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.
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. >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. > 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=64644&t=64362 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
