Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: Error handling in 64/66: Was Re: (Another) Question Regarding Open-Loop

Hi Birdy,

I've thought about this case some more, and I beleive we can go from the
E state to the S state without allowing a 3-bit error condition to occur
by simply adding a variable to the TX and RX processes respectively. If
we have a variable which stores the previous input (eg: rx_prev <=
rx_tobe_decoded) then we can use it as a qualifier to know whether or
not the transition from the E state to the S state is valid. In the case
of the 3-bit error you mentioned where a preamble error causes a jump to
the E state, and a 2 bit error in preamble causes data to look like S,
then the qualifier will catch this and not allow the E to S transition.
I've attached a pdf to further descibe the implementation I was thinking
of. The actual modifications needed are shown in red. Please let me know
what you think, I think this may work.


Bharadwaj (Birdy) Amrutur wrote:
> >Yes you're right, I must have been looking at an older slide deck. But
> >now that I see the new TX process and RX process, I have another
> >question. If you allow for the sequence TZZZZZZZ ZZZZSDDD, what happens
> >if you get an error somewhere. It would appear as if the condition to
> >get out of the error state is to wait for ZZZZZZZZ, but this won't
> >appear between frames given the above circumstance and the next frame
> >will also be "errored". Perhaps I'm mistaken, but it would seem as if
> >the next perfectly good frame will be trashed? Is this correct?
> >
> >-Dave
> Dave, Your observation is correct. The way the state m/c is right
> now, if there is a error, it doesnt get out of the E state till
> a clean Z frame. A problem with this is if you have back to back
> packets with min IPG spacing, you might end up with a S frame
> immediately following a T frame and there wont be any intermediate
> Z frame for a while.
> We probably should modify the state diagram by allowing transitions
> from E state to Z, D, or T states. (We need to disallow transition
> from E to S to prevent a certain 3-bit error condition from
> slipping through).
> Regards,
> Birdy Amrutur