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UK ADSL - How It All Slots Together - DSLAM

Diagram of a typical DSLAM used by BT

PVC1 = Home500 USB users connected at 50:1
PVC2 = Business500 Ethernet users connected at 20:1
PVC3 = Business1Mb Ethernet users connected at 20:1
PVC4 = Business2Mb Ethernet users connected at 20:1

Notes: The Permanent Virtual Connections (PVC's) are where BT intends that contention should take place. They have a minimum size of 10Mbps and carry a mix of traffic to/from 3 ISPs. ie - each PVC would be used by a maximum of 3 ISPs for users on the same service - Home 500 for instance. All PVC's are connected to BT's ATM Network.

Contention (click for more info)

Yep, it's that old bugbear, the bane of UK ADSL users lives - right? Well actually no, but it's a qualified no. BT are currently not contending anyone on the PVC's but at some point in the future they will undoubtedly have to. We know (or at least we believe) that the PVC's are a minimum of 10Mbps, but they could be far larger and on some exchanges almost certainly are.

At the moment there's just not enough ADSL users in the UK to cause BT a problem with bandwidth on their ATM network, but as the popularity of ADSL grows there will definitely come a point where they will have to apply some contention to the PVC's. We can only hope that when that moment arrives enough data has been collected about usage patterns that BT will realise that 50:1 contention is unsustainable. My personal view is that 20:1 is borderline.

Note to readers - you are contended on your Home Gateway. How much you're contended and how it is handled is down to individual ISPs - some are good, some are bad. That's life i'm afraid.

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