tenna Reflector, Microwave J. 8:79. This page intentionally left blank Part 3 Network Technologies This part deals with the primary core network technologies that operate over the physical media. It explains both the transmission network and data overlay network technologies. The transmission network is closely related to the particular media and is used as a common network able to carry a wide variety of signal types. Above the transmission network are a number of overlay networks that are designed to provide a particular range of services to network users. A number of different overlays have been developed; for example, PSTN is an overlay network delivering voice services, while Frame Relay is an overlay network offering low speed data services. These networks are called overlays because they overlay the transmission network. At the transmission level, both SONET and SDH technologies were largely developed by 1990 as a means of providing high-capacity circuit-switched transport when voice and private circuit traffic dominated. Both have a strictly hierarchical structure that well suited the needs of voice transmission. However, as demand for data grew and customer networks widely adopted Ethernet standards, it became clear that SONET and SDH in their original form did not have the flexibility to provide efficient transport for these data signals. For example, SDH bitrates increased by factors of 4, while Ethernet increased by factors of 10-a mismatch that resulted in inefficient fill of SDH channels when data was being carried. These restrictions were overcome with the introduction of Next Generation SDH, which provided much greater flexibility in the way SDH capacity could be used. These two technologies are described in Chapters 10 and 11. The main "legacy" overlay data network technologies are described in Chapter 12. This starts with private circuits, which have been widely used for providing Wide-Area Network interconnections using either SONET/SDH or PSTN-like tech- nology. For access, ISDN is also discus