cat: add sms_pp_download_envelope()

Put a helper for wrapping an SMS-DELIVER TPDU in the ENVELOPE
of TS 102 223 section 7.5.1 the assembly next to SMSPPDownload,
where the IEs already live, so it can be reused by other tooling,
for example for triggering scp81 sessions.

Change-Id: Id23227eac53d697f4f13a84e087c32bf1d60f474

pySim-smpp2sim.py

@@ -55,6 +55,7 @@ from pySim.transport import LinkBase, ProactiveHandler, argparse_add_reader_args
 from pySim.commands import SimCardCommands
 from pySim.cards import UiccCardBase
 from pySim.exceptions import *
+from pySim.cat import sms_pp_download_envelope
 from pySim.cat import ProactiveCommand, SendShortMessage, SMS_TPDU, SMSPPDownload, BearerDescription
 from pySim.cat import DeviceIdentities, Address, OtherAddress, UiccTransportLevel, BufferSize
 from pySim.cat import ChannelStatus, ChannelData, ChannelDataLength
@@ -169,10 +170,7 @@ class MyServer:
         tpdu = SMS_DELIVER.from_smpp_submit(pdu)
         logger.info(tpdu)
         # 2) wrap into the CAT ENVELOPE for SMS-PP-Download
-        tpdu_ie = SMS_TPDU(decoded={'tpdu': b2h(tpdu.to_bytes())})
-        addr_ie = Address(decoded={'ton_npi': {'ext':False, 'type_of_number':'unknown', 'numbering_plan_id':'unknown'}, 'call_number': '0123456'})
-        dev_ids = DeviceIdentities(decoded={'source_dev_id': 'network', 'dest_dev_id': 'uicc'})
-        sms_dl = SMSPPDownload(children=[dev_ids, addr_ie, tpdu_ie])
+        sms_dl = sms_pp_download_envelope(tpdu)
         # 3) send to the card
         envelope_hex = b2h(sms_dl.to_tlv())
         logger.info("ENVELOPE: %s" % envelope_hex)