Bug #6078
closedSI1 rest octests indicated "Band Indicator: 1900" on a 850MHz BTS
100%
Description
I've configured osmo-bsc/bts with a 850MHz cell on ARFCN 136. I then try to register to it from osmocom-bb/mobile via trxcon and get:
20230628205120086 DL1C NOTICE trxcon(0)[0x61200004f720]{BCCH_CCCH}: L1CTL_DM_EST_REQ indicates single ARFCN PCS1900 136 (l1ctl.c:569)
20230628205120086 DTRXC ERROR trx_interface(0)[0x61200004f8a0]{RSP_WAIT}: ARFCN 32904 not defined (trx_if.c:321)
Some research shows that mobile will do something like "ARFCN 136 | PCS" because SI1 rest octests contains "Band Indicator: 1900" for this cell.
Irrespective of any OsmocomBB bugs, it seems wrong for osmo-bsc to encode this in SI1RO.
Related issues
Updated by laforge 5 months ago
Looking at osmo-bsc source shows there is
static int is_dcs_net(const struct gsm_bts *bts)
{
if (bts->band == GSM_BAND_850)
return 0;
if (bts->band == GSM_BAND_1900)
return 0;
return 1;
}
which seems outright wrong, at least in the context of the SI1 rest octets. If the cell is 850 MHz, is_dcs_net returns 'true' and the SI1RO will contain "Band: 1900".
SI1RO only exists to differentiate 1900(PCS) from DCS. 850 MHz shouldn't relate to it at all.
Updated by laforge 5 months ago
TS 45.005 states:
A multi-band MS shall interpret ARFCN numbers 512 to 810 as either DCS 1800 or PCS 1900 frequencies according to the parameter BAND_INDICATOR when received in other than the DCS 1800 or PCS 1900 bands. If received in the DCS 1800 or PCS 1900 bands, those ARFCN numbers shall be interpreted as frequencies in the same band. The BAND_INDICATOR is broadcast on BCCH, PBCCH and SACCH. The most recently received value shall be applied by the mobile station. If the parameter is not broadcast, the default value is DCS 1800 frequencies.
So while it is odd for osmo-bsc to encode band_indicator=1900 when operating a 850MHz BTS, it is actually in itself not yet a bug. The bug would only happen once that BTS tries to broadcast 1800 MHz neighbor ARFCNs or the like.
The bigger bug is hence in osmocom-bb/mobile, using that flag for ARFCN numbers < 512 or > 810.
