Under The Hood: NAS Signaling for AMR-WB

If you are in the business of analyzing UMTS network traces for development, debugging or operational reasons and ever wondered how to find out if a speech call was established as wideband or narrowband or if the codec was changed at some point during the call, I've got an interesting tip for you today that I was recently shown myself: Have a look at the


information element in the RadioBearerSetup or RadioBearerReconfiguration messages of a call. It's quite at the top of the message in RabInfo section so it can be found quickly and the 4 bits of the IE describe the speech codec the MSC wants the mobile device to use for the call:

Narrowband: 1010 –> UMTS AMR-WB (see 3GPP 26.103, Table 4.2)
Wideband:     0110 –> UMTS AMR2 (i.e. narrowband)

Apart from the table in 3GPP 36.103, TS 24.008 gives some more details:

The ME shall activate the codec type received in the NAS Synchronisation Indicator IE.

The NAS Synchronisation Indicator IE shall be coded as the 4 least significant bits of the selected codec type (CoID) defined in 3GPP TS 26.103 [83], subclause 6.3.