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Posted by Gerald Vogt on March 29, 2007, 8:18 am
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ram wrote:
> hello,
> I have one doubt. why the ethernet packet payload minimum size is 46
> bytes. why it sent from 1byte to max. size.
>
> any reason behind it.
Yes. The ethernet protocol was originally designed not collision free
and to run on a shared medium. That means it is possible that two sender
start to send about the same time and it comes to a collision. A sender
first listens to the medium. If it does not receive anything it starts
sending. The sender must listen while sending to notice whether somebody
else started sending, too, or not. If it does then it must continue to
send until the minimum packet length has been send out. The minimum
packet length is designed to make sure that the transmission will reach
all other nodes connected and all nodes are able to recognize the
collision. Obviously it depends on the line speed and the length of the
medium. Just think of a long cable and two senders at each end. If the
would only send 5 bytes each at the same time they would never notice
any collision because they receive the other transmission after they
finished theirs. The minimum packet size is designed to make sure that
the transmissions of one sender ends not before all other nodes
connected. Then all know reliably of the collision and are able to
retransmit later.
For further reading read
http://en.wikipedia.org/wiki/Ethernet
in particular the section about CDMA/CD.
Obviously that does not matter so much in today's switched networks
anymore... ;-)
Gerald
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