Telephony; Echo Cancellation - Harris CM-30 Installation & Operation Manual

2 –Functional Design
2.2.4

Telephony

The CM-30 module and NetXpress LX system allow you to create streams whose endpoints are
Intraplex voice channel modules that support telephone signaling (for example, the VF-15E, VF-16AE,
and VF-25 modules). The voice circuits generate signaling bits which the NetXpress LX system
transports out-of-band and reinserts onto the NetXpress LX internal TDM bus at the receive end. No
more than 20 telephony streams are allowed in-service at one time.
Telephony streams can be easily identified in the stream table by the telephone icon after the stream
name. The color of the icon, (red, yellow, or green) indicates the current status of the out-of-band
signaling transfer. The NetXpress LX Performance menu provides statistics on signaling packet
transmission, and these statistics are cleared when the other stream statistics are cleared.
The TDM bus in T1 systems inherently supports signaling. In E1 systems, the signaling bits are carried
on the internal TDM bus using Channel Associated Signaling (CAS). Telephony streams need to
originate and terminate on busses where the CAS mode is enabled. You can select the specific TDM
busses for the telephony streams on the TDM Bus Edit screen (System Config | TDM Bus | TDM
Bus Edit). When the CAS mode is enabled, the NetXpress LX system uses Time slot 16 to carry the
signaling bits for all the other channels on that bus; this time slot is not available to carry normal
traffic.
2.2.5

Echo Cancellation

The NetXpress LX system can provide echo cancellation for voice circuits using up to 2 optional echo
canceller cards that plug onto the CM-30 module. Each board can provide echo cancellation for up to
30 channels per echo canceller card on the TDM bus.
Echo cancellation can only be enabled for full-duplex streams with identical bus/channel configuration
in the transmit (Tx) and receive (Rx) direction, and both endpoints of the stream need to have an
echo canceller card installed. Echo cancellers are needed in 2-wire modules (for example, the VF-15E
Harris Corporation
Intraplex Products
NetXpress LX & CM-30 Installation & Operation Manual
Figure 2-1. Carrier Multicast Backbone Example
Version 1, September 2010
2-5