Frequently Asked Questions

1. The type I want to decode is not listed when I specify the -listPduId option. What's wrong?

If the -root command-line option is not specified, then the last (or only) module in this right-most ASN.1 input file is taken as the central ASN.1 module in the specification. So, only this last module's PDUs are listed when -listPduId is specified; ASN.1 definitions from other modules are only read if they are referenced from within this last module.

Additionally, only types that are not used to define other types are considered PDUs by ASN-1Step. If you would like to force a referenced type to be considered a PDU, apply the OSS.PDU directive to it. For example, type A below is used to define type B; thus, A is not considered a PDU. However to force ASN-1Step to consider A as a PDU, you can apply the OSS.PDU directive to it:

--<OSS.PDU ModuleName.A>--

ModuleName DEFINITIONS ::= BEGIN
   B ::= A
   A :: = INTEGER
   a-value A ::= INTEGER
END

Also note that sometimes, due to name conflicts, the actual type name could be altered/mangled (e.g., prepended with the module name).

---

2. The value reference I want to encode is not listed when I specify the -valueRefTable option. What's wrong?

Value references of basic built-in types (e.g., INTEGER, IA5String, REAL, etc.) are not generated in the value reference table. Only value references that are based on user-defined types are there. You can convert value references of basic built-in types to those of user-defined types so they are included in the generated table. For example, if you change:

anInt INTEGER ::= 40

to

MyInt ::= INTEGER
anInt MyInt ::= 40

the anInt value reference will be stored in the value reference table.

Also, if the desired value reference is not in the last input module, then specify the -root option to make sure it is included. For example:

asn1step input1.asn input2.asn input3.asn -root -valueRefTable

You must also specify the -root option when encoding the desired value reference.

Also note that sometimes, due to name conflicts, the actual value reference name could be altered/mangled (e.g., prepended with the module name).

---

3. Is the -noUnique option synonymous with -noUniquePDU?

Yes. Any command-line keyword can be abbreviated as much as you like, so long as the resulting abbreviation is unique. If the resulting abbreviation is not unique, then ASN-1Step will list all the keywords that the abbreviation matches so that you can choose a unique abbreviation.

---

4. When I invoke ASN-1Step on my syntax, I get warning messages about duplicate PDU tags. How do I get rid of these warning messages?

You can try specifying the -noUniquePDU command-line option when ASN.1-compiling your syntax. Additionally, note that these messages are only relevant if you plan to encode/decode your specification using BER or DER.

---

5. Do I have to manually add tags to CHOICE and SET components that have the same ASN.1 type? Or is there a way to automatically add the needed tags?

You can add the AUTOMATIC TAGS keywords to your module definition statement to instruct ASN-1Step to automatically add the needed tags for component differentiation.

The following excerpt illustrates the use of the AUTOMATIC TAGS keywords:

ModuleName DEFINITIONS AUTOMATIC TAGS ::= BEGIN
   ChoiceA ::= CHOICE {
      a INTEGER,
      b INTEGER,
      c INTEGER
   }
END

Note how the CHOICE elements do not have any context-sensitive tags applied to them. ASN-1Step provides all necessary tags when the AUTOMATIC TAGS keywords are present.

---

6. Can I instruct ASN-1Step to encode multiple value references with a single command-line invocation?

Yes: specify multiple -encodeValue options on the same command line. However, if you choose to specify multiple options, refrain from specifying any output filenames. Instead, the output filename will be derived from the value reference that is encoded. Also specify the encoding rules to be used with the -ber, -der, -per, -oer, -coer, and -uper options. For example, the command

asn1step mySyntax2.asn -ber -per -encodeValue myPdu1 -encodeValue myPdu2

will cause the myPdu1 and myPdu2 value references to be both encoded once using BER and once using PER. Four output files will be produced in the above example:

• myPdu1.ber - will contain the BER encoding of the myPdu1 value
• myPdu1.per - will contain the PER encoding of the myPdu1 value
• myPdu2.ber - will contain the BER encoding of the myPdu2 value
• myPdu2.per - will contain the PER encoding of the myPdu2 value

---

7. Is there a way to decode a BER/DER encoding contained in a file without passing the corresponding ASN.1 specification on the command line?

No. However, OSS Nokalva has a separate OSSTLV utility (included with the OSS ASN.1 Tools) that does exactly this. Contact info@oss.com for more information about obtaining OSSTLV.

---

8. Is there a way to decode a PER encoding contained in a file without passing the corresponding ASN.1 specification on the command line?

No. As PER has no tag information coded for it, it is usually not possible to decode it without knowing which ASN.1 specification it corresponds to.

---

9. I have an INTEGER value that ASN-1Step says is too big. Is there a way to make ASN-1Step accept large integers?

Yes. Apply the --<ASN1.HugeInteger>-- directive to the INTEGER type that will take on large values. For example:

--<ASN1.HugeInteger LengthModule.LengthData.lengthOfSolarSystem>--

LengthModule DEFINITIONS ::= BEGIN
	LengthData ::= SEQUENCE {
		lengthOfSolarSystem  INTEGER,
		units  ENUMERATED
				{mm(0), cm(1), m(2), km(3), lightyears(4)} DEFAULT mm
	}
END

---

10. I am not comfortable modifying the ASN.1 source file that I have. Is there a way to add value notation to a separate file and pass both files to ASN-1Step?

Yes. There is no need to modify your ASN.1 source in order to specify values using value notation. Simply add value notation to a separate file and specify it later on the command line. However, you will need to IMPORT the modules containing the types for which you are writing value notation into this last file. For example:

originalASN1.asn:
        D DEFINITIONS ::= BEGIN
                I ::= INTEGER
                B ::= BOOLEAN
        END

myvalues.asn:
        F DEFINITIONS ::= BEGIN
            IMPORTS I FROM D;
            q I ::= 133
            r I ::= -27
        END

Then, invoke ASN-1Step as follows:

asn1step originalASN1.asn myvalues.asn -test

You can also add values to a separate file without enclosing them in an ASN.1 module. For example, use the originalASN1.asn file above and the following:

myvalues.txt:
   q I ::= 133
   r I ::= -27

Then invoke ASN-1Step as follows:

asn1step originalASN1.asn -encodeValue myvalues.txt -ber

First, the 133 value is encoded using the Basic Encoding Rules (BER) and placed at the beginning of the output file. Then, the -27 value is encoded using BER and the result is appended to the end of the output file as a concatenated PDU. The output file will be named myValues.txt.ber by default. Note, the values are specified in the form <valueName> <PduTypeName> ::= <value_notation>. So the PDU identifier can be determined from the value.

You can omit the start of value assignments and add values in the form <value_notation> as follows:

myvalues.txt:
133 -27

Then invoke ASN-1Step as follows:

asn1step originalASN1.asn -encodeValue 1,myvalues.txt -ber

You must specify the PDU identifier, 1, obtained by specifying -listPduIdentifiers, in the -encodeValue option. The output file will be named myValues.txt.ber and contain two concatenated BER encoded values of INTEGER values 133 and -27.

---

11. I use the XML encoding rules. Is there a way to apply a stylesheet to the ASN-1Step output?

Yes. See -xsl for details.

---

12. I use the XML encoding rules. Is there a way to generate a reference to a DTD in the XER output?

Yes. See -dtd for details.

---

13. Is there an easy way to convert a message from one set of encoding rules to another?

Yes. You can specify an encoding rules option along with the -decodePdu option to convert an input encoding into another format. For example, to convert a BER encoding contained in myValue.ber to XER, type:

asn1step input.asn -decodePdu 0 myValue.ber -xer

The output file is named myValue.ber.xer by default. Since the -decodePdu 0 option is implied by default, you can omit it, except when converting from PER. In that case, you must specify the PDU type corresponding to the encoding as an argument to -decodePdu.

---

14. Is there a way to retrieve the ASN.1 value notation corresponding to an encoding?

Yes. Specify the -asn1Value option along with the -decodePdu option to convert an input encoding into its ASN.1 value notation equivalent. For example, to convert a BER encoding contained in myValue.ber to ASN.1 value notation, type:

asn1step input.asn -decodePdu 0 myValue.ber -asn1Value

The output file will be named myValue.ber.txt by default. Since the -decodePdu 0 option is implied by default, you can omit it, except when converting from PER. In that case, you must specify the PDU type corresponding to the encoding as an argument to -decodePdu.

---

15. How can I reduce the number of PDU identifiers printed by -listPduIds?

Specify the -listPduIdentifiers true option to print a subset of all PDU identifiers for unreferenced PDU types and some types with PDU directives. In this case, PDU types that are referenced in information objects or types with contents constraints will not be printed. Use -listPduIdentifiers without the argument to view the full list of PDU identifiers.

---