Cellular Network Infrastructure » Radio Access Network » OsmoBTS

Should I implement static or dynamic variant? What should happen if new value for "channel-descrption bs-ag-blks-res" is entered in OpenBSC vty?

Static variant sent for review as gerrit #1047. Not strictly speaking required, but handy for debugging is extended logging from gerrit #1043.

Preliminary variant sent for review in gerrit #1099. Missing bits:
- it's unclear how/if octphy support different number of AGCH (equivalent of lch_par->agch.u8NbrOfAgch)
- it's unclear if/how we have to activate/deactivate lchan for osmo-trx (equivalent of sapi_deactivate_cb())

Current implementation is somewhat ugly because we 1st unconditionally activate CCCH before knowing SI3 with proper number of AGCH. When we finally receive AGCH value from SI3 we deactivate channel and than activate it again. osmo-bts-trx seems to be different in this regard - maybe we do not have to use reactivation hack in there. Also, once this early auto-activation hack is removed we can streamline this code.

Support for lc15 and sysmo has been merged.
Octphy: waiting for vendor response.
Osmo-trx: waiting for vendor response.

I guess you need to follow-up with Octphy and Osmo-TRX folks regularly to make sure this doesn't stay stalled forever.

Related gerrit 3067 has been sent for review.

patches in gerrit under review, not stalled.

The octphy support requires vendor cooperation.
The virtphy support is planned. Might make sense to add corresponding TTCN-3 test if time permits.

please add it to -virtual and extend the TTCN-3 test to cover it.

I've dropped the OCTPHY topic. I presume you had inquired them regarding this at the time you were working on the code? If not, please make sure they are aware we're waiting for them to add support to the PHY.

I presume you had inquired them regarding this at the time you were working on the code?

Yes, twice actually. Although it's been a while ago so I'm pretty sure they forgot about it by now. Dexter, can you please ping them again? The question is basically "how do we instruct octphy l1 that it should use X AGCH channels (as in 3GPP TS 45.002 §3.3.2.3 a)BS_AG_BLKS_RES)?" or "is that supported by some fw version already?".

I wrote Jason an email, maybe he can give us more information about this or even point us to the right places in the documentaion/header files.

Regarding Octphy: The response was that there is no different L1 SAPI between AGCH and PCH, so we don't ned to inform/instruct the PHY about where higher layers put the split.

Instead, both AGCH and PCH are leading to PH-RTS.ind of type cOCTVC1_GSM_SAPI_ENUM_PCH_AGCH. AFAICT, in osmo-bts-octphy/l1_if.c:chan_nr_by_sapi() we map this to cbits=0x12 which is "Downlink CCCH (AGCH/PCH)". So it "should simply work".

I've pushed a change Ic1038b8dc57bdaf05493cd8479355b960275ea41 that simply removes the related warning, see https://gerrit.osmocom.org/5148

osmo-bts-virtual (and related test case) thus remains the only open tasks here. Please get that done.

I have figured out how the paging and access grant channels should be laid out. I hope my understanding is correct here:

=====================================================
== TDMA frame mapping for FCCH + SCH + BCCH + CCCH ==
=====================================================

F = FCCH
H = SCH
B = BCCH
C = CCCH
D = SDCCH
S = SACCH
I = Idle

A = AGCH
P = PCH

          1         2         3         4         5
012345678901234567890123456789012345678901234567890
            v            v            v            

FHBBBBCCCCFHCCCCCCCCFHCCCCCCCCFHCCCCCCCCFHCCCCCCCCI
      0000  11112222  33334444  55556666  77778888  (B0 ... B8)
      PPPP  PPPPPPPP  PPPPPPPP  PPPPPPPP  PPPPPPPP  BS_AG_BLKS_RES = 0
      AAAA  PPPPPPPP  PPPPPPPP  PPPPPPPP  PPPPPPPP  BS_AG_BLKS_RES = 1
      AAAA  AAAAPPPP  PPPPPPPP  PPPPPPPP  PPPPPPPP  BS_AG_BLKS_RES = 2
      AAAA  AAAAAAAA  PPPPPPPP  PPPPPPPP  PPPPPPPP  BS_AG_BLKS_RES = 3
      AAAA  AAAAAAAA  AAAAPPPP  PPPPPPPP  PPPPPPPP  BS_AG_BLKS_RES = 4
      AAAA  AAAAAAAA  AAAAAAAA  PPPPPPPP  PPPPPPPP  BS_AG_BLKS_RES = 5
      AAAA  AAAAAAAA  AAAAAAAA  AAAAPPPP  PPPPPPPP  BS_AG_BLKS_RES = 6
      AAAA  AAAAAAAA  AAAAAAAA  AAAAAAAA  PPPPPPPP  BS_AG_BLKS_RES = 7

=======================================================================================
== TDMA frame mapping for FCCH + SCH + BCCH + CCCH + SDCCH/4(0...3) + SACCH/4(0...3) ==
=======================================================================================

F = FCCH
H = SCH
B = BCCH
C = CCCH
D = SDCCH
S = SACCH
I = Idle

A = AGCH
P = PCH

          1         2         3         4         5         6         7         8         9         0
012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901
            v            v            v            v            v            v            v           

FHBBBBCCCCFHCCCCCCCCFHDDDDDDDDFHDDDDDDDDFHDDDDDDDDIFHBBBBCCCCFHCCCCCCCCFHDDDDDDDDFHDDDDDDDDFHDDDDDDDDI
      0000  11112222                                     0000  11112222                                 (B0 ... B2)
      PPPP  PPPPPPPP                                     PPPP  PPPPPPPP                                 BS_AG_BLKS_RES = 0
      AAAA  PPPPPPPP                                     PPPP  PPPPPPPP                                 BS_AG_BLKS_RES = 1
      AAAA  AAAAPPPP                                     PPPP  PPPPPPPP                                 BS_AG_BLKS_RES = 2

Example #1:
FN: MOD102:  BLOCK:
165 63         B1
216 12         B1
220 16         B2
267 63         B1
271 67         B2
318 12         B1
322 16         B2
369 63         B1
372 66         B2
420 12         B1
424 16         B2
==> B0 is never used for paging ==> BS_AG_BLKS_RES = 1

Example #2:
FN: MOD102:  BLOCK:
573 63         B1
577 67         B2
624 12         B1
628 16         B2
675 63         B1
679 67         B2
726 12         B1
730 16         B2
777 63         B1
828 12         B1
832 16         B2
==> B0 is never used for paging ==> BS_AG_BLKS_RES = 1

I did an experiment. I have set BS_AG_BLKS_RES = 2 via the osmo-bsc VTY and ran BTS_Tests.TC_paging_imsi_200percent. The result looks like BS_AG_BLKS_RES = 1, so there is indeed something wrong here.

At least for the osmo-bts-trx/faketrx/trxcon case I can say that it works now. The reason why it did not work last time is that the setting for bs_ag_blks_res is quite confusing. It is not set via OML, so setting it via osmo-bsc.cfg does not change anything. The parameter must be changed in BTS_tests.ttcn as well at two places (for the test and for the SI3). Once this is changed correctly osmo-bts receives the parameter correctly and behaves accordingly.

For the ttcn3 test I have simplified the things a bit: https://gerrit.osmocom.org/10707

I have added now a unit-test. We see the expected behavior for all possible bs_ag_blks_res settings, which is very good.

https://gerrit.osmocom.org/#/c/osmo-bts/+/10724 paging: add unit-test to check different bs_ag_blks_res settings

I have also checked the support for osmo-bts-virtual with an experiment. I have set different values for bs_ag_blks_res, started osmo-bts-virtual and observed the frame number of the emitted GSMTAP frames. The frame numbers of the paging channel (dummy pagings) all fell into the correct range. I have tested with bs_ag_blks_res=1 and bs_ag_blks_res=2.

Since the decision when to send pagings to the phy is made in l1sap.c:l1sap_ph_rts_ind() it is common code we can be very sure that the behavior of osmo-bts-virtual will not be different from other BTS. So I think we can conclude that bs_ag_blks_res is correctly handled in osmo-bts-virtual.

To increase the test coverage a bit I have added a check to BTS_Tests.ttcn that checks if the pagings on L1CTL actually fall into the PCH and not in the AGCH or even BCCH. We currently only check with bs_ag_blks_res=1. See also:

https://gerrit.osmocom.org/#/c/osmo-ttcn3-hacks/+/10726 BTS_Tests: check paging channel fn (bs_ag_blks_res)

While osmo-bts-trx seems to work just fine inside the TTCN3 tests we still have problems. When I set

bts 1
 channel-descrption bs-ag-blks-res 2

The phone does not register anymore. So I think we have another problem that is related to bs-ag-blks-res. Here is a sample from the log:

Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802121/1359/09/36/33 Too many contiguous elapsed fn, dropping 297
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802142/1359/04/06/02 Too many contiguous elapsed fn, dropping 49
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802144/1359/06/08/04 Too many contiguous elapsed fn, dropping 420
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802175/1359/11/39/35 Too many contiguous elapsed fn, dropping 31
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802205/1359/15/18/13 Too many contiguous elapsed fn, dropping 596
Fri Aug 31 17:43:58 2018 <0000> rsl.c:2672 (bts=0,trx=0,ts=0,ss=4)(NONE) is not active . Dropping message.
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802207/1359/17/20/15 Too many contiguous elapsed fn, dropping 32
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802225/1359/09/38/33 Too many contiguous elapsed fn, dropping 584
Fri Aug 31 17:43:58 2018 <0007> scheduler.c:877 1802285/1359/17/47/45 Too many contiguous elapsed fn, dropping 143
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802337/1359/17/48/45 Too many contiguous elapsed fn, dropping 130
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802339/1359/19/50/47 Too many contiguous elapsed fn, dropping 984
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802352/1359/06/12/08 Too many contiguous elapsed fn, dropping 231
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802426/1359/02/35/30 Too many contiguous elapsed fn, dropping 74
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802446/1359/22/04/50 Too many contiguous elapsed fn, dropping 447
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802460/1359/10/18/12 Too many contiguous elapsed fn, dropping 255
Fri Aug 31 17:43:59 2018 <0000> rsl.c:2672 (bts=0,trx=0,ts=0,ss=4)(NONE) is not active . Dropping message.
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802533/1359/05/40/37 Too many contiguous elapsed fn, dropping 87
Fri Aug 31 17:43:59 2018 <0007> scheduler.c:877 1802535/1359/07/42/39 Too many contiguous elapsed fn, dropping 109
Fri Aug 31 17:44:00 2018 <0007> scheduler.c:877 1802621/1359/15/26/21 Too many contiguous elapsed fn, dropping 747
Fri Aug 31 17:44:00 2018 <0007> scheduler.c:877 1802655/1359/23/09/03 Too many contiguous elapsed fn, dropping 195
Fri Aug 31 17:44:00 2018 <0007> scheduler.c:877 1802712/1359/02/15/08 Too many contiguous elapsed fn, dropping 57
Fri Aug 31 17:44:00 2018 <0000> rsl.c:2672 (bts=0,trx=0,ts=0,ss=4)(NONE) is not active . Dropping message.
Fri Aug 31 17:44:00 2018 <0007> scheduler.c:877 1802744/1359/08/47/40 Too many contiguous elapsed fn, dropping 459
Fri Aug 31 17:44:00 2018 <0007> scheduler.c:877 1802748/1359/12/00/44 Too many contiguous elapsed fn, dropping 411
Fri Aug 31 17:44:01 2018 <0007> scheduler.c:877 1802787/1359/25/39/35 Too many contiguous elapsed fn, dropping 75
Fri Aug 31 17:44:01 2018 <0007> scheduler.c:877 1802828/1359/14/29/24 Too many contiguous elapsed fn, dropping 80
Fri Aug 31 17:44:01 2018 <0007> scheduler.c:877 1802902/1359/10/01/46 Too many contiguous elapsed fn, dropping 677
Fri Aug 31 17:44:01 2018 <0007> scheduler.c:877 1802938/1359/20/37/30 Too many contiguous elapsed fn, dropping 194

I run the following channel combinations:

   timeslot 0
    phys_chan_config CCCH+SDCCH4
    hopping enabled 0
   timeslot 1
    phys_chan_config SDCCH8
    hopping enabled 0
   timeslot 2

bs-ag-blks-res = 2 should be a legal setting, even if I would only use CCCH+SDCCH4 with no SDCCH8. I have tested on a sysmobts as well, there it worked, but the image on that BTS is not very recent.