By the way, the document I have on Cisco HDLC (which I can no longer find on a Web site) not only doesn't mention any flags but also doesn't mention an FCS. It must have an FCS. We know it drops bad frames.
Cisco HDLC is starting to sound as big as PPP! :-) I'm not sure it really wins in the "who has a shorter header/trailer war." I think there's a good chance it really does have the flags and I'm sure it has an FCS, which would make it the same size as PPP's header/trailer. Oh, I would kill for a WAN sniffer! :-) Priscilla Priscilla Oppenheimer wrote: > > MADMAN wrote: > > > > Priscilla Oppenheimer wrote: > > > MADMAN wrote: > > > > > >>Stuart Pittwood wrote: > > >> > > >>>It has been mooted to me that we might get better > performance > > >> > > >>from our > > >> > > >>>1Mb line by using HDLC rather than PPP. > > >>> > > >>> > > >>> > > >>>Is this correct? > > >> > > >> HDLC is more efficient so I guess yes. > > > > > > > > > In what way is HDLC more efficient than PPP? > > > > Since there is a little less overhead it is more efficient > > but not to > > the extent that one should be concerned. > > Cisco HDLC may have a couple bytes less than PPP in its header. > Not a big deal, as you say. > > > > > > > > > >>If I recall > > >>correctly, > > >>(someone will let me know if not;) PPP rides on top of HDLC. > > > > You definately know more details than I but I did a quick > > search and > > the second item on this URL mentions the PPP/HDLC relationship > > so I > > somewhat in the ballpark no? ;) > > OK, in the ballpark. :-) One way to look at it is that PPP > specifies the 2-byte protocol field, but then uses an HDLC-like > header for the other parts. The older RFC for PPP (1331) > specifies the PPP header: > > Flag 1 byte (01111110) > Address 1 byte > Control 1 byte > Protocol 2 bytes (not present in most HDLC derivatives, though > added by Cisco for Cisco HDLC) > info (variable) > FCS 2 bytes > Flag 1 byte (01111110) > > The current RFC for PPP (1661) just says this: > > "encapsulation requires framing to indicate the beginning and > end of the encapsulation. Methods of providing framing are > specified in companion documents." > > Real helpful. :-) Sort of implies you could do something > shorter if desired, though? > > Now, notice that if you do use the wording that "PPP rides on > top of HDLC," as you did, it's not quite right and it's > referring to the generic HDLC, not Cisco HDLC. 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. Howard would know for > sure, but I thought it was necessary in order for the other end > to synch up. Don't cringe, Howard. :-) Bit stuffing is required > to make sure it doesn't show up in the actual data. Well, that > might explain why Cisco dropped it! > > Priscilla > > > > > > > > http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/ppp.htm > > > > Dave > > > > > > > > > > > I would be glad to correct you. :-) > > > > > > HDLC is really more of an architecture than a specific > > protocol and there > > > are many derivatives of it. PPP is just one of them, as is > > Cisco's HDLC. > > > Other derivitaves include LAPB, LAPD, and LLC2. > > > > > > The standard PPP and Cisco HDLC are so similar in frame > > format you can > > > barely tell them apart. > > > > > > Cisco HDLC encapsulation has: > > > > > > one-byte address field, which is set to 0x0F for most > frames > > > one-byte control byte that is always set to 0x00 > > > two-byte protocol type field > > > > > > > > > Guess what PPP has? Essentially the exact same thing: > > > > > > one-byte flag field set to 0x7F > > > one-byte address field, set to 0x11 > > > one-byte control field set to 0xC0 > > > one or two-byte protocol field > > > > > > > > > Both HDLC and PPP also have a control protocol for keeping > > the link up. HDLC > > > has SLARP. It sends keepalives. PPP has the Link Control > > Protocol. It brings > > > the link up and send echos and echo replies. > > > > > > Cisco HDLC can also use SLARP to assign an IP address to the > > other end. > > > > > > PPP has the Network Control Protocols in many different > > varieties. The IP > > > variety can assign IP addresses. > > > > > > PPP also supports authentication, which Cisco HDLC doesn't. > > > > > > _______________________________ > > > > > > Priscilla Oppenheimer > > > www.troubleshootingnetworks.com > > > www.priscilla.com > > > > > > > > > > > > > > >>> > > >>> > > >>>If so is it just a case of changing the Encapsulation PPP > to > > >>>Encapsulation HDLC on both ends of the link? > > >> > > >> Assuming you have a Cisco on both ends, yes. > > >> > > >> > > >>> > > >>> > > >>>Are there any implications I should be aware of? > > >> > > >> One big advantage of PPP in the ability to authenticate. > > >>Though 1M > > >>seems odd I assume it's a dedicated link and authentication > is > > >>not an issue. > > >> > > >> Dave > > >> > > >> > > >>> > > >>> > > >>>Thanks > > >>> > > >>> > > >>> > > >>>_________________________ > > >>> > > >>>Stuart Pittwood, MCSE > > >>> > > >>>IT Technician > > >>> > > >>>Amery-Parkes Solicitors > > >> > > >>-- > > >>David Madland > > >>CCIE# 2016 > > >>Sr. Network Engineer > > >>Qwest Communications > > >>612-664-3367 > > >> > > >>"You don't make the poor richer by making the rich poorer." > > >>--Winston > > >>Churchill > > -- > > David Madland > > CCIE# 2016 > > Sr. Network Engineer > > Qwest Communications > > 612-664-3367 > > > > "You don't make the poor richer by making the rich poorer." > > --Winston > > Churchill > > > > > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=64548&t=64362 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
