Cellular Network Infrastructure » OsmoMGW

some updates.

• I.460 multiplex doing 16k but also 8k sub-slots implemented in https://gerrit.osmocom.org/c/libosmocore/+/18247 (merged)
• new API for TRAU frame parsing/encoding: https://gerrit.osmocom.org/c/libosmo-abis/+/18249 (WIP)
• new code for TRAU frame synchronization: https://gerrit.osmocom.org/c/libosmo-abis/+/18250/5 (WIP)
• new code for TRAU frame to RTP conveersion: https://gerrit.osmocom.org/c/libosmo-abis/+/18382/3 (WIP)

Build verification of all patches passes now. Next would be some kind of verification.

I'm planning to fire up my good old BS-11 tomorrow, set up a call and do a capture of at least FR and EFR on 16k TRAU sub-slots. This test data can then be used for an end-to-end test through the full chain of I.460, trau_sync, trau_parse, trau2rtp and the resulting RTP played back for verification.

I've just pushed an update to the laforge/trau branch of libosmo-abis, which contains the new code modules for E1 / TRAU usage, and also a small test program and test data.

The test program takes a raw binary dump created from an E1 timeslot with TRAU frames captured of a BS-11 and converts that to RTP packets. You can feed those RTP packets into osmo-gapk to play them back. The audio is clear, there are just a few seconds of quiet at the beginning before anything is audible.

The patches also add an "I460" mode to e1_input. So from the osmo-mgw side,
I think we need to

• link against libosmo-abis and call e1inp_init + e1inp_vty_init()
• use existing VTY code to configure E1 line numbers / drivers / ports
• call e1inp_ts_config_i460() on every timeslot we want to use (this should not happen on all timeslots, as some of them are used for signalling by the BSC)
• call osmo_i460_subchan_add() / osmo_i460_subchan_del() for each i.460 sub-slot we want to use
• build the processing chain like in libosmo-abis/contrib/trau2rtp/trau2rtp.c via i460 -> trau_sync -> osmo_trau_frame_decode_16k/osmo_trau_frame_decode_8k -> osmo_trau2rtp

I believe this should be at a stage where it can be integrated into osmo-mgw. The API may not be 100% stable yet, but it shouldn't change completely. The main missing bits are:

• allow selection of sync pattern in trau_sync.c (or auto-detect?)
• handling of T-bits for alignment of air-interface timing with downlink TRAU frame timing

I will next be hacking on a small tool that will speak the 'osmo-e1d' protocol but use binary capture files that are looped over and over again. This way you should be able to use libosmo-abis e1_input without any physical card and
still get TRAU frame data.

At a lower priority, I'd like to look into support for HRv1 and AMR TRAU frames on 16k sub-slots.

my most recent work has been in osmo-e1d, adding support for a "virtual E1 interface pair".

In e1d language, an "interface" can have multiple lines (e.g. a 4-port interface would have 4 E1 lines, each having 32 timeslots).

The so-called "vpair" driver for osmo-e1d will create two virtual E1 interfaces with each an identical number of lines. The lines will be mapped 1:1 and provide something like a "virtual E1 loopback cable". So you can have two applications written for talking to E1 lines (e.g. via libosmo-abis) but without any hardware and a physical cable in between.

I've also written a osmo-e1d-pipe command line program which can be used to connect stdin/stdout of the program with an E1 timeslot. It has the capability to read a file containing a raw timeslot dump and transmit that via a E1 timeslot. It will loop after EOF.

So putting things together, we now can build a system where a capture (such as my 16k TRAU captures for FR and EFR taken on a BS-11) can be played back into one side of a vpair, and the other side can be opened by software (the future osmo-mgw) an it will resceive an endless loop of recorded TRAU frames for testing.

Likewise, we can use this as a vehicle in our TTCN3 tests, where the TTCN3 test would need to get a test port that talks to osmo-e1d in order to send TRAU frames or other E1 streams to the IUT (osmo-mgw, osmo-bsc).

I'll put some documenation to the osmo-e1d project wiki.