 12 Legacy Packet-Switched Data Overlay Networks Matthew R. Thomas University of Essex Paul A. Veitch BT Design, Adastral Park David K. Hunter University of Essex Introduction to WAN Services Leased lines have been used extensively to carry corporate data since the 1970s. Mainframe networks of this era used analogue leased lines of a very low datarate (2400 baud) over the Public Switched Telephone Network (PSTN) with modems at either end. During the 1990s, many corporations rented digital leased lines from telecom providers to construct their own data networks. These lines were typically of E0 (64-kbit/s) or T1/E1 (1.544/2.048-Mbit/s) datarates; however, for many customers the cost was prohibitive. A typical company might require data communication over a spread geographical distance (perhaps 1000 km); although it is easy to provision a circuit over such a distance, it would be very expensive. The telecom providers responded by offering data overlay networks. In essence, layer-2 and layer-3 networks were constructed, initially over Plesiochronous Digital Hierarchy (PDH) and later over Synchronous Digital Hierarchy (SDH/SONET). Leased lines were still required for access to the nearest point-of-presence (POP) where the service provider had located layer-2 switches, which themselves were interconnected into Wide-Area Network (WAN) clouds. In their simplest form these overlay networks could be shared by customer traffic over dispersed geographical distances, thus reducing the price Chapter 12 Legacy Packet-Switched Data Overlay Networks point (namely, the price at which demand is relatively high) for the customer. In general, the trade-off for this was lower average bandwidth per customer and differing Service Level Agreements (SLAs). By the late 1990s and early 2000s, Frame Relay (ITU-TQ.922) had expanded its take-up by corporate customers to become a main player in data overlay networks (ITU-T, 1992). Despite the enormous take-up of Frame Relay, analysts expect the service to give way almost entirely to newer Mult