DSPs provide four major functions in a voice gateway:

- Voice termination: is the process of digitizing and packetizing the audio stream on a TDM interface. A DSP is required to convert the traditional audio stream to VoIP.

- Trancoding: is the process of matching two disparate VoIP streams. This is required when the codecs or sampling rate of two VoIP streams do not match.

- Conferencing: The DSPs in a voice gateway cna be registered to a CallManager to provide hardware based conferencing.

- Media termination point (MTP): You insert MTP's into a call to provide supplementary services if H323v2 is not supported end to end. Supplementary services include hold, converence, or other signaling instructions that are invoked during the call.

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_configuration_example09186a0080334294.shtml

Here is the DSP calculator:

http://www.cisco.com/cgi-bin/Support/DSP/dsp-calc.pl

Cisco voice gatewasy use various DSP models with different capabilities. Some GW and Network Moduls (NM) use modular DSPs, or Packet Voice DSP Modules (PVDM).