bt ADSL

Posted on Oct 5, 2004

The Register reported a few days ago that BT is to double the speed for its broadband connections for business users (unless you already have 2Mb, in which case it will just get cheaper). This sounds like good news, especially when I received a letter yesterday saying that it should happen to me some time in December. Moving from 512Kb to 1Mb will be very welcome.

More problematic though is the general latency of BT ADSL connections. The hop from my house into the BT network has a round-trip time (RTT) of between seventeen and twenty milliseconds. Add in the hops to the Sun VPN server and then a couple of jumps inside SWAN and the path to “the office” is around 35ms.

That’s not so bad for most interactive use. X11 sessions can be a bit awkward, but it’s tolerable. NFS really needs a bit more bandwidth to use before it’s practical, but I’m sure that the latency will bite. Perhaps NFSv4 with caching will help.

Whilst 35ms is okay, most of the time the link is being used for things other than just pushing my emacs sessions around, so the RTT is quite a bit higher. Getting more bandwidth might help this, but it’s not going to half it. To truly work anywhere I think that the RTT needs to be down below 10ms, at which point you probably can’t tell the difference between the machine in the next room and the one at the office for most purposes.

How will this be done? Is the 17-20ms latency in the first hop as low as ADSL can get? Maybe running PPP over ATM wasn’t such a good idea, but perhaps there just aren’t viable alternatives today. Do other ADSL services have a similar first hop latency?

In the end many people will not care about interactive use over their broadband connection, but to a home worker it can be a significant factor. Fibre to the kerb or similar alternatives all sound great, but bandwidth isn’t everything - latency is important as well.