Data Communications Networking Devices - 4th Ed.pdf

346 ________________________________________________________ LOCAL AREA NETWORKSFigure 3.41Logical Link Control frame formatprotocol data unit and consists of either three or four ®elds, depending upon theinclusion or omission of an optional information ®eld. The control ®eld is similarto the control ®eld used in the HDLC protocol and de®nes three types of framesÐinformation I-frame) are used for sequenced messages, supervisory S-frames) areused for status and ¯ow control, while unnumbered U-frames) are used forunsequenced, unacknowledged messages. Both Service Access Points SAPs) andDestination Service Access Points DSAPs) function in the same manner aspreviously covered in our examination of Ethernet. In addition, Token-Ring, likeEthernet, supports the same types of connectionless and connection-orientedservices at the data link layer.REVIEW QUESTIONS3.1.1 Discuss the differences between LANs and WANs with respect to their geographicarea of coverage, data transmission and error rates, ownership, government regulation, anddata routing.