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TCSM2 PCM circuit types

A single unit of Nokia TCSM2 has a minimum of 1 and a maximum of 7 E1 or T1 circuits on the A interface toward the MSC. Each of these A-side or MSC-side E1/T1 circuits is also called a PCM circuit: they carry A-law or mu-law PCM samples on full 64 kbit/s timeslots. (Interestingly enough, TCSM2 supports A-law in both ANSI and ETSI configurations, but allows the option of mu-law only in ANSI mode. The "global" ANSI vs ETSI setting must match the type of ET module used for Ater.)

A further quirk of TCSM2 architecture is that each A-interface E1 (and in a minimal lab setup, you will typically have only one such E1) must be configured as being one of 6 possible PCM circuit types:

  • With circuit type A, each A-side timeslot maps to a 16 kbit/s subslot on Ater, and the supported GSM call types are FR speech, EFR speech and CSD. And according to documentation, it even supports CSD 14.4 kbit/s mode!
  • With circuit type B, each A-side timeslot maps to an 8 kbit/s subslot on Ater, supporting half-rate speech channels. It is not clear whether or not Nokia ever supported half-rate CSD calls.
  • With circuit type C, the documentation says that each A-side timeslot maps to a 16 kbit/s subslot on Ater, which supposedly supports all of FR, HR, EFR and maybe CSD.
  • With circuit type D, the documentation says that each A-side timeslot maps to a 32 kbit/s subslot on Ater, which then supports everything that is supported with circuit type C, plus two-GSM-timeslots HSCSD.
  • With circuit type E, the documentation says that each A-side timeslot takes up a full 64 kbit/s timeslot on Ater, which then supports everything that is supported with circuit types C and D, plus 4-GSM-timeslots HSCSD.
  • With circuit type F, each A-side timeslot maps to a 16 kbit/s subslot on Ater just like with circuit type A, but the transcoder supports AMR instead of classic codecs, and the support for TFO goes away.

According to Nokia's documentation, they made 4 different transcoder DSP firmware images:

  • One DSP image supports circuit type A;
  • Another supports circuit type B;
  • Another supports circuit types C, D and E - supposedly with the same DSP image;
  • Yet another supports circuit type F.

However, the TRCO card that holds all of the firmware can only hold up to 3 DSP fw images, hence it is not possible to support all 6 PCM circuit types at the same time. Our TRCO card came with fw images that support PCM circuit types A, B and F - but not C, D or E.

Because we don't have DSP fw for circuit type C which supposedly (according to documentation) supports both FR/EFR and HR, we will have to switch our configuration between circuit types A and B in order to alternate between playing with FR/EFR/CSD and playing with HR codec - but oh well. It is also interesting how Nokia split off AMR support into a separate PCM circuit type of its own - and it is not clear how they supported AMR-HR, if at all. Once we get our TCSM2 hooked up to our test rig with E1 interfaces (see #6464), we will do experiments to test if circuit type F is indeed AMR only, or if it also supports the older codec types. Actual experimentation with AMR will be a much lower priority for the indefinite future.

Updated by falconia 1 day ago · 1 revisions

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