use "Direct TLLI" method to confirm IMMEDIATE ASSIGNMENT messages towards PCU
To assign downlink TBFs, the PCU sends an IMMEDIATE ASSIGNMENT MAC block through the PCU SOCK interface to the BTS. The BTS confirms the sending of this IMMEDIATE ASSIGNMENT to the PCU by sending the MAC block back to the PCU. The MAC block essentially serves as a reference for the confirmation message. This method has some disadvantages so we decided to replace it.
The new method attaches a TLLI (and a paging group field) to the MAC block when it send to the BTS. The BTS then uses the TLLI instead of the MAC block as an identifier to confirm the sending of the IMMEDIATE ASSIGNMENT. The OsmoPCU already supports the method but OsmoBTS still have to be upgraded.
The related PCU/BTS TTCN3 tests also require an update.
Updated by fixeria 22 days ago
A patch for osmo-bts can be found here:
https://gerrit.osmocom.org/c/osmo-bts/+/31600 pcuif_proto: version 11: rework sending IMM ASS over PCH [WIP]
Some highlights (proposed changes):
PCU_IF_MSG_DATA_CNF_DT, there is already
PCU_IF_MSG_DATA_CNFwhich already contains the TLLI;
PCU_IF_SAPI_AGCH_DT, DATA.req with
PCU_IF_SAPI_PCHcontains the TLLI now;
struct gsm_pcu_if_data_cnf: remove meaningless fields like
arfcn(why would they be present in CNF?!?);
struct gsm_pcu_if_data_cnf: add padding after
sapito facilitate aligned access;
struct gsm_pcu_if_pch_data: new structure, which is included in DATA.req with
struct gsm_pcu_if_pch_data: this structure carries
IMSI % 1000(needed for paging group calculation);