d. In this way the codec used on the Ater interfaces (e.g. EFR or AMR) can be forwarded over the backbone, improving voice quality. No transmission savings would be obtained in this way as the codec on an interface is G.711 and the transcoders will override part of the G.711 frame with the EFR codec and fill the rest with padding. Some MGWs have the feature to recognise the use of TFO between the TCs. TFO is activated through in-band signalling during the call establishment procedure. Forced Payload Compression over the Backbone When TFO is not supported by the TCs or when the MGWs operate fixed bandwidth reservation at the call establishment phase and it is not possible to detect the use of TFO, the MGW may be configured to use a default codec (e.g. EFR or AMR) in the Nb interface or backbone, introducing supplementary transcoding operations. This enables transmission savings but it reduces the voice quality perceived by the end user as up to four transcoding operations can take place in tandem for each communication path. G. 711 Codec Directly Transmitted Using AAL2 Bearer Channels This results in a 64 kbps transparent channel for voice, even in the UMTS packet backbone, and can be used when TCs do not support TFO or when the MGW has a fixed reservation for GSM channel bandwidth and the voice quality cannot be reduced using forced compression. In fact, if the MGW is reserving bandwidth at the call setup and is not able to modify dynamically the reservation made, then detection of the TFO mode used by the transcoders has no benefit as the MGW will consider a 64 kbps channel as permanently allocated to the call even if only a fraction of the capacity is needed during the call. User Plane Dimensioning User data are carried over AAL2/ATM on the Iu-CS interface and over RTP/UDP/IP or AAL2/ATM on the Nb interface. The protocol stacks for user data transport over Iu-CS and Nb interfaces are depicted in Figure 4.15. The MGW supports several voice codecs, among them UMTS AMR, GSM EFR, GSM FR and G.711 A and u law codecs