Layer 3 is not the same as the OSI network layer, though. ISDN has a
different protocol stack. It sounds like the writer of the study guide
question might have been confused by this gotcha.
Your answer of "a unique datalink layer address" is better than the answer
in the book of "a unique network layer address."
TEI is a device identifier. SAPI is like an LLC SAP and identifies the
point where Q.921's Layer 2 provides a service to Q.921's Layer 3. For
example, SAPI 0 = call control, etc. We're still at the OSI datalink layer,
though.
Cisco Press's CIT book has all sorts of details. It might be a good place
to check.
Priscilla
At 05:12 PM 7/27/00, Taylor, Don wrote:
>Your post made me wonder, so I went a-searchin'. The only thing I was able
>to find on it that might be somewhat authoritative is the following URL:
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/dial_
>r/drprt1/drbri.htm
>
>Scroll to the bottom of the page and look at the table (or just do a search
>for "sapi" - it only shows up once). Note that SAPI doesn't enter the
>equation until layer 3.
>
>- Don
>
>-----Original Message-----
>From: Ole Drews Jensen [mailto:[EMAIL PROTECTED]]
>Sent: Thursday, July 27, 2000 1:16 PM
>To: '[EMAIL PROTECTED]'
>Subject: TEI & SAPI
>
>
>I had a question in my study guide about what the combination of TEI & SAPI
>gives you.
>
>My answer was 'A unique datalink layer address', but the answer in the book
>said 'A unique network layer address'.
>
>I had the understanding that this was info in the address field of the LAPD
>protocol (Q.921) which is operating on the ISDN D-channel in the Datalink
>layer.
>
>Who's right, and if I'm wrong - could someone please explain why?
>
>I hope this is a typo in the book, because just when you think you know what
>the .... it's all about, you get thrown a "No, you're wrong" in the face...
>
>Thanks in advance,
>
>Ole
>
>P.S. Besides the few typo's I've seen, so far this BCRAN book (from McGraw
>Hill) is excellent!!!!
>
>~~~~~~~~~~~~~~~~~~~~~~~~
> Ole Drews Jensen
> Systems Network Manager
> CCNA, MCSE, MCP+I
> RWR Enterprises, Inc.
> [EMAIL PROTECTED]
>~~~~~~~~~~~~~~~~~~~~~~~~
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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