ZyXEL Communications P-27xx Series User Manual page 48

Chapter 6 Specifications
For a horizontal installation, do not place the device in an inverted position;
5a
make sure the holes on the back of the ZyXEL Device are positioned at the top
and that they match the positions of the holes drilled into the wall. Ensure that
the two side vents remain unblocked.
For a vertical installation, ensure that the holes on the ZyXEL Device match
5b
the positions of the holes drilled into the wall and that the two side vents
remain unblocked.
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P-27XX Series User's Guide