band. Baseband transmission can be defined as the direct application of the baseband signal to the transmission medium. Broadband transmission, in this context, is where the baseband signal from the data device is translated in frequency to a particular frequency slot in the RF spectrum. Broadband transmission requires a modem to carry out the trans- lation. Baseband transmission may require some sort of signal-conditioning device. With broadband LAN transmission we usually think of simultaneous multiple RF carriers that are separated in the frequency domain. Present broadband technology comes from the cable television (CATV) industry. The discussion in this section will essentially cover baseband LANs. An important aspect is that only one user at a time may access a LAN segment at a time. We expect segments to be isolated one from another by smart bridges or switched hubs. 11.3 LAN TOPOLOGIES There are three types of basic LAN topology1 bus, ring, and star. These are illustrated in Figure 11.1 along with the tree network, which is a simple derivative of the conventional bus topology. A bus is a length of transmission medium from which users tap into, as shown in Figure 11.1a. Originally the medium was coaxial cable. Today coaxial cable is being phased out in favor of unshielded twisted pair (UTP) or fiber-optic cable. A ring is simply a bus that is folded back onto itself. A ring topology is illustrated in Figure 11.1b. User traffic flows in one direction around the ring. In some other approaches a second transmission medium is added for flow in the opposite direction. Such a dual counterrotating ring concept improves reliability in case of a failed station or a cut in the ring. 1 Topology means the logical and/or physical arrangement of stations on a network. In other words, topology tells how these assets are connected together. 11.3 LAN TOPOLOGIES BIU-Baseband interface unit WS-Workstation HSP-High-speed printer CPU-Central processing unit Figure 11.1a A typical bus network. Comp. Comp.-Computer WS-Workstatio