I found this on Google. Found here
http://www.csd.uoc.gr/~hy431/old/project/cellular.htmlI think this should answer the question.
Air Pressure Primer
This primer discusses air pressure equipment, which telephone companies use to keep underground and aerial lines free from moisture. It's a fascinating and little known aspect of telephony and it will give you more insight into what makes up the local loop and outside plant.
I've tried defining OSP before. Mud Demon comments that "where I work, the outside plant extends up the tip cables from the vault to the termination point at the coils on the main distribution frame.
Anything out from the coils is considered "outside plant" and is maintained by our group. Also the air compressor/dryers, pipes, meter and pipe panels, computer, and even the 48 volt power supply for the computer are considered "outside plant" even
though they are inside."
Inside means inside the central office, most typically in the cable vault below the building. One thing outisde the primer article's scope are the gas bottles you see tied to telephone poles on the outskirts of some cities. These must also be used to keep lines dry. I asked about this and what kind of gas they might use. He replied that "Nitrogen is inert,and relatively inexpensive. Liquid nitrogen
is also inert, and relatively compact, but not used much. Bottles lashed to poles are a sign that an air pipe system is not in place. To achieve a dual feed system without an air pipe, there must be a source of some pressurized gas at the end of the run, so bottles are placed. By comparison, with an air pipe system such as the article describes, air pipes routed through the duct system provide pressurized air at the end of a cable run."
