Harald wrote:
In AoIP there's
no BSC-colocated transcoder... while in classic TDM based A interface
there normally is, and you have PCMA/PCMU.
I decided to dive into this rabbit hole once again, and I discovered
some details which differ from what has always been taught in Osmocom
land. The purpose of this post reply is to share these findings with
the community.
As it turns out, in Nokia's implementation of GSM BSS the transcoder
(bank of TRAUs) is not enmeshed inside the BSC, nor is it enmeshed
inside the MSC. Instead they introduced a third interface named Ater,
joining the ranks of A and Abis. The voice path looks like this:
MSC <---A i/f---> TCSM <---Ater i/f---> BSC <---Abis i/f---> BTS
The bank of TRAUs (the term TRAU strictly means a single voice/CSD
channel handler) is called TCSM in Nokia land, which stands for
Transcoder and Submultiplexer. This TCSM sits between MSC and BSC in
the voice path, while its physical placement can be either at the MSC
site or the BSC site. (Locating the TCSM at the MSC site results in
4x savings on TDM leased line costs, hence I would imagine this choice
was the popular one, but what do I know, I wasn't there...) Whatever
the physical placement of the TCSM, the A interface is a bunch of
E1/T1 circuits, and so is Ater. But the codecs are different: on A
each call takes up a full timeslot in G.711, on Ater each call is 1/4
of a timeslot in the same format as on Abis.
I have yet to learn how Siemens and Ericsson implemented this part in
their respective BSS architectures, but at least old Nokia gear has a
sane & sensible design in this regard.
I am now looking to acquire a Nokia TCSM, either TCSM2 or TCSM3i.
Based on my reading of the docs found online, it should be possible to
fire up a TCSM by itself, having neither an MSC nor a BSC of TDM kind.
What would one do with a TRAU by itself, without those other network
elements? Answer: play with TFO! Without TFO, a TRAU is quite
UNinteresting: it doesn't do anything more than what is contained in
reference C sources for HR/EFR/AMR codecs from ETSI, i.e., practically
free code which we can run and study as we please. But if a TRAU
supports TFO and if that feature is enabled, then it is a complex
wonder with a stealthy in-band messaging protocol (messages transferred
by stealing the lsb of every 16th sample), lots of state machines and
timers - this is the kind of stuff where it would be impractical to
develop a new implementation and have any confidence about its
correctness without being able to test against someone else's existing
implementation.
If anyone here has any leads to Nokia surplus GSM equipment dealers, I
would greatly appreciate some pointers. I know there are people who
run Nokia InSite/MetroSite/UltraSite BTSes with Osmocom - perhaps the
same dealers who sell that BTS gear would also have some components
from higher up the signal chain?
M~