iprotocol Label Switching (MPLS) and Ethernet solutions by 2011. In 2003, U.S. customers spent $7.4 billion on Frame Relay services, while in 2005 Frame Relay services in the United States represented a $6.5 billion market. Analysts are expecting a five-year compound annual growth rate (CAGR) of -12.1 percent for the sector (IDC, 2006). In Europe analysts are expecting customer spending to fall at a CAGR of -17 percent from $2.9 billion in 2006 to $1.1 billion in 2011 (IDC, 2007). Frame Relay differs from earlier offerings such as X.25 (ITU-T, 1996) because it was developed as an OSI (International Standards Organization) layer-2 service designed to relay a frame to its destination (ISO, 1984). Apart from Frame Relay and X.25, there are two other data services introduced in this chapter that differ significantly from these standard overlay services-namely, ISDN and ATM. Integrated Services Digital Network (ISDN) (ITU-T, 1993a, 1993b, 1993c, 1995, 1997, 1998) is distinctive due to its inclusion within the PSTN. ISDN provides dynamic services, hence, signaling is required, which is provided by Signaling System Seven (SS7). ISDN is a part of the telephone system, and it utilizes bandwidth provided by the telephone trunks. Asynchronous Transfer Mode (ATM) was designed with a slightly different set of aims in mind. ATM is an extremely efficient way to move data across a network (De Prycker, 1991). From the 1990s onward, ATM was touted by some industry analysts as a possible replacement technology for the underlying SDH/SONET network. The general intention was the convergence of voice and data over a combined infrastructure. The idea was the replacement of the SS7 signaling system with IP voice/data conversion gateways, resulting in the phasing out and removal of the SS7 telephone signaling system with its associated circuit bearers. It was envisaged that if voice traffic were running over ATM, there would be no need for SDH/SONET to provide bandwidth for both voice and data networks, and SO the under- lying SDH/SONET 