PRINCIPLES OF OPERATION
AT&T Pub 43801 or BT’s SSDC5A). The recommendation is for at least 4
kHz of overhead bandwidth on Minimux/Univ. MM+ V4 aggregate links.
Additional bandwidth is required if two or more CELP channels sharing
an aggregate are likely to be dialing simultaneously.
Due to the complexity of calculating the overhead bandwidth
requirements of such systems, GDC does not recommend pulse dialing
via the CELP Voice Channel Module (-001) on TMS-3000/TMS Compact
systems .
E&M Signaling
(-002, -003, -004,
-005 Versions)
The -002, -003, -004 and -005 versions of the CELP Voice Channel Module
accomplish E&M signaling using true in-band techniques. The
signaling information is passed end-to-end via the voice channel without
the need for any extra bandwidth (there will be short breaks in the voice
channel when E&M signaling is being passed). The pulse distortion (625
micro secs max) is considered to be acceptable for all applications. The
only constraint is that signaling pulse widths must be greater than 20 ms
(20.625 ms at 4.8 kHz). This is sufficient for all known regular pulse
dialing systems (but 20 pps “speed dialing’’ systems will not be
supported).
Note that for in-band E&M signaling, both ends of the voice link must be
the -002, -003, -004 or -005 CELP Modules. If one end of the CELP voice link
has a -001 CELP Module then in-band signaling will not be supported.
The “Overhead Control Bandwidth’’ method of E&M signaling must be
used in this case.
Piggyback Card
Operation
Serial PCM data at 64 kHz is sent from the base card and converted into
a low bit rate data stream for transmission to the multiplexer via the base
card. Simultaneously, low bit rate encoded voice is received from the
multiplexer channel and converted back to serial PCM encoded voice
data at 64 kHz and returned to the base card.
The speech compression algorithm selection is based on the speed of the
channel rate clock (either 9.6 kHz, 6.4 kHz or 4.8 kHz).
Software informs the piggyback card which data rate is currently se-
lected. The piggyback card detects a change in this bit rate and selects the
new algorithm.
Dialing and call progress tones are passed through the channel. The
CELP Voice Channel Module supports DTMF, MF, SF and 1-kHz test
tones.
The CELP Voice Channel Module piggyback card also performs the func-
tion of an echo canceller. The echo canceller calculates the amount of
echo in the input signal and attempts to cancel the echo with an equal
and opposite computed echo signal. The echo canceller performance is
equal to (or better) than CCITT G.165 specifications.
The -002, -003, -004 and -005 versions of the CELP Voice Module transfers
E&M signaling information via the voice band. There is a short break
in the voice path every time the piggyback detects a change in the
condition of the M lead input.
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