Technical Helpweb

RTP latency may be greater than desired in certain Dialogic^® Integrated Media Gateway (IMG) call scenarios where there are multiple VoIP legs within a local area network.

Reason for the issue:

 

The IMG VoIP modules use default minimum and maximum Jitter Buffer Delay values of 75 and 150ms respectively, which are suitable for most IP network environments; however, in some cases it may be desired to alter the Jitter Buffer Minimum and Maximum values to reduce overall latency on a VoIP call that includes the IMG.  
Because it is usually not necessary to override the default Jitter Buffer parameters, these settings are not configurable via the Dialogic^® Gate Control Element Management System (GCEMS) ClientView graphical user interface. 

Fix / Solution:

 

A custom API file can be created and downloaded to each IMG to adjust the minimum and maximum allowed jitter buffers. 

Note that this configuration change is particularly applicable to configurations where multiple VoIP 'hops' take place in a local area network and, therefore, the latency in the network and the quality of service are predictable.

Before applying this change, it is important to understand the possible implications of the change when wide area network or the Internet connections are involved since these networks may have unpredictable latency or varying quality of service.  Therefore, it is advisable to thoroughly test changes that override default settings prior to deployment in a production network.

To make the changes, there are three steps:

 - create the IMG raw API file
 - apply the file to the IMG
 - verify the results

To create the raw API file, you will need to know the IP addresses of the IMG VoIP module(s) that is/are being modified.   These addresses are displayed in ClientView, in the VoIP Module object under the Network Interfaces object.

Once the module IP addresses are known, convert them to hexadecimal format.  For example, 10.10.10.2 is converted to 0x0a 0x0a 0x0a 0x02.

Create and download the raw API file as described on this page of the IMG Webhelp, substituting the required IP addresses in bytes 21-24. 

In the examples below, the last byte represents the value of the parameter being changed:
 

' For Module 0, IP address 10.10.10.2, set Minimum Jitter Buffer Delay to 10ms, 0x0a 

00 1b 00 e3 00 00 ff    00 01 01 01 04    00 02    00 09 00 06 3e 04 0a 0a 0a 02 01 c2 00 01 0a
 

' For Module1, IP address 10.10.10.3, set Maximum Jitter Buffer Delay to 20ms, 0x14

00 1b 00 e3 00 00 ff    00 01 01 01 05    00 02    00 09 00 06 3e 04 0a 0a 0a 03 01 c3 00 01 14

To verify the changes from the IMG Telnet Advanced Debug prompt, type v, then M0 for module 0 and M1 for module 1.  The current values will be displayed, with any overridden parameter values marked with a preceeding asterisk. 

In the following example, the minimum has been overridden, but the maximum remains at its default:
 

*Minimum JB Delay: 20ms
Maximum JB Delay: 150ms 

Product List

Dialogic^® Integrated Media Gateways

Related Documentation

Configuring VoIP on the IMG Gateway:  www.dialogic.com/webhelp/IMG1010/10.5.1/WebHelp/Procedures/IP/con_voip.htm
Using a raw API file:  www.dialogic.com/webhelp/IMG1010/10.5.1/WebHelp/Procedures/General/down_raw_file.htm