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The problem of tandem vocoders and their effect on voice quality is well-known. In the case of international mobile-to-mobile calls, there can be up to three vocoding stages - between each mobile and its base station controller, as well as a Digital Circuit Multiplication Equipment (DCME) or Voice over IP (VoIP) stages on the international link. The ITU standards for 2G and 3G networks define the concept of Tandem Free Operation (TFO) for a mobile-mobile call. The same concept can be applied to the three-vocoder-stage international call, if the DCME or VoIP devices are capable of simulating TFO-capable mobile equipment.
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