[INTERNAL] refactored parts of ISN

Tried to cleanup that messy code. Found nice FIXMEs while doing so.

Change-Id: Ia2c7274c4e7d877b563d074146702c04d8664359
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1661
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>