Voice audio testing in osmo-gsm-tester

historical

Dear all,

osmo-gsm-tester is currently capable of setting up an entire network
with one or more BTSs, Modems, NITB, etc. and test signaling + SMS.

Voice testing has always been on the wishlist, but further down on the
TODO list.  One issue here is that

a) many modems don't do voice at all (data-only modems for laptops)
b) some modems expose voice only as analog audio (difficult to interface,
   would require custom hardware next to the modem, e.g. using USB
   soundcard, calibrate the levels, ...)
c) some modems expose the voice as PCM bus.  Similarly, would require
   some external codec chip and/or USB soundcard or the like, plus a
   custom circuit
d) some modems expose voice as "GSM codec frames over UART" or other
   highty proprietary formats
e) some modems expose voice as USB audio device.  Most convenient, but
   only found in some Qualcomm LE based devices such as Sierra Wireless
   or Quectel.

Over the weekend I was thinking of yet another method to make this much
simpler:  Every phone is supposed to include a voice loop-back mode.  In
this mode, the phone siply loops back all voice frames received in the
downlink and sends them back in the uplink.  This functionality is
mandatory by the spec, and used to test the receiver performance of the
phone during development, manufacturing and service. IT is specified in
3GPP TS 44.014
(http://www.etsi.org/deliver/etsi_ts/144000_144099/144014/14.00.00_60/ts_144014v140000p.pdf)
which used to be GSM TS 04.04
(http://www.etsi.org/deliver/etsi_ts/101200_101299/101293/08.06.00_60/ts_101293v080600p.pdf)
before.

The idea is that one puts a special "Test SIM" (as specified in TS
51.010-1 Annex 4, where EF.AD first byte == 0x80 is the criteria in this
context) into the phone, and then sends some specific commands on Layer3
to activate the loop.

So if we can activate that loop-back functionality from the Osmocom
stack, we could test if we get back the same voice data that we send in
downlink (minus some occasional bit error, but those should be super low
given we're operating omso-gsm-tester over coaxial cabling between MS and
BTS).

Regards,
	Harald

-- 
- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
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