Will communication towers be rendered obsolete by satelittes
performing the present communication tower function - in the next 20
years? I am told that the present communication tower infrastructure
will be used for decades to come. What is the consensus of educated
people in this field? Person answering should be knowledgable in this
field. I am asking this question as part of due diligence for
investment purposes.

There has been an answer in the past few years and neither of your
communication systems, towers or satellites has won.  A few years ago,
a satellite system, Teledesic, was proposed to provide high speed
digital commumication between any points on earth.  It failed in that
after spending more than $100 million on design, it was not
constructed because an alternative system could provide higher speed,
higher capacity and lower cost.  This winning system is optical fiber.
 According to a Cisco website:
http://www.cisco.com/univercd/cc/td/doc/product/mels/cm1500/dwdm/dwdm_fns.htm
over one fiber at one wavelength, it is possible to transmit data at
2.5 to 40 Gbs (2.5 to 40 times 10^9) bits/second and one can send 16
to over 160 different closely space wavelengths for potentially 40 x
160 = 6400 Gbs over one optical fiber.  This capacity is incredably
higher than any satellite or tower system.  This capacity growth is
what severely damaged the fiber optic industry, in that a single fiber
can carry an immense flow of data, so who needs more fibers.  You can
find many old Teledesic web sites via a Google search.  They all paint
a rosy picture of a failed idea and business plan.

A satellite system that was actually built is Iridium, that provides
satellite phone service world-wide.  Service started 11/98, entered
chapter 11 bankruptcy 8/99.  Iridium cost ~$5 billion and was
purchased by Iridium LLC in 2001.  Iridium is now in operation, in
large part supported by US military users.  Iridium provides low data
speed, about 2400 bits/second.  Find Iridium info at:
http://en.wikipedia.org/wiki/Iridium_(satellite)

I would view investment in either towers or satellite data
transmission systems as generally not competitive with buried optical
fiber.

My question centered on whether or not Towers would be obsolete in 20
to even 30 years. I realize there is no "absolute" answer. But, if
fiber optic is not feasible due to costs to install; Iridium's costs
are in the billions and the only one who can afford it is the military
> seems that Towers are needed to service the mass population for a
long time to come.
Your conclusion that that towers or satellite is NOT competetive with
buried optical fiber is a bit confusing from a practical/reality
perspective. The cost of burying the fiber optic cables is
enormous...most likely more than the satelittes if considered on a
national basis.

I would like more comment on this subject.

Every system has it's limits, an boundaries that it can reach &
everything has a potential for failure, back-ups to modern systems
will continue to b in use for along time to come, communication towers
are expected 2 b used as emergency backups to the satalite system, as
of when they may fail, the military also has plans to continue using
comunication towers. As the general use of the towers decreases the
military will have a free channel to comunicate with nothing else
using the system, as the analogue system will of been replaced by the
digital, & the interference caused by so many of these signals will be
a thing of the past & of a great use 2 the military in both tracking a
signal & comunicating over vast distances, comunication towers still
have a big part 2 play as it will be the cheapest of them all, & will
always have a major use

Making 20 year predictions in technology is risky, but I?ll take a
crack anyway.  I would argue that there are some fundamental physical
reasons why satellite is unlikely to do away with comms towers.

Different techs are suited to different purposes.  Satellite is best
where ubiquitous coverage is vital (think satellite phones for
emergency workers) or where many users are being broadcast the same
information (think satellite TV).

However, satellite has some disadvantages.  It is distant, so anyone
sending signals up to the satellite needs a relatively powerful
signal.  This can be an issue if the device is mobile since it impacts
battery life.  Thus cell phones are unlikely to switch to satellite
any time soon.

Also, because of the distance satellite links have higher latency ? a
lag between sending a signal and getting a response.  This doesn?t
matter for TV, but is annoying for voice and can be detrimental for
data.  (One solution to the latency problem is to have many low-orbit
satellites, like Iridium, but this is an expensive and complex
solution).

Finally, because of the distance, the frequency of the radio signal
needs to be lower.  This inherently limits the amount of bandwidth
available for satellite comms ? the higher frequencies used for some
terrestrial transmissions are not suitable.  This is not an issue at
satellite traffic volumes today, but if you were discussing a scenario
where a significant percentage of wireless traffic migrated to
satellites, it might be a problem.

Terrestrial wireless solutions do require many towers, but they avoid
the problems above, and can make more efficient use of spectrum (since
a given frequency can be reused many times in different locations).

One coming technology that will make additional usage of communication
towers is wimax, which can be crudely thought of as long-range wi-fi. 
See

http://www.intel.com/netcomms/technologies/wimax/
http://en.wikipedia.org/wiki/Wimax

for more details.  A number of major companies are investing in wimax,
which will use comms towers, so I think you can take some comfort that
the industry consensus is against the idea that satellites will make
them irrelevant.

Incidentally, I currently work for a major US telco, but I am not an engineer.

I don't know about obsolete, but Im in South Mississippi and we've
just experienced the devastation of our landline, fiber optic and cell
tower infrastructure by Hurricane Katrina.  The only way to
communicate in most places is with a SATELLITE PHONE!

Please refer to the following blog:

http://spinynebula.blogspot.com/2004/10/limits-of-technology-example-by.html