In a
telecommunication network
A telecommunications network is a group of nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit switching, messa ...
Transcoder free operation, or TrFO, also known as Out of band transcoder control is the concept of removing
transcoding
Transcoding is the direct digital-to-digital conversion of one encoding to another, such as for video data files, audio files (e.g., MP3, WAV), or character encoding (e.g., UTF-8, ISO/IEC 8859). This is usually done in cases where a target ...
function in a call path. In legacy
GSM
The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...
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.
With the introduction of
NGN and 3G networks the
Radio Network Controller
The Radio Network Controller (RNC) is a governing element in the UMTS radio access network (UTRAN) and is responsible for controlling the Node Bs that are connected to it. The RNC carries out radio resource management, some of the mobility managem ...
was connected to
MGW through
ATM or
IP instead of
TDM. Therefore, this external transcoder was removed and transcoding function was moved up to the MGW. NGN also introduced Nb interface over IP such that it became possible to carry compressed voice codecs such as AMR in the Nb interface. In a call scenario such as this the transcoding functionality in the MGW could be eliminated such that voice quality can be improved and resources in MGW also could be saved. Concept of TrFO became applicable for 2G networks also with the "A interface over IP" implementation.
References
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Mobile telecommunications
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