Data Communications Networking Devices - 4th Ed.pdf

Data Communications Networking Devices: Operation, Utilization and LAN and WAN Internetworking, FourthEditionGilbert HeldCopyright # 2001 John Wiley& Sons LtdISBNs: 0-471-97515-X (Paper); 0-470-84182-6 (Electronic)APPENDIX ECSMA /CD NETWORKPERFORMANCEEthernet is a carrier sense multiple access with collision detection CSMA/CD)network. Each station on the network listens for a carrier and attempts to transmitdata when it senses the absence of that signal. Unfortunately, two stations mayattempt to simultaneouslytransmit data, resulting in the occurrence of a collision.Even when one station thinks there is no carrier, it is quite possible that a carriersignal is propagating down the transmission path. Thus, a station transmitting datawhen its sampling of the line indicates the absence of a carrier mayalso result in acollision.Because of the random nature of collisions, Ethernet bus performance is notdeterministic and performance characteristics and message transmission delays arenot predictable. However, over a period of time you can determine average and peakutilization, data elements which you may use to split one Ethernet LAN into two ormore LANs via the use of bridges to increase individual network performance. Inthis appendix we will focus our attention upon determining the rate at whichvarious length frames can ¯ow on an Ethernet network. This information can beused bynetwork managers and LAN administrators in conjunction with networkmonitoring tools to determine if a network should be segmented via the use of oneor more bridges, upgraded to a higher operating rate network, or if LAN switchesshould be used to enhance network performance.Determining the network frame rateThe top portion of Figure E.1 illustrates the IEEE 802.3 Ethernet) frame format.In this illustration the seven-byte preamble ®eld and the one-byte start of frame