Path: utzoo!utgpu!watserv1!watmath!att!att!ima!dirtydog!jimm
From: jimm@ima.isc.com (Jim McGrath)
Newsgroups: comp.protocols.iso
Subject: Re: Datapac Priority Calling Feature?
Message-ID: <1991Jan02.154652.4253@dirtydog.ima.isc.com>
Date: 2 Jan 91 15:46:52 GMT
References: <139433@pyramid.pyramid.com>
Sender: news@dirtydog.ima.isc.com (NEWS ADMIN)
Reply-To: jimm@ima.isc.com (Jim McGrath)
Organization: Interactive Systems, Cambridge, MA 02138-5302
Lines: 32

In article <139433@pyramid.pyramid.com> csg@pyramid.pyramid.com (Carl S. Gutekunst) writes:
>Can anyone out there in netland give me a correct description of the Datapac
>Priority Calling facility? As I have it in some existing code, it appears to
>be a facility 0x01 0x01 following the national marker, and its presense forces
>the packet size to 128. I don't know if this is correct, or if it invokes more
>things that I need to be aware of. For example, should my Call Accept daemon
>give it special preference?
>
Some of the old X.25 hacks seem to be getting lost in antiquity.
There are two encodings, depending on the plenary year of the
implementation.   

In the 1976 plenary year, it is encoded as part of the Fast
Select/Reverse Charge facility parameter, which was _not_ a CCITT X.2
defined facility at the time, although reverse charging was defined as
a facility 'for further study for inclusion in Recommendation X.2'.
The facility code is 0x01. 

The facility parameter is parsed in the following manner.  Using CCITT
bit numbering, where bit 8 is MSB, bit 8 indicates fast select, bit 7
indicates restricted response to a fast select call, bit 2 is Datapac
specific and indicates priority class (packet size of 128) when set,
and bit 1 indicates reverse charging.

In the 1980 plenary year, traffic class is treated as a Datapac
national facility, and follows the national options marker which is
0x00 if Datapac is the calling PDN, and 0xFF if Datapac is the called
PDN.

The facility code for traffic class is 0x01.  The parameter value is
0x01 if Priority class (packet size of 128) is requested, and 0x00 if
not requested.