See the manual of ippsMan.pdf=> FIRMR_Direct
Parameter associated with the number of samples to
be filtered by the function. The (numIters *
downFactor) elements of the source vector are filtered
and the resulting (numIters * upFactor) samples are
stored in the output array.
In addition, there are some sample code about almost all of FIR function usage in \ipp-samples\signal-processing\signal-processing-functions. IPP Downloads, Registration and Licensing.It isin MSVC 2005 project if windows. You may refer to them.