Late collision (Last chance! help ASAP)

Answered Question
May 30th, 2007

I was reading a pdf note from cisco website (Preparing to take CCNA exam) and find this out.

The question is attached (question.jpg) and the answer which was given too.

befor you open them : Do Late collision happen before this 64 bytes are transmitted ? or after?

In the topic some explaination are shown and we have to drag and drop each explaination in the matching case either routine collisions or late collisions.

Then they match the explaination "Before this 64 bytes are transmitted" to Late collision .

I was always thinking that Late collisions (THEY HAPPEN LATER) happen after 64 bytes transmitted on the media by its sender

I have this problem too.
0 votes
Correct Answer by ankbhasi about 9 years 8 months ago

Hi Friend,

You are absolutely right. To allow collision detection to work properly, the period in which collisions are detected is restricted (512 bit-times).

For Ethernet, this is 51.2us (microseconds), and for Fast Ethernet, 5.12us. For Ethernet stations, collisions can be detected up to 51.2 microseconds after transmission begins, or in other words up to the 512th bit of the frame.

When a collision is detected by a station after it has sent the 512th bit of its frame, it is counted as a late collision

512th bit of frame is equivalent to 64 bytes of frame.

So I will say never trust the docs answers as they are many times misleading and many a times when you read dumps you will find this issue especially test kings. So just apply your logic and trust your answers and if you have any doubts you can discuss with your peers or post your doubts on this forum and someone will try to answer in best possible way.

HTH

Ankur

*Pls rate all helpfull post

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scottmac Wed, 05/30/2007 - 19:15

I believe you are correct, late collisions would imply a collision occurring after the first 64 bytes are on the wire.

The reason that Ethernet was originally spec'd with a minimum frame size of 64 bytes was because that's how long it took to cover the distance, end-to-end, of the longest possible (by spec) cable and detect if a collision occurred during that transmission.

If you see late collisions on a protocol analyzer, they usually show up as a cluster of "AA"s (bin: 10101010) or fives(bin: 01010101) ... indicating that a "colliding" preamble (alternating ones and zeros) was detected during the transmission of the current (captured) frame.

So, I think the Q&A are ambiguous at best, or flat out wrong at worst.

Good Luck

Scott

Correct Answer
ankbhasi Wed, 05/30/2007 - 20:10

Hi Friend,

You are absolutely right. To allow collision detection to work properly, the period in which collisions are detected is restricted (512 bit-times).

For Ethernet, this is 51.2us (microseconds), and for Fast Ethernet, 5.12us. For Ethernet stations, collisions can be detected up to 51.2 microseconds after transmission begins, or in other words up to the 512th bit of the frame.

When a collision is detected by a station after it has sent the 512th bit of its frame, it is counted as a late collision

512th bit of frame is equivalent to 64 bytes of frame.

So I will say never trust the docs answers as they are many times misleading and many a times when you read dumps you will find this issue especially test kings. So just apply your logic and trust your answers and if you have any doubts you can discuss with your peers or post your doubts on this forum and someone will try to answer in best possible way.

HTH

Ankur

*Pls rate all helpfull post

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