rated a range of equip- ment from more than one switch vendor. Trunks between the DSE overlay switches were Chapter 12 Legacy Packet-Switched Data Overlay Networks Converged Services MSIP Tiered ATM Cloud Tier 1 (28 locations) STM-16, 2.5-Gbit/s Connections Tier 2 (~95 locations) STM-4, 622-Mbit/s Connections Tier 3 (~95 locations) STM-4, 622-Mbit/s/STM-1, 155-Mbit/s Connections Customer Customer Fig. 12.10 MSIP platform. implemented in SDH/SONET and included STM-16 rates in the core. As of late 2007, the network encompassed a geographic footprint of approximately 200 distinct switching locations. The current WAN network has two different tasks. The first is that of a corporate data service offering designed to connect customer locations from geographically dispersed regions without the high cost of leased-line access. In this respect traffic enters and exits the WAN cloud from customer CPE to customer CPE. The second purpose of the converged ATM network is as a feeder network to BT's IP Internet backbone. The Internet backbone in the United Kingdom is dominated by the Internet exchanges in Manchester, London, and Scotland. Connected to the Internet exchange is a purpose-built high-speed router backbone, incorporated into which are nine major POPs that host the BT access servers. Internet traffic travels over this backbone from other ISPs via the Internet exchanges to the BT IP access servers located at the points of presence. A home Internet user can attach to the LE using a digital subscriber line. This line is terminated over copper pairs into the LE. The voice and data signals are separated at the exchange, with the voice component patched onto the SS7 digital exchange. The termination of the data part of the signal is then performed on a DSLAM. Currently the DSLAMs are ATM enabled, and the data signal is carried over an ATM virtual path using the MSIP data overlay network to BT's IP points of presence. The BT access servers can authenticate the customer and then forward authorization requests to ISDN Basic R