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tmo/CSD-tests: document Experiment 7
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 28 Nov 2024 02:53:59 +0000
parents ed314cc25b8d
children
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71:ed314cc25b8d 72:afebef67e8d4
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225 Thus we now know that NT mode working or not is not a question of firmware 225 Thus we now know that NT mode working or not is not a question of firmware
226 version, but some kind of law of chance: time of day, space weather, phase of 226 version, but some kind of law of chance: time of day, space weather, phase of
227 the moon... 227 the moon...
228 228
229 Experiment 7
230 ============
231
232 Earlier in Experiment 4 I observed that T-Mobile network does not pass through
233 CSD call indication: when I dialed CSD calls from subscriber line A to
234 subscriber line B, the destination MS would receive a voice call, while the
235 calling side presumably invoked modem emulation IWF. Just for the sake of
236 completeness and due diligence, I decided to also test the case of ISDN-mode
237 CSD as opposed to modem emulation, i.e., ITC set ot UDI instead of 3.1 kHz. In
238 this experiment I dusted off two of my legacy T-Mobile SIMs that aren't my
239 "personal" one in the Pirelli DP-L10, put those two SIMs into a pair of FCDEV3B
240 boards, and powered up both FCDEV3Bs simultaneously. I then set AT+CBST=71,0,0
241 on both MS and tested dialing calls from one to the other in both directions,
242 both voice and CSD. Results:
243
244 * On voice calls, the destination MS once again received MT SETUP message
245 containing _no_ bearer capability IE.
246
247 * ISDN-mode CSD calls were rejected by the network. The MO call got as far as
248 CALL PROCEEDING message and channel assignment (the network assigned TCH/F9.6
249 to the MO leg), but then came a DISCONNECT message with cause 21 (call
250 rejected) and indicated location 0011 ("transit network").
251
252 * I didn't test analog-mode CSD, as that mode was already tested earlier in
253 Experiment 4.
254
229 NT mode: need for further debugging 255 NT mode: need for further debugging
230 =================================== 256 ===================================
257
258 (This section was written following Experiment 6.)
231 259
232 The flaky nature of NT mode calls for further debugging: since we see 260 The flaky nature of NT mode calls for further debugging: since we see
233 MS-initiated DISCONNECT on those tries when it fails, we really need to know 261 MS-initiated DISCONNECT on those tries when it fails, we really need to know
234 exactly what is making the MS unhappy. But this debugging is made difficult by 262 exactly what is making the MS unhappy. But this debugging is made difficult by
235 the misfeature of TI's DSP: in Experiment 5 I tried capturing TCH DL bits, but 263 the misfeature of TI's DSP: in Experiment 5 I tried capturing TCH DL bits, but
343 or altered in some way. The remaining memories of everything else that 371 or altered in some way. The remaining memories of everything else that
344 happened after this logically-deduced MT SETUP should be treated as 372 happened after this logically-deduced MT SETUP should be treated as
345 unreliable. 373 unreliable.
346 374
347 * When I tried replicating in the present time (2024-11) the just-described 375 * When I tried replicating in the present time (2024-11) the just-described
348 result from many years ago, I had no success: see description of Experiment 4 376 result from many years ago, I had no success: see description of Experiments
349 above. Now when I dial a CSD call from one of my legacy T-Mobile SIMs to 377 4 and 7 above. Now when I dial a CSD call from one of my legacy T-Mobile
350 another, the receiving MS gets an MT SETUP message that contains no BC IE at 378 SIMs to another, the receiving MS gets an MT SETUP message that contains no
351 all, same as with calls coming from outside PSTN, the call rings as voice, 379 BC IE at all, same as with calls coming from outside PSTN, the call rings as
352 and gets assigned TCH/AHS like other speech calls. And because the modem- 380 voice, and gets assigned TCH/AHS like other speech calls. And because the
353 emulating CSD IWF in the MO direction initially emits silence while waiting 381 modem-emulating CSD IWF in the MO direction initially emits silence while
354 for the answering modem, silence is what the receiving phone hears in this 382 waiting for the answering modem, silence is what the receiving phone hears in
355 errant setup. 383 this errant setup. (The just-described behavior happens when ITC is set to
384 3.1 kHz on the MO CSD call; if I change it to UDI which I never tried in the
385 past, the network rejects the MO call instead.)