A Transcoder and Rate Adaptation Unit ( TRAU ) performs transcoding function for speech channels and RA (Rate Adaptation) for data channels in the GSM network . The Transcoder/Rate Adaptation Unit (TRAU) is the data rate conversion unit. The PSTN/ISDN switch is a switch for 64 kbit/s voice. Current technology permits to decrease the bit-rate (in GSM radio interface it is 16 kbit/s for full rate and 8 kbit/s for half rate). Since MSC is basically a PSTN/ISDN switch its bit-rate is still 64 kbit/s. That is why a rate conversion is required in between the BSC and MSC.
10-428: Transcoding is the compression of speech data from 64 kbit/s to 13/12.2/6.5 kbit/s in case FR/EFR/HR (respectively) speech coding. Rate adaptation without transcoding allows Tandem Free Operation (TFO), allowing the original encoded speech data to be carried in a 64 kbit/s channel. TFO offers benefits because transcoding can lead to a degradation of speech quality and requires computational resources. TRAU
20-562: A well defined state machine. Based on the sequence of events, the state machine table defines a sequence of actions to be performed. As a part of this sequence, the local transcoder sends TFO In-band Signalling messages (IS_Messages) on the LSBs of the PCM samples. The protocol is very well defined in the specification documents. The remote transcoder equipment that receives these messages, acknowledges (ACKs) with its own IS_Messages. Upon initial exchange,
30-435: Is a part of ETSI's 3GPP standard specification, which has been included from R99 of the standards specifications onwards. In traditional GSM networks, a call between two Mobile Stations (MS) involve a dual encoding/decoding process. Speech signals are first encoded in the originating MS, converted to G.711 in the local transcoder, converted back to a GSM codec in the remote transcoder and finally converted back to speech at
40-530: Is an 8-bit value and is sampled at the rate of 8 kHz. Hence, the bandwidth requirement is 64 kbit/s, with each bit corresponding to 8 kbit/s. TFO is a mechanism that steals least significant bits (LSBs) of PCM samples to literally embed the bits from encoded stream. Since most GSM/UMTS codec rates are around the range of 8 kbit/s to 16 kbit/s (and higher rates of up to 32 kbit/s for 16 kHz sampled Wide Band codecs), one needs to steal only about 1 or 2 LSBs of total 8 bits. This aspect
50-535: Is performed in the TRAU network element which connects RAN and CN. 16 kbit/s for FR (Full Rate), Redundancy (Channel Coding)= 9.8 kbit/s => Gross data rate after adding redundancy = 22.8 kbit/s => 12.2 kbit/s for EFR (Enhanced Full rate) => Gross data rate after adding redundancy = 11.4 kbit/s. This computing article is a stub . You can help Misplaced Pages by expanding it . Tandem Free Operation Tandem Free Operation ( TFO )
60-555: Is very important, as, if there is a breakage in TFO connection, the upper most significant bits (MSBs) can still be used to carry transcoded G.711/PCM sample values. The degradation due to loss of 1 or 2 LSBs is not much. The remote transcoder equipment then extracts the encoded stream from LSBs of PCM samples and reconstructs it as codec frames and then sent as though it was encoded by itself, thereby virtually avoiding two iterations of trancoding. The transcoder equipment that supports TFO runs
70-494: The edge of the core networks that acts as a gateway between mobile core networks ( IP based) and digital PSTN networks. Mobile core IP networks carry voice encoded in the form of one of the GSM/UMTS codecs (e.g.: GSM AMR). When this has to be carried over a G.711/PCM based PSTN network, the gateway equipment transcodes from GSM/UMTS codec to G.711 PCM samples. This results in a certain loss of voice quality. A single G.711/PCM sample
80-402: The terminating MS. In this configuration the two transcoders are operating in tandem introducing a voice quality degradation. It is possible to eliminate this problem by removing the two transcoding operations in the voice path if the two MS are using the same codec. Broadly, the equipments that are en-route two end mobile sets can be categorized into two types: These equipment are typically at
90-505: The two transcoders also exchange their capabilities (the codecs they support, etc.). Once a common codec is decided, they both start streaming PCM samples with LSBs containing encoded stream. As described earlier, these are not active equipment that does transcoding. These typically come in the path of two transcoders. To ensure that the TFO stream that is embedded in the LSBs of PCM samples are not touched by these equipments, even these have to be aware of TFO. The specifications define
100-401: Was also the term used for the frame format used in transport of the compressed bits from these speech coders. For an MS-to-MS call, the transmission path covers the radio access network (RAN) as well as the core network (CN). Since the transmission modes and coding standards are different for RAN and CN, speech data is converted/transcoded at the transition points from RAN to CN. This conversion
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