Cellular Network Infrastructure » Radio Access Network » OsmoBTS

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.