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detecting end/length of Ethernet II frame?

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Anonymous
April 28, 2005 3:03:28 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

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!

Jim
jpartan [at] gmail.com
Anonymous
April 29, 2005 2:25:07 AM

Archived from groups: comp.dcom.lans.ethernet (More info?)

Jim Partan wrote:

> 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?
>

The transmitter simply stops, at the end of the frame. The gap allows the
receivers to recognize that the transmitter has stopped.
Anonymous
May 6, 2005 6:01:28 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

In article <683821bb.0504281003.18e300cc@posting.google.com>,
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. 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. 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....

-P.

--
Peter Saward
Centre for Applied Research in Education
School of Education & Lifelong Learning
University of East Anglia, Norwich NR4 7TJ, UK.
Related resources
May 7, 2005 8:17:54 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

"Peter Saward" <p.saward@uea.ac.uk> wrote in message
news:p .saward-12755C.14012806052005@cpca14.uea.ac.uk...
> In article <683821bb.0504281003.18e300cc@posting.google.com>,
> 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
Anonymous
May 8, 2005 7:47:14 AM

Archived from groups: comp.dcom.lans.ethernet (More info?)

stephen wrote:

(someone wrote)

>>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.

>>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).

Well, 10Mbps is all that Ethernet II allowed.

It includes the preamble so that clock recovery logic (such
as PLLs) have time to synchronize to the data stream.

Yes, later standards use synchronous data streams which identify
the end of the packet in a different way, but that isn't Ethernet II.

-- glen
Anonymous
May 9, 2005 10:40:37 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

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!
>
> Jim
> jpartan [at] gmail.com

Doesn't the voltage on the cable drop to totally 0 - (no carrier, no
nothing) when the frame is done? At least for ethernet II? Somewhere
along the line I got it in my head that is how the end of frame is
signalled in 10mbps ethernet
April 2, 2009 3:59:18 AM

Quote:
Archived from groups: comp.dcom.lans.ethernet (More info?)


Yes, later standards use synchronous data streams which identify
the end of the packet in a different way, but that isn't Ethernet II.

-- glen


What way in Fast Ethernet?
!