds anywhere near to a gigabit. There are two ways to deploy this technology to multiple customers, either through hotspots like they have at Starbucks or on a point-to-multipoint basis. Let’s look at the actual performance of 802.11ac in these two cases.
There is no doubt that an 802.11ac WiFi hotspot is going to perform better than the current hotspots that use 802.11n. But how much better in reality? A number of manufacturers have tested the new technology in a busy environment, and with multiple users the new 80211.ac looks to be between 50% and 100% better than the older 802.11n standard. That is impressive, but that is nowhere near to gigabit speeds.
But let’s look deeper at the technology. One of the biggest improvements in the technology is that the transmitters can bond multiple WiFi channels to make one data path up to one 160 MHz channel. The downside to this is that there are only five channels in the 5 GHz range and so only a tiny handful of devices can use that much spectrum at the same time. When there are multiple users the channel size automatically steps down until it ends up at the same 40 MHz channels as 802.11n.
The most important characteristic of 5 GHz in this application is how fast the spectrum dies with distance. In a recent test with a Galaxy S4 smartphone, the phone could get 238 Mbps at 15 feet, 193 Mbps at 75 feet, 154 Mbps at 150 feet and very little at 300 feet. This makes the spectrum ideal for inside applications, but an outdoor hotspot isn’t going to carry very far.
So why do they call this gigabit WiFi if the speeds above are all that you can get? The answer is that the hotspot technology can include something called beamforming and can combine multiple data paths to a device (assuming that the device has multiple receiving antennas). In theory one 160 MHz channel can deliver 433 Mbps. However, in the real world there are overheads in the data path and about the fastest speed that has been achieved in a lab is about 310 Mbps. Combine three of those (the most that can be combine), and a device that is right next to the hotspot could get 900 Mbps. But again, the speeds listed above for the Galaxy S4 test are more representative of the speeds that can be obtained in a relatively empty environment. Put a bunch of users in the rooms and the speeds drop from there.
But when companies talk about delivering rural wireless they are not talking about hotspots, but about point-to-multipoint networks. How does this spectrum do on those networks? When designing a point-to-point network the engineer has two choices. They can open up the spectrum to deliver the most bandwidth possible. But if you do that, then the point-to-multipoint network won’t do any better than the hotspot. Or, through techniques known as wave shaping, they can design the whole system to maximize the bandwidth at the furthest point in the network. In the case of 5 GHz, about the best that can be achieved is to deliver just under 40 Mbps to 3 miles. You can get a larger throughput if you shorten that to one or two miles, but anybody who builds a tower wants to go as far as they can reach, and so 3 miles is the likely networks that will be built.
However, once you engineer for the furthest point, that is then the same amount of bandwidth that can be delivered anywhere, even right next to the transmitter. Further, that 40 Mbps is total bandwidth and that has to be divided into an upload and download path. This makes a product like 35 Mbps download and 5 Mbps upload a possibility for rural areas.
If this is brought to an area that has no broadband it is a pretty awesome product. But this is nowhere near the bandwidth that can be delivered with fiber, or even with cable modems. It’s a nice rural solution, but one that is going to feel really tiny five years from now when homes are looking for 100 Mbps speeds at a minimum.
So it’s unfortunate that these companies are touting gigabit wireless. This technology only has this name because it’s theoretically possible in a lab environment to get that much output to one device. But it creates a really terrible public expectation to talk about selling gigabit wireless and then delivering 35 Mbps, or 1/28th of a gigabit.