l groups of countries, and CT1 exchanges switch traffic between continents. CT3 exchanges connect switch traffic between the national PSTN and the international gateway. Transmission Nodes Transmission nodes, which are part of the transport infrastructure, provide com- munications paths that carry user traffic and network control information between the nodes in a network. The transmission nodes include the transmission media discussed in Chapter 2 as well as transport equipment, including amplifiers and/or repeaters, multiplexers, digital cross-connect systems, and digital loop carriers. Service Nodes Service nodes handle signaling, which is the transmission of information to control the setup, holding, charging, and releasing of connections, as well as the transmis- sion of information to control network operations and billing. A very important area related to service nodes is the ITU standard specification Signaling System 7 (SS7), which is covered later in this chapter. Chapter 4 The PSTN The Transport Network Infrastructure The transport network includes two main infrastructures. The first is the PDH, also known as T-carrier, E-carrier, and J-carrier wideband transmission standards. This infrastructure was first introduced in the early 1960s. The second infrastructure of the transport network is the Synchronous Digital Hierarchy (SDH; ITU terminol- ogy), also known as Synchronous Optical Network (SONET; ANSI terminology), which was first formalized and standardized in 1988. SDH/SONET is the second generation of digital hierarchy, and it is based on a physical infrastructure of optical fibers. PDH and SDH/SONET are voice-centric circuit-switched network models that switch millions of 64Kbps circuits between various switching points. Each circuit is multiplexed numerous times for aggregation onto transmission facilities. Aggre- gation occurs at many points in the network: in the access network, within the local exchange, and throughout the interexchanges. Hence, a significant portion of the cost of a network g