more I symbols Start Delimiter Frame Control Destination Address Source Address Frame Check Sequence End Delimiter T symbol Frame Status A third type of frame transported via FDDI is the Logical Link Control (LLC) frame. LLC frames transport information on the FDDI network. Whereas MAC and SMT frames are local to a ring, LLC frames can be bridged or routed off a ring. RING SCHEDULING AND SERVICES FDDI supports the opera- tion of a timed-token protocol (TTP), which defines the means by which a station acquires access to a ring. As we will soon note, several timers gov- ern the operation of an FDDI LAN. However, before discussing the role of timers, we shall describe the two types of services supported by FDDI: asynchronous and synchronous. ASYNCHRONOUS SERVICES In FDDI terminology, asynchronous transmis- sion refers to the ability of a station to transmit data during periods when bandwidth is not reserved by synchronous services. Thus, we can note that in the FDDI context the term asynchronous services represents the transmission of information that is not extremely delay-sensitive (i.e., not time-critical). SYNCHRONOUS SERVICES A second type of service specified by FDDI is synchronous transmission services. In synchronous services, each sta- tion is guaranteed a portion or slice of the 100-Mbit/s FDDI band- width. The actual amount of synchronous bandwidth allocated to a station is negotiated using an allocation procedure defined by SMT. As we will soon note, when a station does not need its guaranteed band- width for synchronous services, it can use that bandwidth for asyn- chronous services. Chapter Five TOKEN PASSING If you are familiar with the 4- or 16-Mbit/s Token Ring LAN frame format, you are probably mystified as to the location of the token in the FDDI frame. Unlike Token Ring, which includes a token bit in its Frame Control field, FDDI uses a special six-symbol frame as illustrated in Figure 5.7. The token frame consists of four fields: a pream- ble field of 16 or more I symbols, an SD field of J and K