Retronetworking » Retro-GSM

h1. 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 [[TCSM2_ET_modules|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 [[TCSM2_firmware|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 [[TCSM2_TRCO_card|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.