Dialogic IP Media Library API Programming Guide and Library ...

Using the AMR-NB Audio Coder
8.3 Sample Scenarios
The following sections show sample scenarios for using AMR-NB and for setting CMR and FT
values:
Sample Scenario: Transmit at Bit Rate Requested by Remote Side
Sample Scenario: Transmit at Bit Rate Less Than Preferred Value
Note: Using the AMR-NB resource in connection with one or more Dialogic products does not grant the
right to practice the AMR-NB standard. To seek a license patent agreement to practice the standard,
contact the VoiceAge Corporation at www.voiceage.com/licensing/php.
8.3.1 Sample Scenario: Transmit at Bit Rate Requested by
Remote Side
In this example, the host application wants to transmit at the bit rate requested by the incoming
CMR value. The following diagram depicts this use case, where the local side is the host
application. FT refers to Frame Type and CMR refers to Codec Mode Request.
Local Side Remote Side
FT=4 (7.4 kbit/s), CMR=5 (7.95 kbit/s)
FT=5 (7.95 kbit/s), CMR=6 (10.2 kbit/s)
FT=6 (10.2 kbt/s), CMR=5 (7.95 kbit/s)
Note: While the diagram above implies an immediate reaction to a CMR from the other side, in reality,
the other side’s response to a CMR may take a few packets.
The sequence of activities is as follows:
1. The application on the local side:
a. Sets its preferred bit rate via the remote audio coder settings, in this case 7.4 kbit/s.
b. Sets its CMR value via the local audio coder settings, in this case 7.95 kbit/s.
c. Sets the RTP payload format to bandwidth efficient and its CMR rule to “CMR Tracking”
via the coder options settings.
d. Starts RTP streaming using ipm_StartMedia( ).
2. The IPM device on the local side transmits at the preferred bit rate, in this case 7.4 kbit/s.
3. The IPM device on the local side receives its first packet from the remote side with a CMR
value which is higher than its preferred bit rate. In this case the CMR value of 10.2 kbit/s is
higher than the preferred bit rate of 7.4 kbit/s.
42 Dialogic ® IP Media Library API Programming Guide and Library Reference
Dialogic Corporation