Unless Nissan has changed their design drastically from the 1999 Sentras,
the CamPS is an integral part of the distributor.  On the 1999, the
distributor is driven directly by the camshaft. I had to replace mine with
a new one because of two problems:

The distributor timing is an electronic sensor of the rotor position via a
metal disk with many small slots and a photosensor. In mine, the seal
between the distributor shaft was worned out, allowing oil from the cam
area to seep through and coated this disk, thus making it unable to send
correct rotor position. The engine started to misfire and eventually it
wouldn't run at all. If you let the oil to drip down for an hour or so,
the  engine would start up again, but in about 10-15 minutes, the same
problem will return. Cleaning this disk with a carb cleaner would make it
work and run for many more miles until the oil started to do the same
thing. Sometimes after that I started to get code for bad CamPS. Bought a
NEW and complete distributor from eBay for $150. That did the trick.

download the 2001 fsm from http://www.turbo6.net/vqtech/ESM/Nissan/Sentra/
look in the file ec.pdf on page ec-356 you will find a picture of its
location (its under the engine near the starter,according to the pics)