This document covers the procedures for configuring conferencing and transcoding support on Cisco IOS voice gateways. The procedures that you perform depend on the type of voice network module you are using to allocate DSP resources.
Determining DSP Resource Requirements
DSPs reside either directly on a voice network module, such as the NM-HD-2VE, on PVDM2s that are installed in a voice network module, such as the NM-HDV2, or on PVDM2s that are installed directly onto the motherboard, such as on the Cisco 2800 and 3800 series voice gateway routers. You must determine the number of PVDM2s or network modules that are required to support your conferencing and transcoding services and install the modules on your router.
STEPS 1. Determine performance requirements.
2. Determine the number of DSPs that are required.
3. Determine the number of network modules that are supportable.
4. Verify your solution.
5. Install hardware.
In regards to calculating DSP`s, there is already a tool for that