Trend AuroraPlus User Guide
1-12 429850
Notes
In a typical BRI installation NT1 and NT2 functions are
combined in single NTE (Network Termination Equipment).
ISDN Standards
ISDN is subject to standardisation by the ITU-T and ETSI,
which issue recommendations and specifications covering
ISDN equipment and interfaces. Standards also exist for
types of service, protocols and ISDN numbering.
Pulse Code Modulation
Pulse Code Modulation (PCM) is a method of converting
analogue signals into digital code. An analogue waveform is
sampled at regular intervals and a measurement is taken of
the sample point at various levels. The number obtained is
digital. The receiver recovers the original analogue signal by
reversing the conversion.
The sampling rate is 8kHz, and the sample value is translated
into an 8 bit code, to conform with ITU-T G.711. Therefore
each speech signal results in a continuous bit stream at a rate
of 64 kbps.
The process of converting between analogue and digital is
known as encoding. An encoding device when combined
with a decoder is called a Codec.
Methods of Voice Encoding
The number of levels used in the sampling, and the way they
are spread, affect the amount of signal noise and distortion.
There are two methods of encoding within the ITU-T G.711
recommendation: A-Law and µ-Law.
A-Law encoding
This is the type of voice encoding used in Europe. Sampling
consists of 4096 levels, divided into 7 segments.
Chap01.P65 30/04/01, 16:2612
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