tree e2cfac9fba1a32cc3f6b230c577b7655937c5040
parent 40d3c793ceb4c77116551ff0b04d2af9c61bd09b
author Neels Hofmeyr <nhofmeyr@sysmocom.de> 1679508195 +0100
committer neels <nhofmeyr@sysmocom.de> 1680702466 +0000

3G: decapsulate IuUP to AMR at the MGW; allow 3G<-AMR->2G

For all 3G calls, convert IuUP <-> plain AMR/RTP on the MSC's MGW hop
like this:

Before this patch:

 hNodeB <--IuUP--> MGW@hnbgw <--IuUP--> MGW@msc <--IuUP--> other call leg

After this patch:

 hNodeB <--IuUP--> MGW@hnbgw <--IuUP--> MGW@msc <--AMR--> other call leg
                                                    ^

This allows, in principle, 2G to 3G calls without expensive transcoding,
like this:

 hNodeB <--IuUP--> MGW@hnbgw <--IuUP--> MGW@msc <--AMR--> MGW@msc <--AMR--> MGW@bsc <--AMR--> 2G-BTS
                                                    ^
(So far only proven to work with AMR-FR at 12k2.)

3G to 3G calls now look like this:

 hNodeB <--IuUP--> MGW@hnbgw <--IuUP--> MGW@MSC <--AMR--> MGW@MSC <--IuUP--> MGW@hnbgw <--IuUP--> hNodeB
                                                    ^

Implementatino: get rid of the shim that was put in place to still send
IuUP (VND.3GPP.IUFP) to the CN. So now, for all 3G voice, the IuUP gets
decapsulated to plain AMR/RTP at the MSC's MGW hop.

What is proven to work with this patch:
successful voice call between 2G and 3G with these conditions:
- a hNodeB that stubbornly accepts only 12k2 AMR;
- a 2G BTS configured to use only TCH/F and only FR3, with only 12k2 as
  allowed AMR rate.

We have not yet seen a call working for TCH/H HR3 <-> 3G, because of the
lab hNodeB's limitation to 12k2.

Future work we probably need:
- properly request and negotiate AMR rates via SDP fmtp:mode-set.
- request more RFCIs in our RANAP RAB Assignment requests
  (see I61e0e9e75e3239662846fd797532acdefa9f73dc).
- Convert IuUP to AMR already at the HNBGW's MGW?
Solving this is not part of this patch.

Related: SYS#5092
Change-Id: I386a6a426c318040b019ab5541689c67e94672a1