Reading First 64 byte == Detecting the collision

[Iluvnetwork]

How collsions should be detected during the first 64 bytes of the frame according to Ethernet specifications?  

[Georg Pauwen]

Hello,

not sure I understand what you are asking: IEEE specifies that the length of an Ethernet frame needs to have a minimum of 64 bytes to make sure the frame stays on the line long enough for both sender and receiver to detect a collision. So I don't think a frame can even be shorter than that...

[Iluvnetwork]

I was taught that the minimum packet size is 64 byte. But I wonder how 64 byte is the minimum packet size. L4(Assuming TCP) header + L3 header + L2 header + CRC = 58 byte. Where are the rest of 6 byte coming from?

[PaulP · Accepted solution]

in short answer: using CSMA/CD which is the same way it detects any other collisions. There is nothig special about the first 64 byte frame (minimum frame size) or any other frames following that. If during the first 64byte frame the switch detectes a collision (using CSMA/CD signalling) it calls that a "collision" and if the collision is detected after the first 64byte frame but before the last frame in a payload then this is called "late collision". Anything under 64byte frame would be a runt.

Edit: To answer your last question about the remaining 6 byte: "Frames must be at least 64 bytes long, not including the preamble, so, if the data field is shorter than 46 bytes, it must be compensated by the Padfield"

Interesting readings here:

http://www.dcs.gla.ac.uk/~lewis/networkpages/m04s03EthernetFrame.htm

or something that can "ruin" your weekend because of the details it goes into... :) but there are some very useful diagrams with timing events which is probably what you are after.

http://www.erg.abdn.ac.uk/users/gorry/course/lan-pages/csma-cd.html

I hope it helps,

Paul

[Iluvnetwork]

Thank you very much. Two links you commented were really helpful and interesting :)