TFO - Tandem Free Operation

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In Early legacy telephone network, A call between the two mobiles involved with two transcoding function at both the end which decrease the voice quality. The Transcoding Function done by the TRAU (Transcoding and Rate Adaptation Unit) to compress and decompress the speech. 

TFO (Tandem Free Operation) enables to avoid the traditional double speech encoding / decoding in MS to MS call configurations. TFO uses in-band signalling and procedures for transcoders to enable compressed speech to be maintained between a pair of transcoders. 
So the main objective of TFO is the improvement of the voice quality for calls between 2 mobile subscribers, but no resource optimisation is introduced  as transcoder functions are always present in the path.

If TFO is activated between two end nodes, TFO Frames with compressed speech (e.g. AMR in LSB) as payload are carried over 8 or 16 kbit/s channels mapped onto the least or two least significant bits of the 64 kbit/s PCM speech samples. It is also carried the G.711 codec in MSB if it is not possible to achieve the TFO. So we can say that TFO is used to achieve the voice quality if possible.
Tandem Free Operation is activated and controlled by the Transcoder Units after the completion of the call set-up phase at both ends of an MS-MS, MS-UE, or UE-UE call configuration. The TFO protocol is fully handled and terminated in the Transcoder Units. For this reason, the Transcoder Units cannot be bypassed in Tandem Free Operation. This is the key difference with the feature called Transcoder Free Operation (TrFO).

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