Hack34.Test Your Internet Connection Latency


Hack 34. Test Your Internet Connection Latency

Test to make sure Skype's latency needs are met.

Works with: all versions of Skype.

From Skype's point of view, latency is a measure of the delay that occurs between the moment you speak and the moment your recipient hears what you said. During face-to-face conversations, the delay is obviously not noticeable because the distance your voice has to travel is so short.

However, as this delay lengthens, conversation becomes more and more stuttering and broken, and the likelihood of the call participants talking over each other increases.

The Apollo moon landings provided a dramatic demonstration of latency. The moon orbits Earth at an average distance of 238,900 miles and the speed of light is 186,000 miles per second. This meant that there was a minimum round-trip delay of about 2.6 seconds between someone in ground control asking a question and his receiving an answer from an astronaut standing on the moon. This meant that conversations between ground control and the astronauts on the moon were necessarily made more difficult by the long pauses caused by latency.

Similarly, people who get their Internet connection via satellite (specifically, geostationary satellite) face a minimum 0. 24-second round-trip delay between transmission and receipt of data because of the long signal path through space. Electronics and cabling along the end-to-end path between sender and recipient simply add to this delay. Some Skype users talk over a satellite linkif you live somewhere really remote, it might be your only option to connect to the Internet and they report that even though the sound quality might be good, the link latency is noticeable and makes for difficult conversation.


3.11.1. Skype's Latency Needs

Skype works by connecting one machine to another over great distances, sometimes on opposite sides of the globe. Between the two machines is the Internet. Not surprisingly, depending on the speed of your Internet connection and how data is routed over the Internet, end-to-end or one-way latency (between caller and recipient) can grow quite long. Anything over 0.3 seconds will make your conversation more and more forced, in the sense that you will have to pause to know with certainty that the speaker has stopped speaking, or risk talking over one another. A latency of between 0.15 and 0. 3 seconds is noticeable, but can be accommodated fairly easily provided the callers cooperate; this is especially true if the callers know each other, as each will be somewhat familiar with the other's speaking habits. Anything below 0.15-second latency and your conversation will be little different from a good landline connection.

All of these latency rules of thumb are derived from my experience. If you want a more objective source of guidelines for the impact of latency on voice-call quality, you should consult the International Telecommunication Union (ITU) at http://www.itu.int/ and get a copy of "Recommendation G. 114: One-way transmission time." At the time of this writing, the cost was 25 Swiss francs. For the curious, here are some summary numbers:

  • Latency less than 0.15 seconds: mostly acceptable.

  • Latency between 0.15 and 0.4 seconds: acceptable (maybe).

  • Latency more than 0.4 seconds: unacceptable.

  • And for comparison, here are some typical numbers for regular telephones and mobile phones:

  • Long-distance telephone: less than 0.05-second latency.

  • International telephone: about 0.1-second latency.

  • Telephone to mobile phone: about 0.15-second latency.

  • Mobile phone to mobile phone: between 0.3- and 0.4-second latency.


3.11.2. Measure Your Connection Latency

The easiest way to get a handle on your one-way and round-trip Internet connection latency is to use an online measurement tool. One such tool is the MaxSpeed test, which you can find at http://www.numion.com/ (refer back to Figure 3-13).




Skype Hacks
Skype Hacks: Tips & Tools for Cheap, Fun, Innovative Phone Service
ISBN: 0596101899
EAN: 2147483647
Year: 2005
Pages: 168

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