TAILIEUCHUNG - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 32

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 32 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 279 Tuesday May 20 2003 2 53 PM Ethernet Operation 279 reach Station 1 it also truncates the current transmission and substitutes a 32-bit jam signal in place of the remainder of the frame being transmitted. Upon sending the 32-bit jam signal Station 1 ceases all transmissions. A jam signal can be composed of any binary data as long as it does not form a proper checksum for the portion of the frame already transmitted. The most commonly observed data pattern for a jam signal is simply a repeating 1 0 1 0 pattern the same as the preamble. When viewed by a protocol analyzer this pattern appears as a repeating hexadecimal 5 or A sequence. The corrupted partially transmitted messages often are referred to as collision fragments and sometimes by the slang term runts. Compared to late collisions normal collisions are less than 64 octets in length and therefore fail both the minimum length test and the FCS checksum test. Types of Collisions Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. Collisions are reported by event counts by most diagnostic tools but they might be reported separately as single collisions or multiple collisions when a switch or other station is queried with SNMP. A single collision refers to a collision that was detected while trying to transmit a frame but on the next attempt the frame was transmitted successfully. Multiple collisions indicate that the same frame collided repeatedly before being successfully transmitted. This is different from frames with deferred transmissions because the deferred transmission frame did not collide. The medium was busy when the station or switch sought to transmit and it was required to wait its turn to transmit. Because of repeated collisions of the same frame the frame might not be transmitted at all which would be reported as being aborted as the result of excessive collisions. The results of collisions partial and corrupted .

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