5.5.4 DSP Resource Usage
Description
To digitally process audio signals, such as a telephone call, the PBX must use a certain number of DSP (Digital
Signal Processing) resources. DSP resources are provided by the DSP cards installed in the PBX. Since there
are a limited number of DSP resources, no further operations (e.g., telephone calls, playing an OGM) can be
performed if all resources are being used.
The following list shows some of the basic operations that require DSP resources.
• IP extension call
• IP trunk call
• Conference
• Accessing the Unified Messaging system (including recording calls)
• OGM playback
• Echo canceller (for trunk-to-trunk analogue calls)
For IP extension and trunk calls, the number of required resources differs depending on the codec (G.711 or
G.729) used.
Note
The examples in this section are intended to illustrate the concept of DSP resource usage. More complex
situations may necessitate additional resources, and in some cases fewer resources may be necessary.
Examples of DSP resource usage
Fundamentally, the number of resources required for a given situation is the sum of the resources required for
each individual operation. The following examples illustrate DSP resource usage in various situations.
[IP trunk to IP extension]
If an IP trunk call using the G.729 codec requires X number of resources, and an IP extension call using the
G.711 codec requires Y number of resources, then the number of resources required for a call from an IP
extension to an IP trunk requires X + Y number of resources.
[Unified Messaging access]
Playing back messages from or recording messages to the Unified Messaging (UM in the figure above) system
requires DSP resources, X in this example, in addition to the resources required for the G.711 codec (Y). The
total cost is X + Y.
476 Feature Guide
5.5.4 DSP Resource Usage
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