Priscilla Oppenheimer wrote: > > Hmm. Well maybe I didn't really want you to get your scope out > then, but rather a protocol analyzer. That didn't sound as > "appealing" though. :-) > > I'm most interested in the fields in the Cisco HDLC header. OK, > I guess I'm curious about the signal too, now that you picqued > my interest!
We used to have a good number of WAN PAs laying around, but not anymore. So they're more difficult for me to get my hands on in a lab environment (they're usually out making money somewhere). I've been looking at this sort of thing on o'scopes long enough that I know a 7E7E when I see it. I'll be setting this up in next few hours if there is, in fact, a scope in the "tool crib." > > It occurs to me that on another thread I mentioned that we > could discuss just about anything on this list, except maybe > the ones and zeros sent across a line, and here we are > discussing the ones and zeros! I love it. Understanding the nuts and bolts of ones and zeros transmission isn't terribly difficult once you've been around it a while (but not to be taken for granted!). Not too many folks bother or have the test equipment + opportunity, though. That leaves a profitable niche market out there -- especially if you truly understand timing and synchronization of serial comm. Again, not terribly difficult but rare these days. I love it too! > > THANKS > > Priscilla > > > s vermill wrote: > > > > 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. > > > > > > > > > > > > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=64744&t=64362 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
