Managing and Troubleshooting the Voice Feature Card
Where to Go Next
9
Cisco AS5350XM and Cisco AS5400XM Universal Gateways Software Configuration Guide
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
Table 11
) based on channel conditions during the call.
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 AS5400XM and Cisco AS5350XM cannot initiate CMRs and received CMRs can be processed.
For more information about AMR-NB codecs, see RFC3267.
Restrictions for the AMR-NB Codec
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.
Where to Go Next
At this point you can go to these references:
•
Chapter 7, “Configuring Voice over IP,”
to learn how to configure voice and fax traffic over an IP
network.
•
Appendix C, “Comprehensive Configuration Examples”
Table 11
AMR Codec Modes and Bit Rates
Codec Mode
Bit Rate (kbps)
0
4.75
1
5.15
2
5.90
3
6.70
4
7.40
5
7.95
6
10.2
7
12.2
8
1
1.
Used for Silence Indication Detection(SID) frames.
1.80
