The folks at iFixit, who disassemble and examine new hardware, found a Broadcom chip that uses single-stream 802.11n in the new, faster iPod touch models released last week. Does that mean the iPod touch is suddenly much zippier on the network? Not quite.
The 802.11n standard – finally ratified by the IEEE engineering group on 11-Sep-09 – sped up Wi-Fi network throughput by several factors. Apple started putting a relatively compatible version of 802.11n in all its new computers in October 2006, and in all its new router models in January 2007.
However, 802.11n hasn’t appeared in any major handheld devices, such as the iPhone or other smartphones – at least, until now. I checked a new iPod touch, and Apple’s AirPort Utility shows it connecting only via 802.11g on an 802.11n network. Apple may have plans to enable it through firmware and driver upgrades later. Back in October 2006, Apple started shipping 802.11n chips within Macs, but didn’t release an enabler until February 2007. It’s barely possible that Apple used the chip for cost and integration reasons, but has no plans to enable 802.11n.
The lack of 802.11n before now was partly due to some basic design principles. The flavor of 802.11n that’s in nearly every computer adapter and base station shipped to date uses at least two antennas, and has the equivalent of two separate radios inside for each of the two common unlicensed radio bands. (802.11n devices can use either 2.4 or 5 GHz, but aren’t required to use one over the other, nor support both. Newer base stations, including Apple’s, can broadcast simultaneously over both bands; such base stations have, in the simplest terms, four radios, two each devoted to each band.)
The radios and antennas work together in a system called “multiple in, multiple out” (MIMO) that takes advantages of wireless signal reflection. Each radio carries a unique stream of data, and different power levels sent to each antenna steer the stream’s beam so that a receiver can separately distinguish and decode both streams.
Fitting two radios, two or more antennas, and the necessary chips into a handheld is pretty much impossible. That led to the development of single-stream 802.11n, which uses one radio stream and one antenna. Single-stream has the advantage of faster encoding than its predecessor, 802.11g, giving it a baseline improvement in speed.
Chipmakers also worked to drop the power requirement. Single-stream 802.11n is likely more efficient than 802.11g in battery use, in fact. The Broadcom chip integrates an FM receiver and a Bluetooth radio, which also contributes to a reduction in battery use. (Oddly, the Wi-Fi-less iPod nano is the model that publicly gained an FM tuner, while the iPod touch hasn’t enabled that function on its chip.)
The better battery usage means that a handheld can use 802.11n, and putting 802.11n into a mobile device lets a unit like the iPod touch send and receive 50 percent more data in the same period of time – maybe a net throughput of roughly 30 Mbps instead of 20 Mbps. (For far more technical detail, read my Wi-Fi Networking News article, “Does the iPhone Need 802.11n?” I wasn’t prescient; there was a lot of chatter early this year about single-stream chips, and I thought what turned out to be the iPhone 3GS was a likely first use.)
That higher speed would make it possible to stream movies or transfer data at far higher rates, but would have little practical impact on routine activities. This might be the precursor to iTunes over-the-air sync (via something like the new Home Sharing feature) for media with the iPhone and iPod touch.
But there’s also a “good neighbor” part of single-stream N that improves network efficiency, and makes, say, other video streaming or transfers on the network perform better.
Single-stream 802.11n can be bad for a network, because each packet transferred takes the space of at least two multiple-radio packets. However, there’s a clever way around that called “space-time block coding” (STBC).
Without getting into any of the gory details, STBC lets a base station transmit separate data streams to single-stream devices, one per radio in the base station. Devices with a single-stream chip are more likely to consume data than produce it (most of the time, at least), and thus this effectively restores network throughput instead of halving it when multiple single-radio 802.11n devices are using the network. (STBC isn’t yet built into Apple’s base stations, but it could be as simple as a firmware update.)
The other advantage of Broadcom’s chip is that it allows the use of either the 2.4 or 5 GHz bands. While 2.4 GHz has greater range, it’s far more crowded, and you’re less likely to get anywhere near the full possible throughput with even 802.11g.
Signals sent at 5 GHz can travel shorter distances (using the same signal power), but because of a dramatically lower level of use and more available spectrum, the odds are much better you’ll get something close to the highest possible throughput.
Chipmakers have been pushing single-stream 802.11n to sell new chips at higher profit margins, of course, but also because they want to encourage users to buy newer base stations!
But Apple’s move may spell a greater interest in moving media on and off the iPod touch and (possibly shortly) the iPhone 3GS. Far higher throughput makes it much easier for an iPod touch to act as a media outpost on a local network.