How The Amr-Nb Codec Works; Feature Limitations Or Restrictions; Gsmamr-Nb Codec Compliance With Rfc 3267 - Cisco VGD-1T3 Software Configuration Manual

Configuring the GSMAMR-NB Codec for the High-Density Packet VFC
Table 1
Codec Mode
0
1
2
3
4
5
6
7
1
8
1. Used for Silence Indication Detection (SID) frames.

How the AMR-NB Codec Works

The multirate encoding (or multimode) capability of AMR-NB is designed for preserving high speech
quality under a wide range of transmission conditions. Unlike other codecs, the AMR-NB codec can
adapt to different bit rates (see
To perform mode adaptation, the decoder (speech receiver) sends a signal to the encoder (speech sender)
to indicate which new mode it prefers. This mode-change signal is called codec mode request (CMR).
Because speech is sent in both directions between the two ends in most sessions, the mode requests from
the decoder at one end to the encoder at the other end are sent in a piggyback form over the speech frames
in the reverse direction; there is no out-of-band signaling needed for sending CMRs. The Cisco VGD
1T3 cannot initiate CMRs and received CMRs can be processed. For more information about AMR-NB
codecs, see RFC 3267.

Feature Limitations or Restrictions

•
•

GSMAMR-NB Codec Compliance with RFC 3267

GSMAMR-NB codec support is compliant with RFC 3267. Specifications for RFC 3267 are supported
by the High-Density Packet Voice Feature Card:
•
Cisco VGD 1T3 Voice Gateway Software Configuration Guide
76
AMR Codec Modes and Bit Rates
Bit Rate (kbps)
4.75
5.15
5.90
6.70
7.40
7.95
10.2
12.2
1.80
All the DSPs in the system must be upgraded with the GSMAMR-NB DSPware.
The following message is displayed when an upgrade takes place if a version of DSPware other than
the recommended version is uploaded:
WARNING: Recommended GSM AMR-NB supported DSPware for this Cisco IOS image is X.Y.Z
Where X.Y.Z changes depending on the Cisco IOS image that is used by the customer.
This warning has no impact on the firmware upgrade and calls can be brought up with a version of
DSPware that is not the recommended version.
The Cisco VGD 1T3 does not initiate CMRs. However, these platforms can process the received
CMRs.
Configuring and Managing the High-Density Packet Voice Feature Card
Table 1) based on channel conditions during the call.