The 21st Century Network (21CN) programme is the network transformation project of the UK telecommunications company BT Group plc for data and voice. It had been intended to move BT's telephone network from the present AXE/System X Public Switched Telephone Network (PSTN) to an Internet Protocol (IP) system. As well as switching over the PSTN, BT planned to deliver many additional services over their new data network, such as on-demand interactive TV services.

BT originally stated that it would accrue annual savings of £1 billion when the transition to the new network was completed, and hoped to have over 50% of its customers transferred by 2008. (See External links below for current progress.) Capital expenditure was put at £10 billion over five years, this being 75% of BT's total capital spending plans in that period.

Read more about BT 21CN:  21CN Architecture, Suppliers, Criticism

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BT 21CN - Criticism - Potential Issues
... is only as strong as its weakest link, and in the case of 21CN, its weakest link – the access network – is also its most valuable ... Although the architecture of 21CN simplifies the network plant, it does not look to solve the problem which will have the greatest effect on data rate ... The original 21CN roll out did not look to drive MSANs deeper into the network instead it locates them in each exchange ...