Power use of bluetooth vs Wi-Fi vs wired audio on the iPhone 5

▼ With all this bluetooth audio going on, I wondered: how much power does playing back audio over bluetooth use? So I tested this. The tests then snowballed as I got curious about audio over Wi-Fi, or having bluetooth and/or Wi-Fi on when the phone is idle. All the tests were done on an iPhone 5 with a freshly replaced battery under iOS 7. The results:

• Playing audio through the headphone jack, airplane mode, background app refresh off. In 7:59 the battery went from 50% to 34%, 2% per hour.
• Playing audio through the headphone jack, Wi-Fi off, bluetooth off, background app refresh on. In 8:04 the battery went from 50% to 31%: 2.4% per hour.

So in airplane mode, you should be able to play audio for 50 hours and with 3G enabled but not Wi-Fi or bluetooth and no background app refresh to trigger CPU and network use, it's a bit over 40 hours. I guess this also means that just running the 3G radio uses 0.4% of the battery per hour, so the standby time of an untouched iPhone 5 would be 250 hours. (A week and a half!)

Now let's add some bluetooth and Wi-Fi:

• Playing audio through the headphone jack, Wi-Fi on, bluetooth on, background app refresh off. In 8 the battery went from 44% to 23%: 2.6% per hour.

With these settings, you'd be able to play audio for over 38 hours, three hours less than with Wi-Fi and bluetooth off. But what if we start actually playing audio over buetooth?

• Playing audio over bluetooth, Wi-Fi off, bluetooth on, background app refresh on. In 8 hours the battery went from 72% to 45%: 3.4% per hour.

Now we're down to 29 hours of audio playing. So if you really only use your iPhone to play audio and you need it to last as many hours as possible between charges, using the headphone jack certainly makes a big difference. But you'd have to charge your bluetooth headphones two or three times before the iPhone runs out of juice! In normal use, this is more than good enough to use bluetooth for audio when that's convenient and not worry about the iPhone's battery.

However, there's another way to stream audio from an iPhone: over Wi-Fi.

• Wi-Fi streaming, bluetooth on, background app refresh off. In 9:55 the battery went from 89% to 65%: 2.4% per hour.

So streaming audio over Wi-Fi uses the same amount of power as playing through the headphones. Who knew?

Last but not least, what's the impact of having Wi-Fi on on the standby time when the iPhone is idle?

• Standby, bluetooth on, wifi off, background app refress off. In 8:49 the battery went from 23% to 18%: 0.6% per hour.
• Standby, bluetooth on, wifi on, background app refress off. In 9:44 the battery went from 18% to 11%: 0.7% per hour.

So with just 3G and bluetooth on the standby time would be a week and with 3G, bluetooth and Wi-Fi on six days. In other words: having bluetooth on uses an extra 5% of your battery each day, and bluetooth + Wi-Fi 7% per day.

Please note that I did every test only once and the accuracy and precision of the battery charge indicator are limited, so this is just a rough estimate, don't be fooled by the fact that there are numbers behind a decimal point. But I think the overall conclusion that you can simply leave Wi-Fi and bluetooth on is a pretty safe one.

Permalink - posted 2014-10-21

Disabling logging in to a Mac with an Apple ID

▼ If you have more than on Mac, it can be very useful to mount another system's drives or share another system's screen. Obviously you have to provide a username and a password for this, or the entire internet would be able to get at your Mac once you enable these services in the Sharing pane of the System Preferences.

A few years ago, a new way to log in to another Mac appeared: with your Apple ID. This can be convenient, but it has a big issue: as Mat Honan learned, others may be able to get at your Apple ID using vulnerabilities at third parties and social engineering. And maybe Apple can be forced to release your Apple ID to various governments.

That's bad enough, but now under (Mac)OS X 10.10, it's no longer an option—you're automatically logged in to the other computer if your accounts on both computers are logged in to iCloud using the same account. Which means that there's no good way to log in using different accounts.

So I wanted to disable this behavior, but there's no obvious setting anywhere that governs it. The only way I was able to turn this off was to sign out of iCloud. I'm not a big iCloud user, but it has a few useful services, so this wasn't the solution I was looking for.

Turns out, there is a somewhat hidden way to do this. First, open Users & Groups in the System Preferences. You then probably have to click the lock and authenticate in order to make changes. Then right-click (or click holding the control key) on your user name in the list on the left, and select "advanced options". This brings up the following information:

After deleting the two entries in the list of aliases, I was no longer automatically logged in when connecting to this computer, and the option to log in using an Apple ID was no longer offered.

Permalink - posted 2014-10-20

Internet exchange renumbering: everything old is new again

▼ This week, the Amsterdam Internet Exchange is renumbering its peering LAN.

An internet exchange (IX) is simply a very big Ethernet. Members connect a router port to that Ethernet, and can then exchange packets with each other. When you want to exchange traffic with many other networks, obviously this is more efficient than setting up dedicated connections with all these other networks.

Until this week, AMS-IX used a /22 prefix, allowing for about a thousand connected routers. That was no longer enough, so they got a new /21 prefix, which can accommodate two thousand connected routers. This means that all the currently connected routers must get a new address. No big deal. This is why search-and-replace was invented.

However, sometimes someone makes a mistake. Like configuring <new address>/22 instead of <new address>/21. And then letting that /22 propagate to other networks over BGP. Suppose:

• 192.0.0.0/21 is the prefix used on the IX peering LAN
• 192.0.2.1 peers with 192.0.2.2
• 192.0.2.3 advertises 192.0.0.0/22
• 192.0.2.1 wants to send a BPG keepalive packet to 192.0.2.2. Normally this packet would go to the port where 192.0.0.0/21 is configured. But the 192.0.0.0/22 prefix is "more specific", so the packet is sent on its way to the source of the 192.0.0.0/22 prefix: router 192.0.2.3.
• 192.0.2.3 forwards the packet to router 192.0.2.2
• All's well that ends well? Not so fast. In BGP, there are not supposed to be any intermediate routers between two BGP routers. 192.0.2.2 sees the packet traversed and extra hop, rejects it and resets the BGP session.
• 192.0.2.1 can no longer send traffic to 192.0.2.2

(A more specific prefix is a smaller range of IP addresses. 192.0.0.0/21 is BGP talk for the address range 192.0.0.0 - 192.0.7.255. 192.0.0.0/22 is the range 192.0.0.0 - 192.0.3.255. Because the latter identifies a smaller range of IP addresses, the packets are sent in that direction, just like you'd follow a sign "Paris" rather than a sign "France" if you were going to Paris, even though Paris is part of France so presumably following the sign "France" would also get you to Paris.)

The sad thing is that the exact same thing happened in 2003, when the AMS-IX renumbered from a /24 to a /23. I always warn against this issue during my training courses, and tell students to filter the IX prefixes of internet exchanges they're connected to, as well as all possible subprefixes (more specifics) that fall within that IX prefix. For instance:

!
ip prefix-list import deny 172.16.0.0/12 le 32
ip prefix-list import deny 80.249.208.0/21 le 32
ip prefix-list import permit 0.0.0.0/0 le 24
!

This prefix list will reject incoming updates with your own prefix and all possible more specifics (assuming your prefix is 172.16.0.0/12) as well as the AMS-IX prefix 80.249.208.0/21 and all possible subprefixes. It then allows all prefixes with a prefix length of no more than /24, which is common practice for IPv4.

"le" means "less or equal" so "172.16.0.0/12 le 32" means:

172.16.0.0/12
172.16.0.0/13
172.24.0.0/13
...
172.31.255.254/31
172.31.255.254/32
172.31.255.255/32

Hopefully, by the time AMS-IX connects more than 2000 routers, the issue is moot because we no longer use IPv4. But for now: happy renumbering!

Permalink - posted 2014-10-15