Data Communications Networking Devices - 4th Ed.pdf

3.5 TOKEN-RING _________________________________________________________________ 345Figure 3.40 Beaconing. A beaconing frame indicates a failure occurring between thebeaconing station and its nearest active upstream neighborÐan area referred to as afailure domain®rst step in the auto-recon®guration process is the diagnostic testing of thebeaconing station's adapter. Other steps in the auto-recon®guration process includediagnostic tests performed by other nodes located in the failure domain in anattempt to recon®gure a ring around a failed area.Duplicate address test frameThe last type of MAC command frame is the duplicate address test DAT) frame.This frame is transmitted during a station initialization process when a stationjoins a ring. The station joining the ring transmits a MAC DAT frame with itsown address in the frame's destination address ®eld. If the frame returns to theoriginating station with its address-recognized A) bit in the frame control ®eld setto 1, this means that another station on the ring is assigned that address. Thestation attempting to join the ring will send a message to the ring network managerconcerning this situation and will not join the network.LogicalLink ControlIn concluding this section, we will examine the ¯ow of information within aToken-Ring network at the Logical Link Control LLC) sublayer. Similar to itsuse on Ethernet, the LLC sublayer is responsible for performing routing, errorcontrol, and ¯ow control. In addition, this sublayer is responsible for providing aconsistent view of a LAN to upper OSI layers, regardless of the type of media andprotocols used on the network.Figure 3.41 illustrates the format of an LLC frame which is carried within theinformation ®eld of the Token-Ring frame. As previously discussed in this section,the setting of the ®rst two bits in the frame control ®eld of a Token-Ring frame to01 indicates that the information ®eld should be interpreted as an LLC frame. Theportion of the Token-Ring frame which carries LLC information is known as a