ing primary ring and the incoming secondary ring. In compari- son, port M provides connectivity to a single-attachment station or to another concentrator, while port S connects a single-attachment station or single-attachment concentrator. FDDI Frame Format The basic FDDI frame format is illustrated in Figure 5.6. The preamble is variable in length, consisting of a minimum of 16 4B/5B I symbols. The actual beginning of the frame is the Start Delimiter (SD) field. This field consists of the 4B/5B J and K control symbols. The SD field is followed by the Frame Control (FC) field. This 8-bit field identifies the type of frame and is followed by the Destination Address (DA) and Source Address (SA) fields. Each field is 48 bits in length and corre- sponds to the MAC (Media Access Control) addresses used by Ethernet and Token Ring LANs. Thus, the first 3 bytes represent the manufac- turer of the FDDI adapter, while the last 3 bytes represent the specific adapter number manufactured by the vendor. The source address is fol- lowed by a variable Information field and a 4-byte Frame Check Sequence (FCS) field. The FCS contains a 32-bit cyclic redundancy check (CRC) value that provides integrity for the FC, DA, SA, and infor- mation fields. The End Delimiter (ED) field consists of two 4B/5B T symbols. FRAME TYPES FDDI specifies several types of frames, each of which has a defined function. Those frame types include MAC frames, SMT frames, and LLC frames. MAC frames carry Media Access Control data. Such frames include claim frames used in the ring initialization process and beacon frames; the latter are used in the ring fault-isolation process. SMT frames are used to transport FDDI management information between frames. Such frames operate, control, and maintain the FDDI ring and its stations. As indicated earlier, SMT is responsible for detect- ing a cable fault and wrapping a primary ring to a secondary ring to reroute data. Fiber in the LAN Figure 5.6 Preamble Information The FDDI information frame. Legend: Preamble 16 or 