I have seen a lot of confusion here over cellular telephone technology
latley. I will attempt to explain some of the basic principles of how
cellular telephones work. Please keep in mind that beyond the basic
technology, each Cellular Operatiing Company can change a lot of things.

A cellular system consists of a geographical area, such as a city. I
will use Orlando as an example here. The coverage in Orlando is well
over a hundred miles from edge to edge, however there exist much larger
systems such as the New York/New Jersey area. The size is limited only
by the number of cells, and the system's capacity for number of cells.

A system is divided into "cells". Each cell has it's own transmitter
and receiver, which is connected via leased lines to the cellular
central office (CO).

Each cell has it's own "control" frequency on which it accepts and
issues calls.

Each mobile unit is identifed by two numbers that are encoded in it's
memory. The first is a serrial number, which is guaranteed to be
unique anywhere in the world (kind of like Ethernet), the second is
the ten digit telephone number of the unit that it has been assigned.
(ie: mine is 305-222-xxxx)

Incomming calls:

        When an incomming call arrives at the cellular CO (305-222)
     the system announces the call to ALL CELLS on their control
     channel. A mobile unit is given a time threshold to respond
     on the control channel. If it does not respond, the caller is
     given some message such as "The Bell South Mobility customer
     you have dialed, has travelled beyond the service area, or has
     left the vehicle". If the mobile does respond, it sends it's
     two identification numbers, and is assigned a voice frequency
     by the CO from the nearest cell site (strongest signal). The
     mobile phone then begins ringing. And the call is left to
     complete (ie: either the person answers, or the phone keeps ringing).

Outgoing calls:

        The customer enters the number he wishes to call, and pushes
     a "SEND" button on the phone. The mobile unit transmits it's two
     identification numbers, and the destination number over the control
     channel to the nearest cell, and the call is either validated, and
     a voice channel is assinged for the completion of the call, or the
     call is rejected, and the mobile issues a siren tone.

Traveling within the service area:

        Once the call has been established, the cellular system constantly
      monitors the signal strength of each mobile in each cell that has an
      active call. As the signal drops below an acceptable level, the system
      automatically selectes a another cell and transmits the new frequency
      to the mobile. Then the call is switched in less than a second, and
      the change is usually inaudiable.

Roaming:

        Roaming occurs when a cellular mobile from system A travels into
       the service area of system B (ie: Orlando to Miami). In most cases
       the unit is allowed to place credit card and local calls only. However
       if system B is operated by the same company (ie: Bell South) it's networ
       may authorize billing and allow the mobile to place toll calls.

       The mobile may also receive calls. However the person who is calling
       you must know which city you are in, dial the access number for that
       city, wait for a second dial tone, and then key in your full ten
       digit telephone code (on a touch-tone telephone). Then the call proceeds
       as described above.


>From my own personal experience, I have had a cellular phone in my car for
almost a year. I drove from Orlando to Montreal with it over christmas, and
was able to use my phone without any problems in the following cities: (I
took I-95 to NY and then I-87 [thuway]) Jacksonville, Charleston, Norfolk,
Richmond, Washington/Baltimore, Wilmington, New York/New Jersey, Albany,
and Montreal. It most cities I was able to place toll calls directly, which
later appeared on my regular Orlando bill as "Roamer" changes. I was also
able to receive calls while driving from people who knew which city I
was in (I had supplied them with time estimates and access numbers).

What really surpried me was the size of the service area around Montreal.
I was able to use my cellular telephone from within the Adirondak State
Park on the New York Northway, my service was being provided by the "Bell
Canada Northern Corridor Service", at that time I was over 35 miles away
from the Canadian border! 70 miles away from Montreal! I did however later
find out that this service was not part of the Montreal system.

Cellular phone can get costly too. I pay $30/month for leasing the
equipment, which I get to keep after 36 months. $20/month for service,
and $0.35 peak/$0.22 off peak minus 6% AT&T corporate discount (even
for personal use). A usual month's bill is $100. Roaming gets much
more expensive. It's anywhere from $0.40 to $1 per minute while roaming,
and some [but not many] cities also add a $1 to $5 /day charge for
roamers. If I were a roamer in Orlando, I would be paying $0.55 per
minute for airtime. These charges apply for inbound and outbound calls
alike, and area *VERY* service area dependant. In Orlando, 911 and
811 (Customer Assitance) calls are free.

The cellular companies also make out on mobile to mobile calls, since
then they get two people paying for the call.
--
                                        Mikel Manitius @ AT&T-IS
                                        mikel@codas.att.com.uucp

          Copyright 1987. Redistribution via Stargate PROHIBITED