It seems that I-Frame size in H263 bitstream strongly relies on framerate. Given a QCIF sequence, if set frame interval to 1001 (30fps) in the par file, the I frame size is 2659 bytes; if set frame interval to 4004 (7.5fps) in the par file, the Iframe size become 6110 bytes.
Both 2659 bytes and 6110 bytesa frame are too big for a low bit rate network because one frame will be sent within a few ms which will cause a burst on the network. Some gateways will block this kind of burst. Is there any way to only control the IFrame size without relate to frame rate?