What is "21CN"?
21CN (21st Century Network) is a Next Generation Network (NGN) being deployed by British Telecom over the next 5 years. The core aim of the project is to bring the UK's telecoms network in to the digital age, whilst at the same time allowing BT to save up to £1billion per year in costs . The total cost of the project is estimated at £10billion .
The following are some of the key changes that 21CN will bring:
- The national PSTN phone network will be moved on to a digital IP network
- The retirement of 16 existing legacy networks and the introduction of a single "multi-service" network 
- The national roll-out of ADSL2+, offering services at up to 24Mbps. ADSL2+ Annex M will also be deployed, offering upstream rates of up to 2.5Mbps
- A complete overhaul of the broadband services offered. A range of new configurable QoS (Quality of Service) products will be made available. DataStream products are being removed completely. See the broadband page for more.
In recent years BT has seen its traditional revenue sources (i.e. phone calls) decline considerably, and its "new wave" products (such as broadband) explode in popularity. Ultimately, 21CN should help BT to surf this new wave of demand and reduce their dependence on traditional revenue streams.
How 21CN will affect your existing phone and internet services
Put simply, 21CN will have no immediate effect on any end users. 21CN focuses on the core of BT's network - it makes no changes to your phone line, nor does it require you to change any equipment in your home (e.g. phone, ADSL modem/router, wireless equipment).
The services that are being migrated on to the 21CN network (such as the phone system, IPStream broadband connections, and ISDN connections) will experience a few minutes downtime on the night of the migration. Beyond that, service should be indistinguishable from present.
Services that are not being migrated on to the 21CN network (i.e. LLU, SDSL, DataStream broadband services) will be completely unaffected.