If the isd file is too big, Simplicity Studio can't open the whole file directly. but you can use the below way to open the selected interval of the isd file.
1. open the isd file with Simplicity Studio, it shows as below.
2. disable the "Auto-zoom' option.
3.drag the mouse from start timestamp to the end timestamp to select the interval.
4. click Open interval button, the selected interval of the isd file will be opened.
5.Click the "Save Segments" to save this segment of the packet trace.