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Cisco VISM Installation and Configuration Guide
Release 3.0, Part Number OL-2521-01 Rev. D0, June 2004
Chapter 3 VISM Functional Description
Bearer Processing Function
Bearer Processing Function
The bearer processing function processes raw bearer DS0 streams from the T1/E1 lines in preparation
for packetization and segmentation and reassembly (SAR) processing on the ATM side. Most of the
bearer processing is performed by the VISM daughter card’s DSPs.
The main processing categories are:
• Echo cancellation, voice compression, and A/Mu law conversion
• Voice activity detection and silence suppression
• Fax and modem handling
• Jitter control
• CAS handling
Echo Cancellation, Voice Compression, A/Mu Law Conversion
Bearer DS0 streams are received from the T1/E1 line function, and each is assigned to a DSP configured
for echo cancellation (ECAN). You can configure the following ECAN parameters:
• Residual echo control
• Maximum tail in milliseconds (up to 128 ms)
• Fax and modem tone detection
If voice compression is required, the ECAN voice streams are assigned to a second DSP configured with
the required codec. Available compression schemes are:
• G.711 64 kbps (A/Mu law, user configurable)
• G.726-16k
• G.726-24k
• G.726-32k
• G.726-40k
• G.729a
• G.729ab
• G.723.1-H
• G.723.1a-H
• G.723.1-L
• G.723.1a-L
VISM allows the use of codec templates in which the user selects a template instead of specifying each
allowable codec individually. VISM supports the following codec templates:
• Template 1—Supports clear channel, G.711a, G.711u, G.729a, G.729ab, G.726-16k, G.726-24k,
G.726-32k, and G.726-40k codecs.
• Template 2—Supports clear channel, G.711a, and G.711u codecs.