InputStream inputStream = client.getInputStream();
while(true) {
    CDR_HEADER header = asn1Decoder.decode(inputStream, CDR_HEADER.class);

    byte payload[] = null;
    if (header.isHeaderVersion2Selected()) {
        HeaderVersion2 hv2 = header.getHeaderVersion2();
        payload = hv2.getCdrData();
    } else if (header.isHeaderVersion1Selected()) {
        HeaderVersion1 hv1 = header.getHeaderVersion1();
        payload = hv1.getCdrData();
    }

    ByteArrayInputStream inputBuffer = new ByteArrayInputStream(payload);
    CDR_PAYLOAD pl = decoder.decode(inputBuffer, CDR_PAYLOAD.class);
    if(pl.isASelected()) {
        A_INTERFACE_CDR_VERSION7 a = pl.getA();
        ...
    } else if(pl.isGbSelected()) {
        GPRSGB_XDR_VERSION4 gb = pl.getGb();
        ...
    } else ...

}