DSL traffic shows upstream flow

A couple of weeks ago, I was at a meeting concerned with the standardisation of future xDSL systems. Amongst all the technical stuff about modulation schemes and test procedures was a very interesting little document from Telia, the Swedish network operator. They have been measuring the trend in Internet traffic on their residential ADSL lines for the last two years.

In January 2000, the total amount of downstream data represented 80% of the total. This is entirely what you would expect for typical web browsing sessions. The TCP/IP protocol usually has about a 10:1 ratio for traffic entirely in one direction because of the acknowledgement frames that are sent in the reverse direction. Add to this a few mouse clicks and the odd uploaded file then the numbers come out at close to the 80% figure.

The strange thing is that, since January 2000, the ratio has been steadily changing and Telia now finds that the amount of data sent in each direction is about equal! In fact, the graph indicates that there might even be more upstream traffic than downstream in the near future.

This is extremely interesting because, unless the effect is something special to Sweden, we have all been designing our xDSL services in the wrong way for tomorrow’s residential traffic. We should be looking at symmetrical schemes or even ones that are asymmetical in the reverse sense to those of today.

Telia’s analysis does not yet show the reasons for this trend but they suggest that it may be caused by the increase in peer-to-peer traffic. I think they mean things like voice over IP or the sharing of MP3 files. Perhaps if home web-site hosting is common in Sweden there could be such an effect, especially if the sites attract high traffic levels. In the UK, the ISPs would act to reduce this practice and I believe the Swedish ones would act in the same way. If video-on-demand services ever take off, the trend might be reversed but there is little sign of that at present.

This is another example of the speed of Internet time; an entire nation of broadband Internet users has dramatically changed its usage pattern in less than two years. I find it staggering that things can change this quickly. I had a quick look at my own usage and found that the ratio is still at the 80% level; I must be behind the times or atypical in some way.

I began to wonder whether there is a similar effect in business traffic. The standards writers have always thought that use of the Internet by businesses, other than web hosting companies, was roughly symmetrical and the xDSL services have been based on this. It may be that typical business use has already progressed past the symmetrical level. It would be interesting to see some work on this. Next generation VDSL services are more symmetrical than ADSL but Telia is suggesting that truly symmetrical systems will be more appropriate for residential applications.

If the goalposts can move this quickly we must all think very carefully when designing our networks or ordering telecommunications services.

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