Automatic TTCN3 tests for frequency hopping support in BSC
We currently have no testing at all whether or not the BSC encodes the various "Channel Description IEs" as expected when frequency hopping is in use.
We know it worked once a decade ago when we first implemented it with Siemens BTS, but that's a long time ago.
tnt recently reported he had frequency hopping active on an Ericsson BTS driven by OsmoBSC, but only signalling up to Location Update, no voice channels in use. Also, that is just a single manual test and not an automatic test.
Let's try to cover this part of the BSC codebase with TTCN-3 tests.
- Status changed from New to In Progress
- % Done changed from 0 to 10
I did a quick test, and according to Wireshark (OML dialect is set to ip.access), osmo-bsc encodes a malformed ARFCN list in Set Channel Attributes message. At first glance, looks like a wrong endianness problem:
IPA protocol ip.access, type: OML DataLen: 27 Protocol: OML (0xff) GSM A-bis OML, Radio Channel(00,00,06) Set Channel Attributes Message Discriminator: Formatted O&M (0x80) Placement Indicator: Only (0x80) Sequence Number: 0x00 Length Indicator: 9 FOM Message Type: Set Channel Attributes FOM Object Class: Radio Channel (0x03) FOM Object Instance BTS: 0 FOM Object Instance TRX: 0 FOM Object Instance TS: 6 FOM Attribute ID: Channel Combination FOM Attribute Length: 1 Channel Combination: SDCCH (0x03) FOM Attribute ID: HSN FOM Attribute Length: 1 HSN: 1 FOM Attribute ID: MAIO FOM Attribute Length: 1 MAIO: 0 FOM Attribute ID: ARFCN List FOM Attribute Length: 2051 ARFCN: 26371 ARFCN: 26883 ARFCN: 27395 ARFCN: 27968 [Malformed Packet: A-bis OML] [Expert Info (Error/Malformed): Malformed Packet (Exception occurred)] [Malformed Packet (Exception occurred)] [Severity level: Error] [Group: Malformed]
At first glance, looks like a wrong endianness problem
Actually, 3GPP TS 12.21 defines coding of 'ARFCN List' as TL16V, where where L16 is the length of V. However, BS-11 treats it as TLV, or rather TCV, where C is the amount of ARFCNs in V. I wrote a fix that makes 'ARFCN List' look good (as per 3GPP TS 12.21) in Wireshark. Unfortunately, I cannot test it against the real hardware, because neither I have a BS-11 nor any Ericsson BTS.
https://gerrit.osmocom.org/c/osmo-bsc/+/18565 abis_nm: fix ARFCN list encoding in Set Channel Attributes
tnt, if you have time, could you please test if it does not break your setup with hopping? Thanks in advance!
Ericsson BTS use OM2k, not OML.
Ah, right. Now I see that osmo-bsc has a separate implementation for OM2k.
Nokia BTS use their own stuff as well, not standard 12.21 OML either.
Yep. And unlike OM2k, its implementation is mixed with generic 12.21 OML :/
Thanks for your feedback!