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by Andrew Welch

Latency Sucks

To really appreciate what the Internet has to offer in a satisfying manner, a traditional analog modem just doesn't cut it, however. I remember back, while growing up, I had a 300bps modem. While it may seem archaic by today's standards (because it is), it made me extremely aware, at a rather young age, that there was a whole big world out there beyond the borders of my rural Connecticut home. I didn't know that the Internet would explode in popularity the way it has, but I was aware, very faintly, like a growing shadow, that something big was going to happen.

The thing is, though, that modems are absolutely horrid devices for accessing the content-rich Internet that is so prevalent today. Your computer is digital; the lattice of phone lines that sprawl across our country, and around the world, are for the most part analog lines. That is, there is no router, no method for the phone company to handle directing digital signals.

Modems essentially take the digital data from your computer, and convert it into an audible analog signal that can travel over the phone system to another computer, where the modem there does the same trick, but in reverse. That awful screech you here over your phone line is your modem audiblizing your data. Modem, in fact, stands for "MOdulation/DEModulation", the process of converting a digital signal into an analog one, and then the other way around. There and back again, to quote a particular Hobbit.

Certainly, modems have made great strides since my trusty old 300bps modem -- we're now up to 56Kbps (much "faster"), but there are still some fundamental problems with modems that make them largely unsuitable for a truly enjoyable Internet experience. We need to talk latency.

When I said that modems have gotten faster over the years, I lied. They've gained the ability to transfer more data in the same period of time, but they haven't gotten faster -- the capacity has just increased. I'm going to paraphrase Stuart Cheshire here, and use his airplane analogy. A modem's capacity is analogous to how many people an airplane can hold. A modem's speed is analogous to how fast the plane can fly. While a 747 can fly 2x as many people to London as a Concord can, the Concord will get you there much quicker. Who cares, you might say?

When we talk about a modem's speed, we're talking about latency: the time it takes for a packet of data to go from your computer to its destination. Latency is crucial -- much more important than capacity -- when you are interested in doing anything remotely interactive over the Internet (online gaming, etc.). Modem manufacturers list the modem's capacity (28.8, 56K, etc.), but they don't tell you about the latency, largely because few people understand it (and modems are inherently lousy in terms of latency).

Consider how important latency is for something as mundane as browsing the web. When you access a web site, let's say your computer requests an image that's on the page. The server sends part of it to you, then waits to hear back from your computer to make sure it was received. This back and forth "Send data / Did you get it?" dance continues until everything is transferred. Since modems have very high latency, the back and forth negotiation takes a good deal of time.

It's like having a conversation where every word is repeated by the person who is listening to you -- the delay in how long it takes them to repeat each word to you adds up incredibly over the span of the conversation. That delay is the latency. If you've ever had an overseas conversation where there is an appreciable delay between when you say something and when the other party hears it, you know exactly what latency is, and how frustrating it can be.

You'll notice that a web page with a number of graphics on it (common these days) takes quite a bit longer to transfer than it seems it should; this is because of the back and forth negotiation, and high latency inherent in modems. Over the years, the planes have gotten bigger, but they don't fly any faster... in some cases, they are actually slower. A 56K modem is no "faster" than a 28.8 modem or even a 14.4 modem -- the packets still take just as long to get there, it just has more capacity.

In order to squeeze more capacity out of the venerable analog phone system, modem manufacturers compress the data that modems send, then decompress it on the other end. This allows them to cram more data over the line, but it also takes time to compress the data, and time to decompress it on the other end, further exacerbating the latency problem. Just how bad are they?

You're lucky to get a 200ms ping (that is the time it takes a packet to go from your computer to another computer, and back again) time for a modem. Cable "modems" are in the range of 20-60ms or so, ditto with ISDN, and it gets even better when you're talking about a dedicated link such as a T1 or T3. So we're talking a minimum of 5x-10x better latency from a cable modem or ISDN -- and remember, this affects every single packet your computer sends out.

Do yourself a favor, if your area provides cable modems, or even ADSL, and you spend any decent amount of time on the Internet, look into junking your analog modem. The difference in terms of the speed is just astounding if you're used to an analog modem, and the capacity is much higher too. You won't regret it.

Links for some useful articles on modems and how they work:

V.34+ Modems Minus the Marketing Hype
by S. Saunders and C. Zimmerman Sept. 96
Go there now

56K Modems: The New Spin on Speed
By Kathleen Cholewka May 97
Go there now

56K? No Way
By Deval Shah and Helen Holzbaur August 1997
Go there now

Various short articles on V.90 available from www.V90.com
Go there now

Andrew Welch
Thaumaturgist
Ambrosia Software, Inc.

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