Apple is ahead of the curve when it comes to adopting the IEEE 802.11ac wireless networking standard. While 802.11ac routers are not new, Apple's latest AirPort Time Capsule is the first to include an integrated hard drive. Apple is also one of the first manufacturers to include an 802.11ac client adapter as standard equipment in its computer lineup.

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When I used AccessAgility's WiFiPerf benchmarking tool to measure the 802.11ac Time Capsule, with a 2013 MacBook Air as the client, I measured a very respectable TCP throughput rate of 451.9 mbps at close range (with the router and client in the same room, separated by about nine feet). That's more than twice as fast as the older 802.11n Time Capsule's 218.7 mbps (operating a network on the 5GHz frequency band).

What's more, the 802.11ac Time Capsule delivered TCP throughput in excess of 100 mbps in two other rooms inside my house where the 5GHz 802.11n Time Capsule wasn't able to maintain any connection to the MacBook Air.

Unfortunately, something is crimping Apple's 802.11ac network performance. As I reported in this earlier story, Apple's gear is radically slower when tasked with transferring real-world files over the network (WiFiPerf is a synthetic benchmark for measuring TCP throughput).

When I copied a 10GB collection of files and folders (videos, music files, word documents, spreadsheets, and the like) from the hard drive in an iMac hardwired to the Time Capsule to the SSD in a wirelessly networked MacBook Air (which was again about nine feet from the router), I measured throughput of just 84.8 mbps, meaning the transfer required more than 16 minutes to complete (each test was performed three times and the results were then averaged). Reading those files from the MacBook Air and writing them to the iMac happened at a slightly faster pace: 132.1 mbps (nearly 10.5 minutes).

The Time Capsule's network performance improved only a little when copying a single 10GB file from the iMac to the MacBook Air, to 134 mbps (averaging 10 minutes, 13 seconds). Copying that same file back to the MacBook Air: 163.5 mbps (about 8.5 minutes). Obviously, none of those figures are anywhere close to WiFiPerf's result of more than 450 mbps.

Based on my experience testing 802.11ac routers with Windows machines, I expected the 802.11ac Time Capsule to be considerably faster than the 802.11n model. And that's certainly what my WiFiPerf results indicate. But these real-world file transfers defy that expectation.

The current theory, first published at Anandtech, is that OS X is not properly scaling TCP window size during file transfers to allow the MacBook Air's 802.11ac client adapter to achieve peak performance. While Apple has not confirmed this to be the problem, the company is aware of the performance discrepancy and is reportedly working on a solution.

Backward compatibility

In addition to operating a 5GHz network based on the 802.11ac standard, the Time Capsule can also operate a 2.4GHz network to support older 802.11b-, -g, and -n clients, as well as a 5GHz network to support 802.11a and 802.11n clients. This is important, as the new MacBook Air is one of the few devices to support 802.11ac. Most computers, smartphones, media streamers, and other networkable devices rely on the older standards.

Using WiFiPerf once again, I measured TCP throughput with a mid-2011 13-inch MacBook Pro connected first to the older 802.11n Time Capsule (at 5GHz) and then to the new 802.11ac Time Capsule. TCP throughput was only a little higher at close range--301.7 mbps with the 802.11ac Time Capsule versus 285.3 mbps with the 802.11n model--but the new router was dramatically faster at longer distance: 67.7 mbps compared to just 32.3 mbps with the client separated from the router by 65 feet and three insulated interior walls.

Apple's 802.11ac Time Capsule delivers great performance with an 802.11n MacBook Pro client, especially at range. (Click to enlarge chart.)

Once Apple fixes the file-transfer problem, upgrading to the 802.11ac Time Capsule should also deliver better network performance with older Macs outfitted with 802.11n adapters.

Feature set

I typically cover a product's feature set at the beginning of the review, but in this case I thought it more important to discuss Apple's file-transfer issues first. As I've already mentioned, the 802.11ac Time Capsule is a dual-band model, capable of operating networks on both the 2.4GHz and the 5GHz frequency bands. It's equipped with six antennas: three transmit, three receive for its 2.4GHz radio, and three transmit, three receive for its 5GHz radio. The antennas are mounted near the top of the 6.6-inch-tall columnar device, which likely helps it achieve longer range. It supports three 433.3 mbps spatial streams for a maximum physical link rate of 1.3 Gbps. (The 802.11ac adapter in the new MacBook Air supports two spatial streams for a maximum physical link rate of 867 mbps when connecting to an 802.11ac router.)

The new Time Capsule also supports an optional feature of the 802.11ac standard known as beam forming. With this technology, the router and each of its clients exchange information as to their physical locations. They use this information to concentrate their radio energy to achieve the highest possible throughput.

The Time Capsule is currently available with either a 2TB hard drive or a 3TB drive. Both models are equipped with an accelerometer that will park the drive's read/write heads if the router is dropped or tipped over (older Time Capsules do not have this feature). Unlike some other hard-drive-equipped routers, the Time Capsule uses a SATA interface to the hard drive, versus a USB-to-SATA bridge. Apple, however, declined to disclose the rotational speed of the drive's platters and if the drive has a SATA 6 Gbps interface or a slower SATA 3 Gbps interface.

On the back of the Time Capsule, you'll find a gigbit WAN port (for connecting to the Internet) and three gigabit LAN ports (for hardwired clients). There's also a single USB 2.0 port, to support either a shared printer or additional storage. You'll need to plug in a USB hub if you want both at the same time. The power supply is built into the enclosure, which is much better than having an outlet-hogging wall wart, but an inline power brick would have been a better alternative. Between the 802.11ac chipset, the hard drive, and the power supply, the Time Capsule needs to shed a lot of heat. Apple put a fan inside there to keep things cool, and you can definitely hear it spinning in a quiet room.

Bottom line

The solution to the file-transfer shortcomings I've covered here will most likely arrive in a future OS X update, I really don't think there's anything wrong with the 802.11ac Time Capsule's design. My WiFiPerf benchmarks indicate that this router is at least as fast as the best non-Apple 802.11ac routers I've tested.

If you've purchased a new MacBook Air, you'll get significantly better performance with the 802.11ac Time Capsule than you will with Apple's 802.11n Time Capsule. In my test environment, the new MacBook Air was unable to maintain a wireless connection to the router's 5GHz network at longer range. This was less of an issue on the 2.4GHz band, but that spectrum is so congested in more typical environments that you probably won't want to use it (I happen to live on a 10-acre parcel in a rural area relatively free from other wireless networks).

If you're using an older Mac client with an 802.11n Wi-Fi adapter, my benchmarks indicate the new Time Capsule will also deliver much higher performance--provided Apple fixes the file-sharing issue, that is. So if you've never purchased a Wi-Fi router, and you like the idea of automated local backups, the 802.11ac router is definitely a good buy.