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61200375L1-1
TA 850
Echo Canceller UIG/61200384L1-31A
Page 17 of 20
© 2001, ADTRAN, Inc. TA 850 User Manual
1. ECHO CANCELLER OVERVIEW
The Echo Canceller Module adds ATM functionality to the TA 850 and is used exclusively with the T1 RCU. Voice
over packet/cell applications require that Echo Cancellation techniques be applied to the voice traffic to achieve high
quality voice. The Echo Canceller is used in ATM voice applications that require G.168 echo cancellation.This mod-
ule cancels echo for up to 24 voice ports.
The T1 RCU must be using Voice Over ATM firmware to be able to use this module. Firmware can be updated by
using XMODEM transfer protocol via the base unit’s
CRAFT
port or by using TFTP from a network server. Refer to
the T1 RCU VoATM User Interface Guide (document number 61200376L2-31A) for instructions on updating firm-
ware.
The Echo Canceller (which installs in special slots A and B) is available with or without Adaptive Differential Pulse
Code Modulation (ADPCM). ADPCM is a speech coding algorithm which uses fewer bits than the traditional Pulse
Code Method (PCM), allowing you to get more calls into less space. The Echo Canceller with ADPCM performs
32 kbps ADPCM voice compression on up to 24 voice ports. ADPCM voice compression is per ITU-T G.721. An un-
compressed 64 kbps PCM voice port requires roughly 77 kbps of WAN bandwidth (the bandwidth required above
64 kbps is overhead due to the ATM cell header). For a 1.536 Mbps T1 line, only 19 voice ports can be active simul-
taneously, with practically no bandwidth left over for data. A 32 kbps G.721 compressed voice port requires roughly
38.5 kbps. For a 1.536 Mbps T1 line, 24 voice ports can be active simultaneously (the maximum allowed by the
TA 850). Even with 24 compressed voice ports active, approximately 611 kbps of bandwidth is still available for data.
The Echo Canceller provides the following functions:
• Performs echo cancellation per G.165 and G.168 for up to 24 voice ports
• Enables/disables echo cancellation on a per-port basis
• Provides per port G.164 2100 Hz or G.165 2100 Hz phase reversal tone detection
• Performs voice compression per G.721 for up to 24 voice ports (Echo Canceller with ADPCM only)
• Enables/disables ADPCM on a per-port basis (Echo Canceller with ADPCM only)
2. INSTALLING A MODULE
After installing the TA 850 Base Unit and connecting the required cables, you can install necessary modules.
Individual access modules insert from the front. A locking bar holds the modules in place for added security. Disen-
gaging the captured screw allows removal of the locking bar. All wiring connections terminate on the backplane.
Remove the 20 Hz fuse before exposing backplane or accessing channel units.
Electronic modules can be damaged by static electrical discharge. Before handling modules, wear
an antistatic discharge wrist strap to prevent damage to electronic components. Place modules in
antistatic packing material when transporting or storing. When working on modules, always place
them on an approved antistatic mat that is electrically grounded.