Tandem free operation ("TFO") is an optimisation implemented in
telephone networks to improve audio quality and reduce delay.
Traditionally cellular phone calls involved a certain amount of
transcoding. For example, even if I call Bob in the next room,
who is on the same GSM ("Global System for Mobile communications") network
as me, our conversation could be
transcoded twice. GSM's compressed voice gets decompressed and
encoded as ITU-T PCM A-law voice, and then this gets decoded and
recompressed as GSM compressed voice before it gets as far as
Bob. Suffice to say this is not ideal - the process introduces
delay and degrades voice quality.
If our network implements tandem free operation (1, 2)
then the GSM-coded voice that was compressed on my phone arrives
intact (modulo any bit errors, etc.) at Bob's phone. This
means most of the time the sound coming out of Bob's phone is more
like my voice than it would have been before the days of TFO. For
this to work, both handsets have to run the same voice codec.
TFO is a Good Thing. Bob can hear me better, my operator gets
a higher mean opinion score and everyone is happy. In the world
of GSM at least, TFO is subtly different from transcoder free operation (3, 4).
1. GSM 06.01 version 8.0.1, Release 99, "Digital cellular telecommunications system
(Phase 2+); Full rate speech; Processing functions (GSM 06.01 version 8.0.1 Release 1999)"
2. GSM 06.10 Version 8.2.0, Release 99, "3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
Digital cellular telecommunications system (Phase 2+); Full rate speech; Transcoding (Release 99)"
3. Tdoc S2-030998 "Liaison on eTFO"
Technical Specification Group Services and System Aspects, March 2003
4. 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)"