>Fresh from reading for my Support exam from Cisco Press CIT........ SDLC is
>the granddaddy of them all......... created in the mid-1970's to transport
>SNA. ISO modified SDLC to create HDLC. Then ITU-T modified HDLC to create
>LAP, then later, LAPB. IEEE modified HDLC to get 802.2, which is also known
>as LLC, of which there are 3 types: Type 1 a connectionless version, Type 2
>a connection-oriented version, and Type 3 an acknowledged connectionless
>version.
Other than for Cisco tests, I'd be hesitant to take Cisco Press as
gospel. That time sequence doesn't sound right -- indeed, the
organizations are wrong. I do have some personal experience with
these. Ideally, one would go back to the original standards
documents, or at least some textbooks from the early 70's. Cypser
would be one endorsed by IBM, or I have the original IBM Systems
Journal issue on SNA somewhere here. An author such as Priscilla
could probably check with some of the original authors (at least the
ones still around) -- I ran into Hal Folts a couple of months ago.
I first installed SNA in 1974, which included SDLC.
ITU-T didn't exist yet. CCITT was the ancestor, and its first X.25
standards were in the 1972 books (I forget the color now--probably
yellow or orange). That used LAP. The first commercial X.25
networks deployed in 1972, the first a banking network in Spain and
then Telenet a few months afterward.
I attended Federal Telecommunications Standards Committee meetings in
1976-1980 or so, where we worked on the ADCCP extensions to HDLC. No
one particularly assumed an SDLC heritage, and indeed very carefully
used different terminology and bit encodings.
LAP-B was in the X.25 1976 standards.
IEEE originally was going to accept Ethernet, which didn't have an
LLC equivalent. IBM's proposal to standardize token ring originally
suggested that everything have the equivalent of LLC2, making
everything reliable like SDLC. The compromise was to create
connectionless LLC1 and connection-oriented LLC2.
LLC 3 was developed by the MAP project, primarily General Motors, and
I don't think it ever became a full IEEE specification. It certainly
isn't in my copy of 802.2.
>
>NetBEUI and SNA both need the reliability of LLC Type 2. I wouldn't say
>"Because NetBEUI and SNA are legacy, the use of LLC Type 2 is
>diminishing"......I would say "Because reliability is more frequently left
>to higher layer protocols, such as TCP, the need for the reliable LLC Type 2
>is diminishing".... hehe =)
>
>I don't know if NetBEUI is a good choice to use in this example. Perhaps
>NetBIOS would be a better choice...... Check out:
>
>http://ourworld.compuserve.com/homepages/TimothyDEvans/intro.htm
>http://www.geocities.com/SiliconValley/Monitor/3131/ne/osimodel.html
>http://www.windowsitlibrary.com/Content/386/10/2.html
>
>(watch for wrap)...... These pages have some info about NetBEUI/BIOS....
>from the sounds of it NetBEUI *is* a Connection-Oriented transport layer
>protocol, in which case it would NOT need the LLC Type 2 reliability, while
>the older NetBIOS a Connectionless transport layer procotol that could
>indeed benefit from the use of LLC Type 2........ But I can't say for sure
>=)
>
>Hope this helps! Good luck on your writing!
>
>Mike W.
>
>
>"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
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