Why My Doorbell Now Requires an Advanced Degree

Dear Tom,

My husband and I just replaced our old doorbell — the kind that just rings — with one of those camera doorbells everyone has now. But it doesn't ring anywhere. It just sends a notification to an app on my phone, and half the time I'm in the garden and don't hear it. Why can't it just ring like a normal doorbell? What is the app even doing?

— Carol from Boise, Idaho

Carol,

What a wonderful question, and I want to say up front: you are not alone in wondering this, and you are not wrong to wonder it, and by the end of this column you will understand — fully, not partially, fully — why your doorbell no longer rings, why it was never really "ringing" in any sense you'd recognize even when it appeared to, and why the app on your phone is, in point of fact, doing considerably more work than the entirety of your previous doorbell's electromechanical lifespan combined.

Let's begin, as we always must, with what a doorbell actually is. Your old doorbell was what's called a closed-circuit annunciator — of course — meaning the button didn't "ring the bell" so much as complete a circuit that permitted current to reach a solenoid, which is, of course, just a coil of wire that becomes magnetic when current runs through it, which then yanked a little clapper into a chime bar. One event, one physical consequence, one room. This is what engineers call a tightly coupled system, and tightly coupled systems have a wonderful property: they cannot lie to you. The bell rings because the button was pressed. There is no intermediary. There is, if you'll permit the term, no epistemology involved.

Your new doorbell is not a doorbell. I want to be gentle but clear about this: it is a distributed sensing-and-notification apparatus that happens to be shaped like a doorbell, out of respect for tradition, the way a modern car still has something that looks vaguely like a horse's harness point on the front. What happens when Carol's visitor presses that button is not "a bell rings." What happens is a small computer — because that is what's in there, Carol, a genuine computer, smaller than the joint of your thumb — wakes from a low-power state, checks its wifi association (we'll return to this), and initiates what's called a publish event to a message broker somewhere in a data center that is, in all likelihood, not in Idaho.

Now, of course, the reason it does this rather than simply ringing a chime in your hallway is the fundamental limitation of physical space, which I probably don't need to remind you of, but will: sound decays. A chime in your front hall has what acousticians call a coverage radius, and your garden — lovely as I'm sure it is — sits well outside it. The old doorbell wasn't ringing "for the house." It was ringing for whoever happened to be within roughly thirty feet of it with functioning hearing and no vacuum cleaner running. Your new system solves this by not making a sound at all in the traditional sense, and instead treating "someone is at the door" as a piece of state that must be synchronized across every device you own, wherever you happen to be standing on the earth. This is, if you think about it — and I have thought about it a great deal — a categorically larger promise than the old doorbell ever made you. The old doorbell promised to ring. The new one promises to find you. Naturally that requires more machinery.

This is where the app comes in, and I want to walk through what it's actually doing, because "notification" undersells it badly. When the camera unit publishes its event, that event travels — assuming, of course, standard suburban wifi topology, a single consumer-grade router with no VLAN segmentation, which I'll assume for you, Carol, though I recognize I haven't seen your network diagram — out through your router, through your internet service provider's aggregation point, up to the manufacturer's servers, where it is checked against something called your device token, which is a unique cryptographic handle that says, in effect, "this specific physical phone, and no other phone on earth, should be told about this." That token then gets handed to Apple's or Google's push notification service — an entirely separate company's infrastructure, mind you, layered atop the doorbell company's infrastructure — which maintains a persistent open connection to your phone at all times, even when you are gardening, even when the phone is asleep, so that it can deliver a tiny packet that essentially whispers "wake up, something happened." Your phone then wakes, checks the packet, and decides — locally, on-device — whether to buzz, chime, or light the screen, depending on your notification settings, which I trust you've reviewed (Figure 4 in my forthcoming piece on notification taxonomies covers this in full, though I confess that piece is not yet written and Figure 4 does not currently exist).

What I find genuinely delightful about this, Carol, is that your button press has now become what's called, in distributed systems theory, an eventually consistent fact. It is not instantaneous — nothing broadcast across four separate corporate networks and two continents' worth of server infrastructure can be truly instantaneous, whatever the marketing says — it is merely fast enough that the half-second of relativistic and queuing delay is imperceptible to a human nervous system, which, I should note, has its own signal latency of roughly 100 to 150 milliseconds just getting the "someone's at the door" concept from your eardrum to your conscious awareness in the first place. So really, Carol, even your old doorbell was never instantaneous. You simply weren't equipped to notice the lag, in the same way a goldfish isn't equipped to notice the curvature of its bowl.

I'd be remiss not to mention the camera itself, briefly, since I know I promised to explain the whole system and not merely the notification half of it. The image sensor in that unit is capturing somewhere in the neighborhood of thirty frames per second, compressing each one using a video codec that throws away information your eye wouldn't miss anyway — a wonderfully lossy, almost philosophical process, discarding reality in order to represent it faster — and streaming the result up to the cloud so that when you eventually open the app, what you're seeing isn't live at all, strictly speaking, but a very recent recording of the past, reconstructed frame by frame on your screen. You are not looking through a window. You are looking at a very good rumor.

So to directly answer your question: it doesn't ring like a normal doorbell because it isn't attempting to be a normal doorbell — it's attempting to be present everywhere you are, which is a fundamentally different and much harder engineering problem, one that trades the old system's total reliability within a thirty-foot radius for a new system's partial reliability across the entire surface of the planet, contingent on wifi association, token validity, push service uptime, and cellular handoff, should you wander far enough from your router (see my note above regarding VLAN segmentation, which I raise again only because it continues to trouble me on your behalf).

I hope this has cleared things up thoroughly, Carol — not partially, thoroughly — and that the next time the app buzzes in the garden, you'll feel the full weight of the apparatus that made that buzz possible, rather than simply reaching for your phone as though it were nothing.

Glad I could clear that up!

— Tom