On Wed, 19 Nov 2008, pradeep kashyap s.a wrote:
Hello,
I hav a few questions concerning the ASN.1 syntax. Consider the following
record structure below:
PosPayLoad ::= CHOICE {
tia801payload OCTET STRING(SIZE (1..8192)),
rrcPayload OCTET STRING(SIZE (1..8192)),
rrlpPayload OCTET STRING(SIZE (1..8192)),
...}
I have (4) questions........
(1) Will the CHOICE{} let me select any one among the three ?? i.e. does it
act like an enumumerated{} ?? I suppose it should be yes but want to
confirm.
On the encoding side, you may select any one of the three possibilities.
On the decoding side, you can accept any one of the three, plus (by virtue
of the extensibility marker "...") you can accept yet unknown
possibilities.
For instance, if your side implemented version 1 of your protocol and the
sending side implemented version 2 of the protocol which includes five
possibilities, you would be able to accept them all, both the three you
know about and the two you don't know about. This is not to say that you
can make sense of the two you don't know about. By including the
extensibility marker, you are specifying that the decoder is supposed to
accept unknown choices as valid.
(2) Say I want to use/send only the rrcPayload in the above PosPayload,
which is defined as an OCTET STRING(SIZE (1..8192)), does that mean that it
contains only string bits of 0's and 1's encoded in Octet format of
unaligned PER (I want to use PER in my case)?? I ask this because the
rrcPayload that I want to send is in another document which is written as
below (not in the ASN.1 syntax)
C/A or P on L2 Mandatory Bit string(2)
URA Index Mandatory Bit string(4)
and many more parameters.........so do I have to write this as another ASN.1
syntax record structure and include it within that or do I have to do it
separately and include only the octet string bits there? For example I could
write a record structure for the above parameters as
RRCNavModel::=SEQUENCE{
C/A or P on L2 BIT STRING(SIZE(1..2))
URA Index BIT STRING(SIZE(1..4))
}
Bear in mind that if you specify a field as an OCTET STRING, it has to be
a multiple of 8 bits. That is, you might have to pad out your field if it
is not an integral number of octets. That said, you may insert any payload
into the OCTET STRING, leaving it specified simply as an OCTET STRING. On
the other hand, if you wish to work with the individual fields within the
payload, you would specify them with ASN.1 (assuming they are able to be
specified in ASN.1).
(3) I have a related question here.........does the BIT STRING(SIZE(1..4))
mean that I have a string of 4 bits??......because that is what I want. If
not please correct me.
No. It means the string is no less than 1 nor more than 4 bits. If you
wish to specify that it must have 4 bits, you would use something like
BIT STRING (SIZE(4))
Also I suppose the answer for the 2nd question would be that I have to
include only the OCTET STRING for the rrcPayload.
(4) Again the OCTET STRING(SIZE (1..8192)) means that I can have anything
upto 8192 OCTET strings right???
Not quite. You may not have a null string. If you wish to also allow null
strings, you would use (0..8192).
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