Apple, I Love Ya But You're Shady (Another iPad WiFi Complaint)

If you're reading this post, there's a 100% chance that you have heard about the Apple iPad. There's also at least a 50% chance that you have heard about WiFi problems on the Apple iPad. And there's about a 10% chance that you've heard about the deceptive way that Apple markets the iPad's WiFi capabilities. That's what this post is all about.


Apple's technical specifications for the iPad say that dual-band (2.4 GHz and 5 GHz) 802.11n is supported. And it is. But what Apple doesn't tell you is that the version of 802.11n built in to the iPad is such that you get almost no speed improvement over 802.11g or 802.11a.


To understand what's going on with the iPad's mediocre 802.11n design, you have to understand a little bit about what goes in to 802.11n. There are three 802.11n improvements that bump up the maximum data rate from the 54 Mbps we had with 802.11a/g: more efficient (5/6 instead of 3/4) OFDM coding, double-wide (40 MHz instead of 20 MHz) channels and MIMO antenna systems (for spatial multiplexing). (And yes, I realize that the short guard interval can also improve data rates, but let's pretend we live in a world where there is always a legacy a/b/g device sharing our BSS.) Let's talk about each of these things individually and whether the iPad supports them.


Let's start with ODFM coding improvements. Having more efficient OFDM coding allows the data rate to jump from 54 Mbps to 65 Mbps (assuming a single spatial stream, which I'll talk about later). The ratio in OFDM coding refers to the number of bits read as data over the number of bits transmitted in the code. Therefore, a higher coding ratio means that more transmitted bits will be read as data. In the case of 802.11n, we are getting 83% (5/6) of the number of our transmitted bits read as data rather than 75% (3/4). This is required in 802.11n. When you sniff the iPad's Probe Request frames (informational frames about the station's capabilities) it shows as supported and when you look at the iPad's data frame behavior it is supported as well. (That's good!)


Next is the improvements from having a wider channel. Having double-wide channels allows the data rate to jump from 65 Mbps to 135 Mbps (again, assuming one spatial stream). The normal 802.11a/g channel is 20 MHz wide and 802.11n allows the channel bandwidth to be 40 MHz instead. The way this affects data rates is that you double the rate and then add 5 Mbps for each spatial stream. The reason you get the extra five is that a double-wide channel makes more efficient use of the bandwidth because you only have two channel edges in that 40 MHz space rather than 4 (just think about it for a second, it'll make sense). This is not required in 802.11n and the iPad's Probe Request frames show it as unsupported. This causes the speed to be lowered, but could also cause the battery life to improve. I guess that's good, but I feel deceived. If you want to give me better battery life you can do it, but when I buy an 802.11n product I expect a maximum data rate of at least 300 Mbps. Without double-wide channels, the iPad has no chance of exceeding 130 Mbps.


The last major 802.11n improvement is having MIMO antenna systems for spatial multiplexing. Spatial multiplexing sounds complicated, but it really just means having double (or even triple or quadruple, depending on the device) the data sent over a single channel. It just means that your two (or even three or four, again depending on the device) antennas each send their own stream of data on the same channel at the same time. The way this affects data rates is that they double (or triple or qua-- ...you get where I'm going here). So the 135 Mbps I'd get from a device that also supports more efficient OFDM coding and double-wide channels would jump to 270 Mbps (or 405 Mbps, or 540 Mbps...). This part of 802.11n is required (at least for 2 streams; 3 and 4 streams are optional) and the Probe Requests from the iPad show that it is supported. BUT IT'S NOT ACTUALLY SUPPORTED! How dare you, Apple! You're cheating the 802.11n standard and, more importantly, you're cheating the 0.001% of your most brainwashed loyal customers who buy anything you release the day it goes on sale.


So what does all of this mean? The bottom line is that you are getting 65 Mbps 802.11n with the iPad, not 300 Mbps like with most Apple stations. 


Does this really matter? The reality is that it doesn't right now. As much as I hate to say it, I like what Apple is doing from a pure practical perspective. The reality is that you don't need more than a 65 Mbps data rate for any application the iPad runs. You would want the enhanced range of 802.11n (and the improved receive sensitivity of MIMO will help you with that), but you don't really need the higher speeds. So by killing spatial multiplexing Apple is robbing you of something you don't really need but giving you something you do (better battery life). 


At this point it may seem odd for me to call Apple shady for giving you a version of 802.11n that fits the iPad better than the plain ol' standard. I can see that. But I just wish they'd be a little bit more straightforward in their marketing. If you're going to give us a technically-standard-but-really-not-supporting-what-is-supposed-to-be-required-in-the-standard version of 802.11n, just tell us. Tell us that you're helping our battery life while still giving us adequate speeds for the apps we're gonna run and you'll get us nitpickers on your side.