Transcoder and Rate Adaptation Unit or TRAU, performs
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 devic ...
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...
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
Tandem Free Operation (TFO) is a part of ETSI's 3GPP standard specification, which has been included from R99 of the standards specifications onwards.
Overview
In traditional GSM networks, a call between two Mobile Stations (MS) involve a dual e ...
(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.
Brief explanation
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 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
See also
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Full Rate
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Half Rate
Half Rate (HR or GSM-HR or GSM 06.20) is a speech coding system for GSM, developed in the early 1990s.
Since the codec, operating at 5.6 kbit/s, requires half the bandwidth of the Full Rate codec, network capacity for voice traffic is doubled, at ...
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Enhanced Full Rate
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Adaptive Multi-Rate
TRAU was also the term used for the frame format used in transport of the compressed bits from these speech coders.
References
Audio format converters
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