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Posted by stephen on May 7, 2005, 4:17 pm
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> jpartan@gmail.com (Jim Partan) wrote:
>
> > Hello,
> >
> > How is the end of an Ethernet II frame detected?
> > The frame has no length field in it, and I'm
> > wondering how its end is detected? Does the
> > ethernet adapter detect when voltage transitions
> > stop? Is there an idle state for the differential
> > transmitter which is then detected? Something simpler
> > than that even? The specification appears to include
> > an interframe gap of 96 bit times -- is part of
> > the reason for this gap to allow the ethernet adapter
> > to detect the end of the frame?
> >
> > Thanks very much!
> >
>
> The encoding is such that there *must* be a low-to-high or high-to-low
> transition in the middle of each bit period. This is how the system
> differentiates between 0 and 1 bits. The receiver clocks in a bit
> whenever a transition occurs.
this cant be the complete storey - more complex encodings are used on higher
speed links, such gigabit ethernet over UTP.
After the transmitter has sent its last
> bit, the line returns to its idle state, there are no more state
> transitions and hence the receiver clocks no more bits into its input
> buffer.
maybe you need to look at this from the perspective of Ethernet "layer 2" -
the bit transport mechanism has to give an indication of the end of each
packet - because otherwise the end of a packet doesnt get identified. Peter
describes the mechanism used on co-ax at 10 Mbps (and probably others).
The interframe gap is to allow other stations a chance to
> access the network: without it, a station could send back-to-back
> packets indefinitely, and no-one else would get a look-in....
the other thing to remember is that layer 1 delineates the end of any
packet - the length field isnt used for that purpose, since only some
packets have that field.
>
> -P.
>
> --
> Peter Saward
> Centre for Applied Research in Education
> School of Education & Lifelong Learning
> University of East Anglia, Norwich NR4 7TJ, UK.
--
Regards
Stephen Hope - return address needs fewer xxs
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