First, I'm going to take issue with what I consider an urban legend
that HDLC was based on SDLC. SDLC was first available in the first
SNA release in 1973. The HDLC family was the basis of the X.25 link
standards, including LAP in the 1972 CCITT specifications. But there
was quite bitter feeling between IBM and the CCITT group developing
the HDLC family. It's fair to say that HDLC and SDLC were based on
contemporaneous work on bit-oriented data link protocols.
Second, HDLC was never intended as a standalone protocol, but as a
family of protocols that could be customized for applications. The
fullest standards based HDLC specification I ever saw was ANSI's
Advanced Data Communications Control Protocol (ADCCP), which I don't
think ever was deployed.
The idea of an unreliable link was quite alien to CCITT, and indeed
was not part of the original OSI specification, ISO 7498, which was
formalized in 1984. George Orwell fans, make whatever observations
you wish. Connectionless communications are an annex to ISO 7498.
Yes, I know that connectionless doesn't necessarily mean unreliable,
as in LLC3 or things that just don't quite fit the model, such as OSI
Fast Select or AppleTalk Transaction Protocol.
Roughly:
IBM CCITT/ISO DIX Ethernet
HDLC IEEE 802
|
v
LAP
|
SDLC v
. LAP-B IBM TR Proposal
............................................ LLC 1 and 2
LAP-D
LLC3
LAP-F
>Priscilla,
>
>The only reading I have done on the LLC2 standard is from a book written by
>Roosevelt Giles. From reading the book I got the impression that LLC was a
>subset of HDLC and it was based on HDLC standard not the other way around.
>I would have to respectfully disagree with the statement that hdlc and lapd
>are based on LLC2. But I wouldn't put my career on it. I would also mention
>the reliability factor is because it is an acknowledged connection-oriented
>service versus LLC type 1 is connectionless. Is this part of the Top down
>network design volume 2 edition. If it is please let us know the due date
>cuz its gonna sellout fast.
>
>Hope this helps
>
>George, Head Janitor, CCNA CCDA
>Cisco Systems
>
>
>""Priscilla Oppenheimer"" wrote in message
>[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
>> I found myself writing this paragraph for a new writing project:
>>
>> When NetBEUI and SNA are used on Ethernet networks, they take advantage
of
>> the reliability of LLC Type 2. Because NetBEUI and SNA are legacy
>> protocols, the use of LLC Type 2 is diminishing. However, it is still
>> important to learn LLC Type 2 because WAN protocols, such as High-Level
>> Data Link Control (HDLC) and Link Access Procedure on the D Channel
>(LAPD),
>> also known as ITU-T Q.921, are based on LLC Type 2. (Cisco's HDLC is
>> non-standard and is not based on LLC Type 2, however. Cisco's HDLC is
>> connectionless.)
>>
>> Do I have it backwards? Are HDLC and LAPD based on LLC2, or is it the
>other
>> way around? Any other lies you can pinpoint in my paragraph? I know it's
a
>> bit awkward still. I will polish it. ;-) Thanks for your help!
>>
>> Priscilla
>>
>> Thanks for your help!
>>
>> Priscilla
>>
>> ________________________
>>
>> Priscilla Oppenheimer
>> http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=8294&t=8262
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