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October 18, 2007

Broadband Primer Part 3

Low latency is crucial for some uses of the Internet – and doesn’t matter at all to others. Latency used to be a problem only for Voice over IP (VoIP) and other highly time-critical applications; now it is a problem for routine web browsing as well.

Latency is the time it takes to get something from your computer to where it’s going on the Internet and to get a response back to your computer. Long latency is bad; short latency is always good. Don’t you hate it when you have to drum your fingers waiting for a web page to image and the pictures take forever to show up or perhaps don’t show up at all and are replaced by boxes with little red x’s in the top left corner? If you have a slow connection (low bandwidth – see Part 1), this is par for the course; but, if you have a reasonably fast connection, latency may be the problem.

The major cause of persistent high latency is the connection between you and your ISP. The good news is that DSL, cable, fiber, WISP (Wireless ISP) service, and even dialup all provide reasonably low latency (assuming there is not much congestion – we all see latency when there is congestion). Typically with any of these connections, the connection itself won’t add even as much as 20 milliseconds (twenty thousands of a second) to round trip times.

The bad news is that satellite service has terrible latency AND the problem can’t be fixed. This is a physics problem; not an engineering one.

As you know, your satellite dish, whether its for TV or for Internet access, was installed pointed at a specific spot in the sky. That means the satellite it is aimed at has to stand still with relation to spots on the surface of the earth; its speed has to match the rotational speed of the earth allowing for the fact that the orbit has a larger diameter than the earth. This only possible with satellites which orbit 22,000 miles high. If they are lower, they will have to move faster and, if higher, have to move slower in order to stay in orbit and not either crash or soar off into space. In either case, they would appear to move as far as your dish is concerned. Sometimes they would even go under the horizon. So 22,000 miles it is.

Radio signals move at the speed of light – 182,262 miles per second in a vacuum. Unfortunately no way to speed that up. Data goes from your computer up to the satellite, back down to the rest of the Internet, to whichever other computer you’re communicating with, back to a satellite uplink, back up to the satellite, and back down to your computer before you see a response. That’s 88,000 miles of up and down traveling so almost half a second MINIMUM latency.

So what’s the big deal about half a second? Well, a lot if you’re using VoIP since the human ear can detect delays of a fifth of a second or more. You get annoying pauses between what you say and the answers from whomever you’re talking too. You start to talk over each other. For technical reasons, delay causes echo and you often hear yourself instead of your friend. If you are trapped and need rescue or are far at sea or exploring a wilderness, VoIP over a satellite Internet connection is fine. Otherwise you don’t want to do it.

It used to be that only VoIP and other very time-critical applications like gaming were badly affected by latency. Email is not noticeably affected since you have no idea whether it took an extra half second for your email to begin to download; most of the time is in the actual downloading. Same thing with downloading files; latency is no big deal. When you’re watching satellite TV you don’t care about latency because you have no way to know the broadcast is actually a quarter of a second ahead of you and you couldn’t care less (it’s only doing one up and down so it’s not a half second difference).

It USED to be that web surfing wasn’t affected by latency. Most delay was caused by the time it took a page to download which depends on speed and not latency. Unfortunately for those using satellite access, web browsing is NOW seriously affected by latency. What’s happened is that web pages are written and designed to be as flashy and customizable as possible given the kind of Internet access that MOST people have.

When you request a page from a modern website, there is very little chance that all of the data needed to create a page on your screen will be sent at once. Instead, the first data downloaded contains instructions for various interactions between your computer and the website. The site wants to know if there’s a cookie on your computer indicating you’ve visited before (“Welcome back, Tom”); what purchases you may have made before (“Here are some recommendations for you”); perhaps what type of computer monitor you have so it can format optimally. Pictures are downloaded in batches after the text to request them gets to your computer and one graphic element may contain the request for another. Ads appropriate to you (maybe) are gathered from various sites as part of building your page.

Meanwhile, if you have high latency, many half seconds have passed while the page builds and you’re there drumming your fingers. Some parts of the page may decide that something is broken because of the long interval and simply not show up. It’s not fun.

You can say that websites should be designed knowing that some people have a lot of latency in their connection. You can say that but it isn’t going to happen. Most Internet users don’t use satellite and the designers of web pages want them to be as appealing as they can be to the majority of people who access them – that leaves you out if you have satellite access; they’re not going to dumb down the pages just because of you and they’re not going to create special versions of pages just for you; they’re way too busy trying to make tiny pages for cellphones.

Note to nerds who may have read these-nontechie posts: yes, there are low earth orbit satellites (LEOS) which, being much closer, don’t cause significant latency. They are used for sat phone service and extremely low bandwidth and expensive (altho also low latency) data. They do move through the sky and pop under and over the horizon which means that antennas which receive and send to them can’t be directional. The consequence of this is that the power required to send broadband data streams to them is very high and interference between uplinks would be a significant problem if they were widely used. Also, they’re expensive because they burn up quickly in the upper edges of the atmosphere and fall down. Maybe, though, this is where an engineering breakthrough for satellite access could occur.

See this post and comments for how to measure latency on your connection.

Broadband Primer starts here.

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