Tony Kerr@ECSOFT
07/10/99 10:43

Title:                        ULCS - User data clarification
PDR Reference:                     99080001
Originator Reference:              uls/pr010
SARPs Document Reference:          SV4, 4.3.2.6.2, Note 1
Status:                       RESOLVED
Impact:                       C (Clarification)
PDR Revision Date:            28/09/99 PROPOSED -> RESOLVED
                              13/09/99 ACCEPTED -> PROPOSED
                              24/08/99 SUBMITTED -> ACCEPTED
PDR Submission Date:               03/08/99
Submitting State/Organisation:     Eurocontrol
Submitting Author Name:       Tony Kerr
Submitting Author E-mail Address:  tony.kerr@ecsoft.co.uk
Submitting Author Supplemental Contact Information:
                    ECsoft UK Ltd., Centennial Court
                    Easthampstead Road, Bracknell
                    RG12 1YQ,  UK
                    Tel: +44 1344 867199
SARPs Date:                   DOC 9705/ed 2
SARPs Language:                    English

Summary of Defect:

Implementers of the ATN upper later communications service have been
confused by Note 1 in 4.3.2.6.2, which explains the ISO/IEC 8823-1 choices
for encoding User-data.  The confusion arises because it is not immediately
apparent where the Note ends and the technical provisions begin, since they
are embedded within an ASN.1 definition.

Assigned SME:                 SME 4 (A. Kerr)

SME Analysis:

There is no defect as such in Sub-Volume 4, but there is a real risk of
mis-interpretation, which should be removed, as this would have dire
consequences for interoperability.

Proposed SARPs amendment:

1) Delete 4.3.2.6.2 Note 1, which states:

<italics on>Note 1.- ISO/IEC 8823-1 specifies two choices for the encoding
of User-data:

User-data ::= CHOICE {
     [APPLICATION 0] IMPLICIT Simply-encoded-data,
     [APPLICATION 1] IMPLICIT Fully-encoded-data }

Simply-encoded-data ::= OCTET STRING
<italics off>

2) Insert new Note 1, immediately before existing Note 2, as follows:

<italics on>Note 1.- Note that ISO/IEC 8823-1 specifies two choices for the
encoding of User-data, either Simply-encoded-data or Fully-encoded-data.
For ATN, presentation User Data is equivalent to Fully-encoded-data, and
NOT to ISO/8823-1 User-data.  That is, the bit to indicate the CHOICE of
simple or full encoding is NOT encoded.
<italics off>


Impact on Interoperability:

None, if the existing text is interpreted correctly.  However, if it is
wrongly assumed that the top-level User-data CHOICE must be encoded, then
applications using the dialogue service will not interwork at all with
correct implementations.

SME Recommendation to CCB:

CCB Decision:                 RESOLVED (CCB-10, Spain)