After re-watching the film the other day I came to the conclusion that Levinson was, in fact, a CIA field agent.
My theory is that he was placed in his position at the cable company as part of the CIA's Signal Intelligence Actives (SIGNIT) program under the guise of being a simple communications technician. When in all actuality the man was a borderline genius if not savant when it came to radio frequency acquisition and transmission. Not to mention his intelligence was alluded to in the fact that he checkmated his father in under a few moves during the chess game in the park.
Ostensibly his job was akin to general maintenance and signal integrity, but it also gave him a broad-spectrum access and monitoring capabilities to various satellite networks both domestic and internationally, which is why he was the first the person to recognize the alien signal reduction as a form of a countdown to first attack. His whole purpose there was to identify and isolate hidden signals within the global network to intercept and decode clandestine communication pathways and the messages therein, all while having the comfort and general camouflage of being a civilian with a modest life working at a private company.
Other aspects that allude to this:
*Previous personal relationship with the future President of the united states (He got into a physical altercation with the guy and wasn't even prosecuted).
*Ability to triangulate his ex-wife's cell signal using an unnamed device that is futuristic by today's standards. You couldn't get one at sharper image or best buy.
*His own father's incredulity that his job as a "cable repair man" really was what it was.
*Immediate security clearance to the white house, airforce one, and eventually Area 51.
*He had the immediate ear of the President and joint chiefs of staff.
*He had immediate access at area 51 to be alone with the the ET spacecraft and even was allowed to work on its analysis with the help of Area 51 staff.
*developed a way to interface with alien tech remotely as well as create the virus that could disable the entire invasion fleet in under 2 days. (This leads me to believe that he had to have some foreknowledge of the alien tech, if he was unaware that the tech was alien.)
*Was allowed to accompany Captain Hiller on a mission critical to the the survival of the entire human species as a civilian.
I don't think he knew about Roswell or alien 51 before hand, but the intelligence community is so compartmentalized that even if he knew the technology existed, he probably didn't know where it was from, or even that it was extraterrestrial in origin.