Reference Manual

Speech and Data Services
In order to activate coding/decoding of speech in the RTC, the RBS
issues TRAU frames of type Speech/Enhanced speech.
In order to deactivate speech coding/decoding in the RTC, the RBS
issues TRAU frames of type Idle Speech.
During the exchange of TRAU frames between RBS and RTC carrying
Full Rate Speech (or Full Rate Idle Speech), the RBS regularly
calculates an appropriate time alignment value for controlling the
downlink frame timing. This value is included in the transmitted TRAU
frames.
The TRAU frames exchanged between BTS and RTC during an active
call are of three types:
•
Speech
• Enhanced Speech
• Idle Speech (used by DTX function GSM speech algorithm
version 1 only)
•
Speech containing coded silence information
TRAU frame type Speech uses GSM speech algorithm version 1 and
frame type Enhanced Speech uses GSM speech algorithm version 2 as
coding/decoding of full rate speech.
For TRAU frames of type Speech, improvements of the subjective
speech quality is described under: Subjective Speech Quality
Improvements.
Full rate speech is either terminated by the RF Channel Release
function, or the Mode Modify function when Data or Signalling
services are requested.
Subjective Speech Quality Improvements
The RBS supports subjective speech quality improvements that go
beyond what is required in the GSM recommendations. The objective is
to avoid unpleasant noise effects that would result from e.g. normal
decoding of lost speech frames.
To improve the subjective speech quality uplink in cases of bad frames
reception, the RBS takes local measures such as "substitution" and
"muting" of frames and computes an improved BFI (Bad Frame
Indicator). This calculation takes advantage of quality information from
the channel decoder.
To improve speech quality downlink, filtering of TRAU frames is done
after detection of disturbances in the frame quality. The filtering is
removed when the disturbances have disappeared.
18.3.4 Half Rate Speech
Half Rate Speech is either initiated by the Channel Activation function
or the Mode Modify function.
In order to activate coding/decoding of speech in the RTC, the RBS
issues TRAU frames of type Speech.
182 (421)
EN/LZT 720 0008 R1A
2001-06-26
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