thernet versions, each data frame is encapsulated at the physical layer before transmission and link synchronization is maintained by sending a continuous stream of IDLE code groups during interframe gaps. All 1000Base-X physical layers support both half-duplex and full-duplex operation. The principal differences among the 1000Base-X versions are the link media and the connectors that the particular versions will support, and, in the case of the optical media, the wavelength of the optical signal. 11.6.3 Token Ring and FDDI Token ring and FDDI (fiber distributed data interface) are examples of controlled access protocols. Both protocols are now obsolete. We briefly review the concept SO the reader can gain some knowledge of controlled or disciplined access. A typical token-passing ring is illustrated in Figure 11.11. A ring is formed by physically folding the medium back on itself. Each LAN station regenerates and repeats each bit and serves as a means of attaching one or more data terminals (e.g., workstations, PCs, servers) to the ring for the 11.6 LAN ACCESS PROTOCOLS Figure 11.11 A token-passing ring network. purpose of communicating with other devices on the network. As a traffic frame passes around the ring, all stations, in turn, copy the traffic. Only those stations included in the destination address field pass that traffic on to the appropriate users that are attached to the station. The traffic frame continues onward back to the originator, who then strips the traffic from the ring. The pass-back to the originator acts as a form of acknowledgment that the traffic at least passed by the destination(s). With token ring (and FDDI) a reservation scheme is used to accommodate priority traffic. Also, one station acts as a ring monitor to ensure correct network operation. A monitor devolvement scheme to other stations is provided in case a monitor fails or drops off the ring (i.e., shuts down). Any station on the ring can become inactive (i.e., close down), and a physical bypass is provided for this purpose. 