>
> In Mr. Dubuisson's book ("ASN.1 - Communication between Heterogenous
> Systems") it is suggested (page 236) that if AUTOMATIC TAGS clause is not
> used in the header module, a tag of class context-specific should be
> inserted before each CHOICE type. The example reads:
>
> T ::= CHOICE { a [0] INTEGER,
> b [1] NULL }
This is not really necessary as long as all of the types within the choice
construct are unique. In the example above, if the context-specific tags
were omitted, the universal tags (UNIV 1 and UNIV 5) would be used to
distinguish the elements.
> Does that mean that when (BER) encoding the object, the choice will have
two
> tags in a row: first to tell that the CHOICE is present within its parent
> SET or SEQUENCE type and the second to indicate which is the chosen
> object/type inside the CHOICE?
Yes.
Regards,
Ed Day
Objective Systems, Inc.
REAL WORLD ASN.1 AND XML SOLUTIONS
Tel: +1 (484) 875-9841
Fax: +1 (484) 875-9830
Toll-free: (877) 307-6855 (USA only)
mailto:[EMAIL PROTECTED]
http://www.obj-sys.com
----- Original Message -----
From: "Eduard Lascu" <[EMAIL PROTECTED]>
To: "Asn1" <[email protected]>
Sent: Monday, May 16, 2005 10:28 AM
Subject: [ASN1] Tag of class context-specific inserted before each CHOICE
type
>
> In Mr. Dubuisson's book ("ASN.1 - Communication between Heterogenous
> Systems") it is suggested (page 236) that if AUTOMATIC TAGS clause is not
> used in the header module, a tag of class context-specific should be
> inserted before each CHOICE type. The example reads:
>
> T ::= CHOICE { a [0] INTEGER,
> b [1] NULL }
>
> U ::= SET { x [0] REAL,
> y [1] T,
> z [2] CHOICE { c [1] BIT STRING,
> d [2] OCTET STRING } }
>
> If U were a SEQUENCE, I guess the correct definitions would be:
>
> T ::= CHOICE { a [0] INTEGER,
> b [1] NULL }
>
> U ::= SEQUENCE { x [0] REAL,
> y [1] T,
> z [2] CHOICE { c [1] BIT STRING,
> d [2] OCTET STRING } }
>
> Does that mean that when (BER) encoding the object, the choice will have
two
> tags in a row: first to tell that the CHOICE is present within its parent
> SET or SEQUENCE type and the second to indicate which is the chosen
> object/type inside the CHOICE?
>
> Regards,
> Eddie L
>
> Eddie Lascu
> Senior Software Developer, Associate
> IBI Group
> 230 Richmond St. W., 5th Floor
> Toronto ON M5V 1V6
> Tel. (416) 596-1930 x312
> Fax. (416) 596-0644
> www.ibigroup.com
>
>
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