Page 167 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 153

Table 3.3 AMR codecs and bit rates.

Codec mode Bit rate (kbit/s)

AMR_12.20 12.20 (GSM EFR)
AMR_10.20 10.20
AMR_7.95 7.95
AMR_7.40 7.40 (IS‐641)
AMR_6.70 6.70 (PDC‐EFR)
AMR_5.90 5.90
AMR_5.15 5.15
AMR_4.75 4.75

networks, AMR‐WB uses the G.722.2 codec with a datarate of 12.65 kbit/s over the air
interface, which is about the same as is required for NB‐AMR voice calls.
WB‐AMR requires a Release 4 MSC in the network as the voice data stream can no
longer be converted to PCM without reverting to a narrowband voice signal. For a con-
nection between two 3G AMR‐WB‐compatible devices, no transcoding is necessary
anymore. This is also referred to as Transcoding Free Operation (TrFO). On the radio
network side, only software changes are required as the requirements for an AMR‐WB
bearer are very similar to those of AMR‐NB.
If a call that is established from an AMR‐WB‐capable device terminates to a device
that is only AMR‐NB‐capable, there are several possibilities for handling the connec-
tion. One way to establish the channel is to use AMR‐WB from the originator to the
MGW of the MSC where the channel is transcoded into AMR‐NB for the terminator.
Another implementation possibility is for the MSC to wait with the bearer establish-
ment of the originator until the capabilities of the terminator are known and then decide
whether to establish a narrowband or a wideband connection to the originator. And
finally, it is also possible to establish a wideband connection to the originator during call
establishment and to modify the bearer if the MSC determines afterward that the ter-
minating side supports only narrowband AMR.
While an AMR‐WB connection is established, it can also become necessary to intro-
duce a transcoder temporarily or even to permanently change from a wideband to a
narrowband codec.
If the user uses the keypad to type DTMF (Dual‐Tone Multi Frequency) tones, for
example, to enter a password for the voice mail system, the corresponding tones are
generated in the MSC. Therefore, the MSC interrupts the transparent end‐to‐end con-
nection during this time to play the tone.
Although there are quite a number of AMR‐WB‐capable UMTS networks in practice
today, the support in GSM networks is more limited. If a wideband voice call is handed
over from UMTS to a GSM radio network that does not support the codec, it is there-
fore necessary to switch the connection from AMR‐WB to AMR‐NB during the hando-
ver. Unfortunately, this has a negative audible effect on the voice quality. As many
networks are configured defensively and thus move a voice call from UMTS to GSM
long before it would be necessary from a signal strength point of view, it can be