Summary of computer network errors
Note: This section is more important about the minimum frame length of the CSMA/CD protocol , and I hope to understand it carefully.
original title
Perhaps the most confusing part of this question is, what does a decrease or increase in the length of the smallest data frame have to do with the distance between the two farthest sites ?
In fact, this is the problem of the minimum data frame length .
First of all, it is necessary to understand the concept and calculation formula of the shortest frame length. The shortest frame length requires the sender to continue to send data frames during the data transmission process, that is, the total number of data sent within the contention time. In the CSMA/CD protocol, only if a data frame longer than the shortest frame length is received, it is considered that no data collision is detected.
Minimum data frame length = 2 * propagation delay * data transfer rate
Therefore, when the minimum data frame length is reduced, it is necessary to ensure that the above formula is valid, that is, the protocol is valid, and the propagation delay or data transmission rate needs to be reduced . Therefore, when the distance between the sites is reduced, the propagation delay is reduced. Therefore, A and B are excluded. .
The detailed calculation steps are as follows:
Therefore, the question should choose B, should reduce 80m.
In addition, there is a question that examines the minimum data frame length :
Similarly, when the data transfer rate increases , if the
Minimum data frame length = 2 * propagation delay * data transfer rate
It is still necessary to increase the minimum frame length or reduce the propagation delay (reduce the cable length) .