2.2 Networks Primer 17 This chapter begins with a networks primer and some functional, physical, and architectural models. Then we describe networks according to their classification as either Telecom or Datacom: Pure Telecom networks (global, state, regional, or public networks): ­ Plesiosynchronous Digital Hierarchy (PDH). ­ Synchronized Optical Networks/Synchronous Digital Hierarchy (SONET/SDH). ­ Optical networks. Pure Datacom networks (office or campus-wide private networks): ­ Enterprise Ethernet. ­ Internet Protocol. The network technologies that are the most likely to be used by Network Processors applications are Ethernet (enterprise and carrier class), MPLS, and IP. Therefore, most of the description in this chapter is focused on these networks, with discussion of the other networks provided to fill out the general overview and to provide the reader with a framework of how networks are combined, relate to each other, and develop.