26/08/2005

PlusNet's ADSL Auto-Disconnects

PlusNet's ADSL Auto-Disconnects

This is probably the first of a tranch of ISPs to implement idle time-outs. It's nothing to do with congestion really, but session limits on the IPStream service.

IPStream is the BT Wholesale service that many UK ISPs resell. When BT introduced capacity based charging the price to the ISP for the end user connection was fixed (about £8) and was no longer related to connection speed. The price of the backhaul increase by a magnitude from about £30,000pa to £300,000pa.

For larger ISPs that doesn't matter as the backhaul is shared between lots of users, so the cost becomes less significant. ISPs can over contend the service whereby they add more users sharing the same bandwidth. This is why ISPs tend to give away DSL modems rather than routers, so when the user shuts-down the PC the connection stops.

It's similar to the old modem days when ISPs knew people would disconnect after surfing because the call charges were prohibitive, so they could have more users than modems.

Unfortunately in the broadband world, connections don't have to close and people keep their PCs on all the time, and that's problematic for the ISP, not due to the contention as when the connection isn't being used as there's little or no traffic being generated, but because there are session limits i.e. the total number of users connecting to the ISP simulaneously.

It's all to do with L2TP (layer 2 tunneling protocol) which IPStream uses, and for a 255Mb/s pipe (backhaul) the session limit was about 12,000 i.e. 12,000 connected customers per pipe.
That's OK if all your customers are 2Mb/s (about 6000 sessions used), but at 512Kb/s the pipe should support upto 25,000 users.
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