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

Using the AMR-NB Audio Coder
Local Side Remote Side
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 Limit” 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.
4. Since its CMR rule is set to “CMR Limit”, the IPM device on the local side cannot exceed the
preferred bit rate, so it leaves the transmitted bit rate at the preferred value, in this case 7.4
kbit/s.
5. The IPM device on the local side receives a packet from the remote side with a CMR value
which is less than its preferred bit rate. In this case, the CMR value of 6.7 kbit/s is less than the
preferred bit rate of 7.4 kbit/s.
6. Since the new received CMR value of 6.7 kbit/s does not exceed the limit (7.4 kbit/s), the IPM
device on the local side changes its transmitted bit rate to match the received CMR value (6.7
kbit/s).
Example Code
The following code demonstrates the configuration required to handle this scenario.
...
/* Setup IP address here */
FT=4 (7.4 kbit/s), CMR=5 (7.95 kbits/s)
FT=5 (7.95 kbit/s), CMR=6 (10.2 kbit/s)
FT=4 (7.4 kbit/s), CMR=5 (7.95 kbits/s)
FT=5 (7.95 kbit/s), CMR=3 (6.7 kbit/s)
FT=3 (6.7kbit/s), CMR=5 (7.95 kbit/s)
// Local Audio Coder
ipmMediaInfo.MediaData[unCount].eMediaType = MEDIATYPE_AUDIO_LOCAL_CODER_INFO;
44 Dialogic ® IP Media Library API Programming Guide and Library Reference
Dialogic Corporation