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October 01, 2006

How Broad is Your Band? (continued)

Latency has a lot to do with what kind of kind of experience you have surfing the web and whether you can successfully use applications like VoIP or play interactive online games.  ISPs usually present their services in terms of bandwidth (aka speed) but latency can be at least as important to you as raw bandwidth.

Latency is the amount of time it takes packet of data to go from you to wherever you sent it to and get back to you again.  When latency is high (bad), you sit around drumming your fingers and waiting for pages you’ve requested to actually show up. When latency is high, VoIP conversations are painful or impossible.  And, when latency is high, you are a loser at every online game you play.  Some people even say that high latency causes them stock market losses because they can’t transact fast enough.

How do you know the latency of an Internet connection?  If you’re a nerd, you know the answer, of course, and probably have a folder full of tools on you PC or Mac for measuring latency.  If you’re not a nerd, I’m gonna guide you through an unnatural act (on a PC.  If you’re on a Mac you’re on your own but I’d be glad for a comment that helps Mac users.)

OK. Here’s how we do it:

  1. Go to your start menu.
  2. Click All Programs
  3. Click Accessories (usually the first folder) and then select Command Prompt     Image002

  1. type ping –n 10 yahoo.com

You get back a screen that looks like this (now don’t you feel like a nerd?):

Ping

What happened is that you sent 10 test packets to Yahoo and asked their server to bounce them back so you can time them.  I picked Yahoo because they have lots of servers and respond fast so any delay is likely to be in the connection rather than at the far end server.

The average reported should be less than 100ms (milliseconds or thousandths of a second).  Any more than that and you’re a finger drummer.  I know my own number here is a little higher than that.  I have a problem in my home WiFi connection that I’m still debugging.  To accurately measure your ISP, you should NOT use your WiFi connection but rather plug your PC directly into your router.

VoIP works best when the number is less than 75ms but can work acceptably at latencies up to 250ms if jitter (the variability of the response time) is low (post coming on that). Fast finger gaming needs average latencies of 60ms or less.

Latency doesn’t matter much for email and uploading and downloading.  A tenth of a second here or there isn’t significant if you’re not hunched over your computer waiting for something to happen or listening to someone speak.  Applications like email or file transfer send a stream of packets out without waiting for a response on a packet by packet basis.  The time it takes to get your email or download a file is much more influenced by bandwidth (see previous post on this) than by latency.

But modern websites are built assuming low latency between surfer and site.  There are lots of interactions between your computer and CNN or Fractals of Change before you see the page you want to see.  High latency’ll leave you frustrated.

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