Bug #4501
closedosmo-bts-{sysmo,oc2g,litecell15}: Packet Control Ack (in form of Access Bursts) arrives on PRACH?!?
100%
Description
As was reported by keith (originally in #1526), with 'gprs control-ack-type-rach' enabled in the BSC, osmo-pcu logs:
DL1IF <0001> ../../git/src/osmo-bts-sysmo/sysmo_l1_if.c:251 Rx PH-RA.ind for unknown L1 SAPI PRACH
this is a regression introduced in I482d60a46b9d253dfe0b16140eac9fea6420b30c:
Parent: 17954da5 (pcuif_proto.h: extend RACH.ind with TRX / TS numbers) Author: Vadim Yanitskiy <axilirator@gmail.com> AuthorDate: 2019-10-05 23:45:31 +0700 Commit: Vadim Yanitskiy <axilirator@gmail.com> CommitDate: 2019-11-23 17:42:45 +0700 PTCCH: properly handle RACH.ind for PCU_IF_SAPI_PTCCH Change-Id: I482d60a46b9d253dfe0b16140eac9fea6420b30c Related: OS#1545
I would expect Packet Control Ack on GsmL1_Sapi_Pdtch or rather on GsmL1_Sapi_Pacch, but apparently (for some magic reason) it arrives on GsmL1_Sapi_Prach.
According to 3GPP TS 45.002, section 3.3.3.2.1:
- PRACH stands for Packet Random Access CHannel,
- used to request assignment of one or several PDTCHs,
- belongs to Packet Common Control Channel (PCCCH)!
Therefore the Packet Control Ack has nothing to do with PRACH. Neither we support PCCCH (it has been deprecated by the specifications).
GsmL1_LogChComb_XIII ///< PDTCH/F + PACCH/F + PTCCH/F
This odd behaviour seems to be caused by the following code (see #4500):
static const struct sapi_dir pdtch_sapis[] = {
{ GsmL1_Sapi_Pdtch, GsmL1_Dir_TxDownlink },
{ GsmL1_Sapi_Pdtch, GsmL1_Dir_RxUplink },
{ GsmL1_Sapi_Ptcch, GsmL1_Dir_TxDownlink },
{ GsmL1_Sapi_Prach, GsmL1_Dir_RxUplink }, // <-- ?!?
#if 0
{ GsmL1_Sapi_Ptcch, GsmL1_Dir_RxUplink },
{ GsmL1_Sapi_Pacch, GsmL1_Dir_TxDownlink }, // <-- ?!?
#endif
};
where we enable GsmL1_Sapi_Prach on GsmL1_LogChComb_XIII (again, there is no PRACH in XIII). I guess we should enable GsmL1_Sapi_Pacch on direction GsmL1_Dir_RxUplink instead.
For now I submitted a blind fix: https://gerrit.osmocom.org/c/osmo-pcu/+/17669. Let's keep it WIP for now.
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