How to distinguish optional IEs with the same type in ANSI-MAP message TransferToNumberRequestRes

Question

Specially, this message has got two IEs with the same type, and they both are optional:

-- TransferToNumberRequest RETURN RESULT Parameters
TransferToNumberRequestRes ::= [PRIVATE 18] SET {
    digits-Destination                      [4] IMPLICIT Digits OPTIONAL,
    -- (Destination) M 6.5.2.58 a
...
    -- O 6.5.2.25 e, f
    digits-Carrier                          [4] IMPLICIT Digits OPTIONAL,
...
    }

Given the nature of ASN.1 set type, the question is:

If there's only one IE with the tag value 4 appear in a BER-encoded PDU buffer, how can we judge its actual type (digits-Destination or digits-Carrier) ?

Thanks for any of your answers.

Answer 1

If you look at the document ref ("M 6.5.2.58 a") the first parameter is Mandatory in the specification. I don't remember if I had to make it optional in Wirehark for it to work properly or if that's a bug. This probably works for Wireshark as the tags are in a table and the mandatory parameter is found first. It might not work if there are 3 tags with the value of 4. I'm not sure to what extent ANSI MAP actually adhere to ASN1 specifications I haven't found a complete ASN1 document as you can for GSM MAP. The specification might say that each parameter may only occure once as well I suppose.