detecting end/length of Ethernet II frame?


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]

Reply to
Jim Partan
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The transmitter simply stops, at the end of the frame. The gap allows the receivers to recognize that the transmitter has stopped.

Reply to
James Knott

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....


Reply to
Peter Saward

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

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

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.

Reply to

(someone wrote)

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

Reply to
glen herrmannsfeldt

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

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