encapsulating the IP traffic in MPLS tunnels for transmission over the core. The aim is to implement around 100 metro nodes. Core node The core nodes are MPLS switches, with the MPLS traffic carried over optical (DWDM) transport. They are completely unaware of customer IP traffic, and only switch based on MPLS tags (all customer IP traffic is encapsulated with an MPLS header by the metro node PEs). Native IP is only used by the core nodes for protocols such as MP-BGP, an IGP, LDP, and RSVP to exchange routing and label information between all core and metro nodes. The aim is to have approximately 10 core nodes. iNode iNodes are the logical nodes that provide the control for the services implemented using the other four types of node. BT has announced that it intends to create an IMS based iNode capability, although its initial PSTN replacement will not be an IMS implementation. The iNode will implement a set of standardized functions – common capabilities – that deliver layered services. Common capabilities include session management, authentication, profile, address book, presence and location. Combinations of these capability primitives will be used to deliver different service types and functionality. The iNode is built upon the AXE telephone exchange TSS (Telephony Softswitch Server) and is currently using HP Alpha processors (APZ 212 50) as well as the IS-Blade in the APZ logic. After extensive field testing in the South Wales Pathfinder area, all current logical and intelligent network services now operate in conjunction with the existing PSTN and the MPLS network. Differences The most significant differences between the legacy 20th century network (20CN) and 21CN are: The use of VoIP landline telephone services served digitally, rather than the previous analogue PSTN system. This does not mean telephone services are provided over the Internet with 21CN. In reality this will make no difference to the end user. The availability of ADSL2+ (including ADSL2+ Annex M) broadband services. The 20CN allows services up to ADSL Max with a maximum download throughput of 8 Mbit/s whereas ADSL2+ allows a maximum theoretical download throughput of 24 Mbit/s. The retirement of legacy IPstream data services previously available with 20CN. Evolution towards FTTC Any chain 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