Priscilla Oppenheimer wrote:
>
> s vermill wrote:
> >> Cisco HDLC just
> > > has this:
> > >
> > > Address - 1 byte
> > > Control - 1 bytes
> > > Protocol - 2 bytes
> > >
> > > It's curious that Cisco HDLC doesn't have the flag fields.
> > > Maybe they just aren't mentioned in the only document I have
> > on
> > > Cisco HDLC?? The 0x7E flag is present in most derivatives of
> > > HDLC, including SDLC. It's used to signal the beginning and
> > end
> > > of a frame and can be sent multiple times and during silence
> > to
> > > keep the link up, from what I remember.
> >
> > Every HDLC derivative I've ever worked with uses the ol' 7E7E
> > idle pattern. Next time I have an o'scope out, I'll take a
> > peek at a Cisco HDLC encapsulated link.
>
> Oh, yes, do please get your scope out! :-) I'm really curious
> about Cisco HDLC and expect the doc I have doesn't tell the
> whole story.
>
> I wonder if a scope would strip out the flags, sort of like an
> Ethernet analyzer doesn't show the preamble, though.
Priscilla,
Most WAN protocol analyzers can be set to sync on 7E7E. Further, most can
be set to blank the idle pattern from the display (whether 7E7E or something
else). An o'scope, on the other hand, is completely protocol unaware. It
simply deflects a trace horizontally and vertically based on time and signal
amplitude, respectively. A typical HDLC idle on an o'scope looks like
(hopefully this will align somewhat):
____ ____
____________| |____________| |_________________________________
This assumes a "negative mark" environment. What you see are six bit times
at the mark condition (no voltage) and two bit times at the space condition
(positive voltage), repeated again, and then followed by some quiet period.
I can't remember how many quite bit times there are between 7E7E idles.
Pretty sure it's one 7E7E flag per frame interval (in other words, a frame
of all zeros follows the flag).
You can get an estimate of what an o'scope trace of a digital pattern will
be by simply converting the hex to binary and visualizing the ones and zeros
as the simple voltage/no voltage conditions that they really are. I'll see
if we have a scope handy in one of the labs soon and fire up a Cisco HDLC
interface. I suspect I'll see the 7E7E.
>
> THANKS
>
> Priscilla
>
> >
> > >Howard would know for
> > > sure, but I thought it was necessary in order for the other
> > end
> > > to synch up.
> >
> > Than's the general idea. You don't want to wait until there's
> > data to be transferred before declaring protocol down. Loss
> > of, say, three consecutive idles can trigger a protocol down
> > condition.
> >
>
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