21CN

The 21st Century Network (21CN) programme is the network transformation project of the UK telecommunications company BT Group plc for data and voice. It was intended to move BT's telephone network from the AXE/System X Public Switched Telephone Network (PSTN) to an (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 on the roll-out of optical fibre.) Capital expenditure was put at £10 billion over five years, this being 75% of BT's total capital spending plans in that period.

21CN has been heavily delayed and is yet to roll out in many parts of the United Kingdom, where the legacy circuit-switched network is still in place.

The new network is based on an architectural model of five classes of network nodes. These are:

Premises nodes includes residential, small-medium enterprise (SME) and enterprise. The presumption is that all these will have high speed connections to the network, delivered over copper (in the form of ADSL or other DSL technologies) or over fibre, as either PON or direct fibre in the case of large enterprises. The sole exception to this presumption is for legacy PSTN, where provision will continue to be made for analogue voice.

21CN introduced the concept of the multi-service access node (MSAN). This logical node takes the various access technologies (mentioned above) and, where possible, aggregates these onto a single backhaul network technology. This includes converting analogue voice into voice over IP (VoIP) using the MSAN as a media gateway (MGW). The aim is to implement a few hundred access nodes. Note that these will not have any routing capability, but will essentially be layer 2 Ethernet devices.

...
Wikipedia