One of the benefits of upgrading to MacOS 10.12 Sierra is the ability to unlock my Mac laptop with my Apple Watch. Yet I’m not able to do that. Why? Turns out, the answer involves some pretty cool tech.

Somebody’s Watching You

The tech specs list the 2013 MacBook and higher as the minimum model needed to enable Watch Unlock on your Mac. You also need a few other things, like Bluetooth enabled and a Watch running WatchOS 3. I checked my personal MacBook against the original specs and found everything in order. I installed Sierra and updated all my other devices and even enabled iCloud Two-Factor Authentication to be sure. Yet, when I checked the Security and Privacy section, I didn’t see the checkbox for the Watch Unlock to be enabled. What gives?

It turns out that Apple quietly modified the minimum specs during the Sierra beta period. Instead of early 2013 MacBooks being support, the shift moved support to mid-2013 MacBooks instead. I checked the spec sheets and mine is almost identical. The RAM, drive, and other features are the same. Why does Watch Unlock work on those Macs and not mine? The answer, it appears, is wireless.

Now AC The Light

The mid-2013 MacBook introduced Apple’s first 802.11ac wireless chipset. That was the major reason to upgrade over the earlier models. The Airport Extreme also supported 11ac starting in mid-2013 to increase speeds to more than 500Mbps transfer rates, or Wave 1 speeds.

While the majority of the communication that the Apple Watch uses with your phone and your MacBook is via Bluetooth, it’s not the only way it communicates. The Apple Watch has a built-in wireless radio as well. It’s a 2.4GHz b/g/n radio. Normally, the 11ac card on the MacBook can’t talk to the Watch directly because of the frequency mismatch. But the 11ac card in the 2013 MacBook enables a different protocol that is the basis for the unlocking feature.

802.11v has been used for a while as a fast roaming feature for mobile devices. Support for it has been spotty before wider adoption of 802.11ac Wave 1 access points. 802.11v allows client devices to exchange information about network topology. 11v also allows for clients to measure network latency information by timing the arrival of packets. That means that a client can ping an access point or another client and get a precise timestamp of the arrival of that packet. This can be used for a variety of things, most commonly location services.

Time Is On Your Side

The 802.11v timestamp has been proposed to be used as a “time of flight” calculation all the back since 2008. Apple has decided to use Time of Flight as a security mechanism for the Watch Unlock feature. Rather than just assume that the Watch is in range because it’s communicating over Bluetooth, Apple wanted to increase the security of the Watch/Mac connection. When the Mac detects that the Watch is within 3 meters of the Mac it is connected to via Handoff it is in the right range to trigger an unlock. This is where the 11ac card works magic.

When the Watch sends a Bluetooth signal to trigger the unlock, the Mac sends an additional 802.11v request to the watch via wireless. This request is then timed for arrival. Since the Mac knows the watch has to be within 3 meters, the timestamp on the packet has a very tight tolerance for delay. If the delay is within the acceptable parameters, the Watch unlock request is approved and your Mac is unlocked. If there is more than the acceptable deviation, such as when used via a Bluetooth repeater or some other kind of nefarious mechanism, the unlock request will fail because the system realizes the Watch is outside the “safe” zone for unlocking the Mac.

Why does the Mac require an 802.11ac card for 802.11v support? The simple answer is because the Broadcom BCM43xx card in the early 2013 MacBooks and before doesn’t support the 802.11v time stamp field (page 5). Without support for the timestamp field, the 802.11v Time of Flight packet won’t work. The newer Broadcom 802.11ac compliant BCM43xx card in the mid-2013 MacBooks does support the time stamp field, thus allowing the security measure to work.

Tom’s Take

All cool tech needs a minimum supported level. No one could have guess 3-4 years ago that Apple would need support for 802.11v time stamp fields in their laptop Airport cards. So when they finally implemented it in mid-2013 with the 802.11ac refresh, they created a boundary for support for a feature on a device that was in the early development stages. Am I disappointed that my Mac doesn’t support watch unlock? Yes. But I also understand why now that I’ve done the research. Unforeseen consequences of adoption decisions really can reach far into the future. But the technology that Apple is building into their security platform is cool no matter whether it’s support on my devices or not.