Re: Linux Frame-Relay and misc

[Guy Harris <guy () alum mit edu>]

On Tue, Oct 07, 2003 at 12:21:24AM +0200, Krzysztof Halasa wrote:
> > So are Q.933a and ANSI LMI the same thing, or is one an extension of the
> > other, or are the two different such that, to correctly dissect a frame,
> > you have to look at, for example, the DLCI?
>
> No, they aren't the same thing. You can tell if it's ANSI by looking
> for ANSI locking shift byte, 0x95. It should be just after "call
> reference" and "message type" byte fields.
>
> They are quite similar, though - in addition to lock shift byte, ANSI
> uses 0x1, 0x3 and 0x7 report types while Q.933a uses 0x51, 0x53 and 0x57
> for respectively status, link integrity verification and PVC status.
> There might be other differences, but I'm not aware of any.
>
> Both ANSI (ANSI T1.617 annex D) and ITU-T/CCITT (ITU-T Q.933 annex A)
> use the same DLCI 0 for LMI. Cisco (the original) LMI uses DLCI 1023.

I guess the question I really meant to ask was "are Q.933 and LMI the
same thing?"

I.e., should 0x08 be NLPID_LMI, or NLPID_Q933, or NLPID_Q933_LMI, as,
presumably, a packet beginning with 0x08 could be a Q.933 packet?

> > I assume it's intended to handle the Q.933 "single octet information
> > elements" such as Shift and Repeat indicator, although it doesn't
> > actually do any shifting....
>
> Hmm...  Never heard of such elements.  I will investigate a little
> further. 

See table 4-2/Q.933 on page 16 of Recommendation Q.933 (10/95):

                                TABLE 4-2/Q.933
                        Information element identifier coding
        +-------------------------------------------------------------------+
        |                                                        Max length |   
        | 8765 4321                                  Reference    (octets)  |
        |                                                         (Note 1)  |
        +-------------------------------------------------------------------+
        | 1::: ----    Single octet information elements:                   |
        |  000 ----    reserved                                             |
        |  001 ----    Shift (Note 2)                4.5.3       1          |
        |  101 ----    Repeat indicator              4.5.25      1          |
        | 0::: ::::    Variable length information elements:                |
        |  000 0000    Segmented message             4.5.26                 |

                                ...
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