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594 WIRELESS NETWORKING: EMPHASIS ON MOBILITY
just as your cell phone keeps in touch with the cellular network when it powers - up.
Presto, you ’ re connected, and the quality resembles that of an analog AMPS network,
which is pretty good for a satellite call.
Propagation delay is not much of an issue for several reasons. First, the satellites
are in low - earth orbits of approximately 485 miles, so the uplink and downlink
propagation delays are not signifi cant. Second, the satellites communicate directly
over switched intersatellite links, so only one uplink and one downlink are required.
As the satellites whiz around the earth like electrons whiz around the nucleus of an
atom, however, they have a very short dwell time , so you cannot maintain contact
with any given satellite for very long, and neither can the other party involved in
this telephone call. Therefore, the satellite with which you established the connection
must pass that call off to another satellite before it gets out of view, and so must
the satellite at the terminating end, and so must every satellite in between. Think
of it as a switched cellular network in reverse. In a cellular network, the antennas
at the cell sites are stationary. As you whiz through the cellular network in your
high - speed vehicle, the cellular network maintains the connection through a hand -
off process between cell sites. In an Iridium environment, you are the one who is
(relatively) stationary, while the cell sites whiz around you. Iridium also, by the way,
supports pagers and airplane communications via the same L - band frequencies. If
you call a device that is not on the Iridium network, your call can be connected to
the existing PSTN and cellular networks via 12 regional gateways.
Iridium is an incredible network, and it should be, as the total capital investment
was in the neighborhood of $ 4.7 billion. The system went fully operational on
November 1, 1998. On May 28, 1999, Iridium LLC received a waiver until June 30
of certain fi nancial covenants from all its lenders to enable it to restructure its capitalization.
Those covenants required that the company have at least 27,000 customers
by May 31. It seems as though the actual numbers fell far short of that requirement.
At a cost of $ 3000 or so for an Iridium phone, $ 3795 for a dual - mode phone that
also works on existing cellular networks, $ 500 or so for a pager (one - way, so no
guaranteed message delivery), some modest amount of money for a cable and data
modem, a considerable monthly charge, and $ 3 – $ 7 a minute for the connect time
(depending on which press releases and articles you believe), Iridium was pretty
pricey. Most commercial users just could not justify the cost unless they wanted to
call between very remote areas such as the summit of Mt. Everest (true story) to
the jungles of New Guinea (it is possible if the canopy of the rain forest is not too
thick) [57 – 67] . Iridium ’ s high cost and relatively poor service did not sit well with
prospective users, who failed to appear in numbers even close to those required to
make the network fi nancially viable. Iridium fi led for Chapter 11 bankruptcy in
August 1999 and ceased service in April 2000. At that point, the plan was to let the
satellites naturally degrade in their orbits until they simply burned up in the atmosphere
in a spectacular Iridium fl amb é (at least that ’ s the way I like to think of it).
Rather than let Iridium fl ame out, however, a group of investors purchased the
network for a paltry $ 25 million. Shortly thereafter, in what can only be termed an
interesting coincidence, the U.S. Department of Defense agreed to a $ 72 million,
two - year contract to provide service to 20,000 government users. Other users can
subscribe to Iridium services through one of the 19 or so authorized service partners
in various countries around the world. Costs for airtime have dropped to a range
of $ 1.40 – $ 3.00 per minute, and costs for handsets have dropped to $ 1500 or less.