This depends on the compatibility of sinks (e.g. TV), sources (e.g. Blu-ray player) and the SWW1800.
If both your sink and your source support 1080p@24 Hz, under certain circumstances special configurations might be required for your sink to display 1080p@24 Hz. Although it is device-dependent, in most cases it means that you have to select 1080p@24 Hz manually or, if you have already done so, switch to AUTO resolution and select 24 Hz ON at your source. For details, please consult the instruction manual for your source.
If both your sink and your source support 1080p@50 Hz/60 Hz only, your sink might display 1080i because the SWW1800 does not support 1080p and 50 Hz/60 Hz.