Transcoder free operation

In a telecommunication network, transcoder free operation, or TrFO, also known as Out of band transcoder control is the concept of removing transcoding function in a call path. In legacy GSM networks a call between two mobile stations involved two transcoding functions, one at each BSC. This transcoding functionality was generally implemented in a separate Transcoder and Rate Adaptation Unit, or TRAU. TRAU was connected to BSC and MSC through TDM E1 or STM-1.

Source: Wikipedia — Transcoder free operation (CC BY-SA 4.0)