Transcoder free operation ("TrFO") is an optimisation implemented in telephone networks to improve audio quality and reduce delay, whilst at the same time saving bandwidth.

TrFO is similar to tandem free operation. The essential difference is that in TrFO not only is the original compressed audio sent all the way to the remote user (as in TFO), but also "no transcoder device is physically present in the communication path" (2). With TFO, transcoded audio might get created in the network - it just doesn't make up the stream that reaches the user. With TrFO, no transcoded audio is ever created.

There are also implementation subtleties in the case of GSM that distinguish TFO from TrFO (1), however mostly these are less interesting. The main thing is that GSM ("Global System for Mobile communications") compressed voice (RPF-LTP FR at 13kbps, or whatever) is less bandwith-hungry than ITU-T G.711 PCM (at 64kbps). So you get 4-ish times as many calls across a GSM network with TrFO as you would without it. This makes TrFO a Good Thing. GSM/3GPP standards include TrFO from Release 4 onwards.

1. Tdoc S2-030998 "Liaison on eTFO", Technical Specification Group Services and System Aspects, March 2003

2. 3GPP TS 23.153 V4.9.0 "3rd Generation Partnership Project; Technical Specification Group Core Network; Out of band transcoder control; Stage 2 (Release 4)"