n a slight slowing of such MPLS adoption, but early adopters have reaped huge financial benefits from the IP/MPLS networks as the corporate data market has significantly expanded. Today the market for differing low-level technologies has become more crowded. Pro- vider Backbone Bridging with Traffic Engineering (PBB-TE), a redefined Ethernet technol- ogy, is one of the latest prospects for low-level adoption (see Chapter 14). This is also known as Provider Backbone Transport (PBT) (Allan et al., 2006). Although PBB-TE is a packet- driven service and vastly different from SDH/SONET circuit-oriented services, it can operate as a replacement for SDH/SONET equipment by replacing and upgrading switches and line cards, while leaving intact the underlying requirement for a low-level multiplexing technology. As such, this technology can be more palatable to telecom operators than com- plete replacement by ATM or perhaps IP/MPLS. There is currently debate within the indus- try as to whether or not this technology will be used to update SDH/SONET equipment; it even seems possible that a hybrid GMPLS-signaled PBB-TE solution might be deployed in access networks. Overlay Network Concepts Fiber optic and radio transmission media form the lowest level of a country's telecommu- nications infrastructure. The organization of these media into a coherent architecture is achieved by multiple layers of network infrastructure. The lowest of these layers is most commonly SDH/SONET. The PSTN, Internet, and data connection services offered by service providers can all be categorized as overlay networks. SDH/SONET Networks and Leased Lines The underlying fiber, submarine cable, and microwave transmission media lie at the heart of any telecommunications network. Almost universally the media and the multiplexers are now standardized as SDH or SONET, which has now almost completely replaced the older Plesiochronous Digital Hierarchy. SDH/SONET allows circuits of T1/E1 (1.544/2.048 Mbit/s) and higher rates to be con- figured statically betwee