 a mobile phone and headset to the global operation of the Internet. The key features that distinguish different networking technologies, particularly wired and wireless, are defined at the Data Link (LLC and MAC) and physical (PHY) layers. These features will be covered in detail in Parts III to V which, above all, reveal the fascinating variety of different techniques that have been harnessed to bring wireless networks to life. CHAPTER Wireless Network Physical Architecture Wired Network Topologies - A Refresher The topology of a wired network refers to the physical configuration of links between networked devices or nodes, where each node may be a computer, an end-user device such as a printer or scanner, or some other piece of network hardware such as a hub, switch or router. The building block from which different topologies are constructed is the simple point-to-point wired link between two nodes, shown in Figure 3-1. Repeating this element results in the two simplest topologies for wired networks - bus and ring. For the ring topology, there are two possible variants depending on whether the inter-node links are simplex (one-way) or duplex (two-way). In the simplex case, each inter-node link has a transmitter at one end and a receiver at the other, and messages circulate in one direction around the ring, while in the duplex case each link has both transmitter and receiver (a so-called transceiver) at each end, and messages can circulate in either direction. Bus and ring topologies are susceptible to single-point failures, where a single broken link can isolate sections of a bus network or halt all traffic in the case of a ring. The step that opens up new possibilities is the introduction of specialised network hardware nodes designed to control the flow of data between Chapter Three Figure 3-1: Point-to-point, Bus and Ring Topologies other networked devices. The simplest of these is the passive hub, which is the central connection point for LAN cabling in star and tree topologies, as shown in Figure 3-2. An