Voice-over-IP is interesting but not in the way most people seem to think

Do you want to be a phone company?

It's a tempting thought: In most offices and many homes there are two networks, one for voice and one for data. Why not run voice over the data network? In a lot of places, including here in Minneapolis, it looks like that would be convenient and cheap. It also doesn't look very hard. Voice is easily digitized and compressed. Your cell phone does it and even land-line telephone companies routinely carry voice over Asynchronous Transfer Mode networks. (ATM networks are data networks where the packets are 53 bytes long and are called cells.) The highest data rate that's ever used for voice is 64 Kbps and that wouldn't make much of a dent in my DSL line's 256 Kbps upload speed. In practice you could use much a much lower rate.

So why aren't we all using phones that have Ethernet ports to make calls that don't cost us anything beyond our monthly fee for internet service? Well, actually some people are. And telephone companies are understandably somewhat concerned about losing voice revenue. But there's a little more to it than that. The problem is that we have different expectations for voice networks and data networks. If an email were delayed by three seconds, I wouldn't notice or care. But if my "Hello" were delayed by three seconds, it would be an awkward way to start a conversation. I'm no fan of giant telephone companies but at least part of the reason that voice is expensive and data is cheap (where that's true) comes from the different expectations we have for those networks.

In addition to expecting little latency when we send voice, we also expect our voice networks to be extremely reliable. I'm pretty sure that that annoying "five nines" thing started with telephone companies. If any telephone company actually achieves 99.999% reliability (that's about 5 1/4 minutes of downtime per year) by any meaningful calculation it's news to me. Still, I have to admit that they're much more reliable than even quite good data networks.

Beyond low latency and high reliability, voice networks also have lots of capacity. It's often the case that something causes lots of people to want to make telephone calls all at the same time. Whether it's a holiday or a disaster, it's very possible that I can have a particularly strong desire to make a telephone call just at the time that everyone else does. As Steven den Beste has observed, building capacity that goes unused almost all of the time is expensive, but the capacity of the voice network around here is great enough that I don't remember the last time I got a fast busy signal or an "all circuits busy" recording.

My ridiculously cheap data network can be laggy, it goes down a couple of times a year, and I routinely run into its capacity limits. And that's just fine. We use data networks in ways that mean we can tolerate those things. The interesting thing about voice-over-IP isn't that voice can be sent using some particular protocol over a cheap data network. Rather, it's how cheap you can keep that data network while providing what people want from a voice network. Here's hoping that's pretty cheap.

Update March 3, 2004
The Register reports on similar issues.

Update May 30, 2004
It seems that some other folks have come to the same conclusion from a different direction.

Posted: Sun - December 21, 2003 at 08:05   Main   Category: