shared zero Flag Sequences

["William Allen Simpson" <bill.simpson@um.cc.umich.edu>]

Phil was correct.

 - A Flag Sequence is 6 1-bits followed by one 0-bit.

 - 5 1-bits in normal traffic are followed by a 0 for transparency.

 - 7 1-bits means toss the frame!  "A frame which ends with an all '1'
   bit sequence of length equal to or greater than seven bits shall be
   ignored." [ISO-3309, p 4].

The shared-zero mode comes from overly bit-happy IBM engineers, and is
used only in SLDC.  It allows the next frame to start one bit time
sooner after an extended idle.  Since it is only useful after idle (you
weren't busy anyway), why bother speeding up the next bit time?

In HDLC (ISO-3309) and X.25, the shared-zero mode has been "not
recommended" since about 1978.

It isn't compatible with byte devices.


> From: "Apostolos K. Salkintzis" <SALKI@voreas.ee.duth.gr>
> > From:           Phil Karn <karn@qualcomm.com>
> > Note that HDLC permits "zero sharing" between adjacent flags. These
> > are both legal flag streams:
> >
> > Flag Stream A: 011111100111111001111110 ...
> >
> > Flag Stream B: 0111111011111101111110 ...
> >
> > The big difference comes after NRZI encoding. Flag Stream A, which is
> > generated by the SIO or 8530 found in most TNCs, results in either of
> > two "lopsided" rectangular waveforms being fed to the FSK modulator
> > with 50-50 probability:
> >
> > 0111111011111101111110 ...
> > or
> > 1000000100000010000001 ...
> >
>
> I thing the correct encoding is:
>   0111111101111111011111110 ...
>   or
>   1000000010000000100000001 ...
> Phil, correct me, if I'm wrong.
>
Bill.Simpson@um.cc.umich.edu