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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
• Template 3—Supports G.711u, G.711a, G.726-16k, G.726-24k, G.726-32k, G.726-40k, G.729a,
and G.729ab codecs, clear channel.
• Template 4—Supports G.711u, G.711a, 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 codecs and clear channel.
Within each template, you can specify a preference order for each codec. At call setup time, the codec
to be used from the configured template is either specified by the call agent or negotiated between the
calling and called VISM cards. If the codec is negotiated, the most preferred codec that both VISM cards
can support is selected.
For each codec, VISM supports various packetization periods as described in Table 3-1.
Voice Activity Detection and Silence Suppression
You can configure the VISM card DSPs to monitor the TDM voice stream for voice activity. If the voice
activity detection (VAD) feature is enabled and no voice activity (silence) is detected for more than a
specified period of time, typically 250 ms, the silent voice samples are suppressed. During periods of
silence, parameters defining background noises transmit periodically. You can configure the remote
VISM to use the background noise information to generate comfort noise at the called end while silence
suppression is in operation.
Fax and Modem Tone Detection
You can configure the VISM card DSPs to detect the modem or fax tones on the data lines. For VoIP
operating mode, the action is specified using command line interface (CLI) commands. Refer to
Chapter 5, “CLI Commands,” for more information on CLI commands. For AAL2 connections, the
action is specified in the AAL2 profile. Generally, when a modem or fax tone is detected, VAD and
ECAN are turned off and codec is changed to the specified type (for example G.711 or clear channel).
Note If the codec is already set to clear channel, the DSP cannot detect any tones—fax and modem tones
are not detected.
Table 3-1 Supported Codecs and Packetization Periods
Codec Packetization Period (milliseconds)
G.711u 5 10 20 — — —
G.711a 5 10 20 — — —
G.729a — 10 20 30 40 —
G.729ab —102030 40—
G.726-16k, 24k, 32k, 40k 5 10 20 30 40 —
G.723.1-H — — — 30 — 60
G.723.1a-H — — — 30 — 60
G.723.1-L — — — 30 — 60
G.723.1a-L — — — 30 — 60
Clear channel — 10 20 — — —