Encoding begins with a PDU ("Protocol Definition Unit") type which encompasses the messages for the corresponding specification.
Class PDU (com.objsys.nas.TS24301Msgs.PDU) has the following fields:
public SecProtMsgHeader secHdr; // optional
public L3HdrOptions l3HdrOpts;
public ProtoDiscr protoDiscr = null;
public PDU_pti pti;
public _NAS_PROTOCOL_CLASS_msgType msgType;
public Asn1Type data;
public Asn1Null eom;
The secHdr field is for use with security-protected messages, which
are not currently supported for Java. This field will thus not be used.
l3HdrOpts holds the EPS Bearer identity (for
Session Management messages) or the Security header type (for Mobility Management messages)
protoDiscr identifies whether the message is a Mobility Management or
Session Management message. It will be either
ProtoDiscr.epsSessMgmt() or
ProtoDiscr.epsMobMgmt() (these are static enum values).
pti is the procedure transaction identity. It is only
used for Session Management messages, but for Mobility Management messages
it must be set to "no value".
msgType identifies the message type. This,
together with protoDiscr, determines
the actual type of the object in the data field. Class
_TS24301MsgsValuesdefines constants for the various messages (see
code example below).
data holds the message payload. For example, for an Authentication
Request message, it is an AuthRequest object. A lengthy comment in PDU.java identifies the
Java type that should be in this field for each {protocol discriminator, message type} pair.
eom is not used. It is there as a result of the techniques used to
model these non-ASN.1 messages in a way ASN1C can work with.
The general procedure to encode a message is as follows:
Create an instance of the generated PDU type (e.g.
com.objsys.nas.TS24301Msgs.PDU) and the
specific message type to be sent (e.g.
com.objsys.nas.TS24301Msgs.AuthRequest).
Populate the types. For the PDU, set the header fields described above. Set the message object into the PDU's data field.
Initialize the encode buffer.
Call the PDU encode function
Get the message from the buffer to work with the binary message.
An example with some comments follows. This example is based on the AuthRequest sample.
/* Create the PDU and Message objects */
com.objsys.nas.TS24301Msgs.PDU pdu = new PDU();
com.objsys.nas.TS24301Msgs.AuthRequest data = new AuthRequest();
/* Populate data structure */
pdu.protoDiscr = ProtoDiscr.epsMobMgmt();
pdu.pti = new PDU_pti(PDU_pti._NOVALUE, null);
pdu.l3HdrOpts = new L3HdrOptions();
pdu.l3HdrOpts.set_secHdr(new Asn1Integer(0));
pdu.msgType = new _NAS_PROTOCOL_CLASS_msgType(_TS24301MsgsValues.mt_AuthRequest);
pdu.data = data;
...
/* Create a message buffer object */
Asn1NasEncodeBuffer encodeBuffer = new Asn1NasEncodeBuffer();
/* Encode the PDU into the buffer */
pdu.encode (encodeBuffer);
/* Write message to a file. You could also use encodeBuffer.getMsgCopy()
to get the message into a byte array.
*/
encodeBuffer.write (new FileOutputStream (filename));
Some general tips for encoding:
Fields that are not optional must be assigned or a NullPointerException will occur. In particular, integer and boolean types are represented using objects (Asn1Integer and Asn1Boolean), so even fields having 0 or false as their value need to be assigned explicitly.
Length fields will generally be automatically computed and encoded, but this
is not true in all cases. Fields marked with
--<is3GLength/> in the ASN.1 specification files
will be automatically computed. If in doubt, you can look at the generated
encode function or experiment to determine whether a given field is
automatically computed or not.