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ETHERNET TECHNOLOGY

Part 1: Frames, Collisions, and 10 Mbps LANs

by James Antonakos

Start ý Ethernet Frame Format ý The Interframe Gap ý Collision Or No Collision ý Detecting Errors ý Random Waiting Period ý 10-Mbps Ethernet ý 10BaseFý Errors In Ethernet LANS ý More To Come ý Sources and PDF

RANDOM WAITING PERIOD

Retransmission of a frame is attempted after a random waiting period that is a multiple of an Ethernet slot time, the time required to transmit 512 bits (a total of 51.2 ýs for a 10-Mbps Ethernet). The actual name of the algorithm is "truncated binary exponential backoff" because of the method used to increase the waiting period. A collision counter keeps track of successive collisions (collisions involving the same frame). The higher the number of collisions, the longer the waiting period.

Mathematically, you have 0 < R < 2K, where K equals the minimum (N, 10) and N is the number of successive collisions. R is a random integer chosen from the range 0 to 2K. As K increases, the upper limit on the range increases exponentially (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024). The random integer R is thus chosen from an ever-increasing set of integers. As an example, after three collisions, R is chosen from the set of numbers 0 to 8. For example, if R is 5, a total of 5 ý 512-bit times is used as the delay period. After 10 successive collisions, the waiting period becomes a maximum of 1024 slot times. When 16 successive collisions have occurred, transmission of the frame is aborted and an error reported to the upper networking layer.

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