Data Communications Networking Devices - 4th Ed.pdf

3.5 TOKEN-RING _________________________________________________________________ 339Logical Link Control LLC) or reference physical link functions according to theIEEE 802.5 Media Access Control MAC) standard. A MAC frame carriesnetwork control information and responses, while an LLC frame carries data.The eight-bit frame control ®eld has the format FFZZZZZZ, where FF areframe de®nition bits. The top part of Table 3.15 indicates the possible settings ofthe frame bits and the assignment of those settings. The ZZZZZZ bits conveyMAC buffering information when the FF bits are set to 00. When the FF bits areset to 01 to indicate an LLC frame, the ZZZZZZ bits are split into two ®elds,designated rrrYYY. Currently, the rrr bits are reserved for future use and are set to000. The YYY bits indicate the priority of the LLC data. The lower portion ofTable 3.15 indicates the value of the Z bits when used in MAC frames to notify aToken-Ring adapter that the frame is to be expressed buffered.Table 3.15F bit settingsFrame control ®eld sub®eldsAssignment00 MAC frame01 LLC frame10 Unde®ned reserved for future use)11 Unde®ned reserved for future use)Z bit settingsAssignment*000 Normal buffering001 Remove ring station010 Beacon011 Claim token100 Ring purge101 Active monitor present110 Standby monitor present* When F bits are set to 00, Z bits are used to notify an adapter that the frame is to be express buffered.Destination addressAlthough the IEEE 802.5 standard is similar to the 802.3 standard in its supportof 16-bit and 48-bit address ®elds, almost all implementations of Token-Ring use48-bit addresses. The destination address ®eld is made up of ®ve sub®elds asillustrated in Figure 3.36. The ®rst bit in the destination address identi®es thedestination as an individual station bit set to 0) or as a group bit set to 1) of one ormore stations. The latter provides the capability for a message to be broadcast to agroup of stations.Universally and locally administered addressesSimilar to the IEEE 802.3 standard, universally administered addresses areassigned in blocks of numbers by the IEEE to each manufacturer of Token-Ring