tailieunhanh - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 33

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 33 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 289 Tuesday May 20 2003 2 53 PM Ethernet Operation 289 first hears an FLP burst or some other signaling scheme. In the silent listening state a portable computer is capable of conserving enough battery power to be quite worthwhile although this mode is not supported by the standard. Ethernet Duplex Operation Two duplex modes exist half and full. For shared media the half-duplex mode is mandatory. All of the coaxial implementations are inherently half duplex in nature and cannot operate in full duplex. UTP and fiber implementations can be operated in half duplex but that mode is an administrative imposition. All 10-Gbps implementations are specified for full duplex only. In half duplex only one station can transmit at a time. For the coaxial implementations a second station transmitting causes the signals to overlap and collide becoming corrupted. Because UTP and fiber generally transmit on separate pairs the signals have no opportunity to overlap and become corrupted. Ethernet has established arbitration rules for resolving conflicts that arise when more than one station attempts to transmit at the same time. Both stations in a point-to-point full-duplex link are permitted to transmit at any time regardless of whether the other station is transmitting at the same time. Autonegotiation avoids most situations in which one station in a point-to-point link is transmitting under half-duplex rules and the other is operating under full-duplex rules. Only two methods exist of achieving a full-duplex connection at speeds below 10 Gigabit Ethernet By using autonegotiation By administratively forcing the interface mode If one station in a point-to-point link is autonegotiating and the other is not the autonegotiating station is required to select half duplex. Thus if one end of a link is forced it is incumbent upon the network support staff to force the other end as well. Failure to force both ends results in an artificially elevated error level and poor .