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Telephone Basics; Precall Validation -- Process and Terms
We know that pressing send or turning on the phone conveys
information about the phone to the cell site and then to the
MTSO. A call gets checked with all this information. There are
many parts to each digital message. A five digit code called
the home system identification number (SID or sometimes SIDH)
identifies the cellular carrier your phone is registered with.
For example, Cellular One's code in Sacramento, California, is
00129. Go to Stockton forty miles south and Cellular One uses
00224. A system can easily identify roamers with this information.
The "Roaming" lamp flashes or the LED pulses if you
are out of your local area. Or the "No Service" lamp
comes on if the mobile can't pick up a decent signal. This number
is keypad programmable, of course, since people change carriers
and move to different areas. You can find yours by calling up
a local cellular dealer. Or by putting your phone in the programming
mode. [See Programming].
This number doesn't go off in a
numerical form, of course, but as a binary string of zero's and
ones. These digital signals are repeated several times to make
sure they get received. The mobile identification number or MIN
is your telephone's number. MINs are keypad programmable. You
or a dealer can assign it any number desired. That makes it different
than its electronic serial number which we'll discuss next. A
MIN is ten digits long. A MIN is not your directory number since
it is not long enough to include a country code. It's also limited
when it comes to future uses since it isn't long enough to carry
an extension number. [See MIN]
The electronic serial number or ESN is a unique number assigned
to each phone. One per phone! Every cell phone starts out with
just one ESN. This number gets electronically burned into the
phone's ROM, or read only memory chip. A phone's MIN may change
but the serial number remains the same. The ESN is a long binary
number. Its 32 bit size provides billions of possible serial
numbers. The ESN gets transmitted whenever the phone is turned
on, handed over to another cell or at regular intervals decided
by the system. Every ten to fifteen minutes is typical. Capturing
an ESN lies at the heart of cloning. You'll often hear about
stolen codes. "Someone stole Major Giuliani's and Commissioner
Bratton's codes." The ESN is what is actually being intercepted.
A code is something that stands for something else. In this case,
the ESN. A hexadecimal number represents the ESN for programming
and test purposes. Such a number might look like this: 82 57
The station class mark or SCM tells the cell site and the
switch what power level the mobile operates at. The cell site
can turn down the power in your phone, lowering it to a level
that will do the job while not interfering with the rest of the
system. In years past the station class mark also told the switch
not to assign older phones to a so called expanded channel, since
those phones were not built with the new frequencies the FCC
The switch process this information along with other data.
It first checks for a valid ESN/MIN combination. You don't get
access unless your phone number matches up with a correct, valid
serial number and MIN. You have to have both unless, perhaps,
if you call 911. The local carrier checks its own database first.
Each carrier maintains its own records but the database may be
almost anywhere. These local databases are updated, supposedly,
around the clock by two much larger data bases maintained by
Electronic Data Systems and GTE. EDS maintains records for most
of the former Bell companies and their new cellular spin offs.
GTE maintains records for GTE cellular companies as well as for
other companies. Your call will not proceed returned unless everything
checks out. These database companies try to supply a current
list of bad ESNs as well as information to the network on the
tens of thousands cellular users coming on line every day.
A local caller will probably get access if validation is successful.
Roamers may not have the same luck if they're in another state
or fairly distant from their home system. Even seven miles from
San Francisco, depending on the area you are in. (I know this
personally.) A roamer's record must be checked from afar. Many
carriers still can't agree on the way to exchange their information
or how to pay for it. A lot comes down to cost. A distant system
may still be dependent on older switches or slower databases
that can't provide a quick response. The so called North American
Cellular Network attempts to link each participating carrier
together with the same intelligent network/system 7 facilities.
Still, that leaves many rural areas out of the loop. A call
may be dropped or intercepted rather than allowed access. In
addition, the various carriers are always arguing over fees to
query each others databases. Fraud is enough of a problem in
some areas that many systems will not take a chance in passing
a call through. It's really a numbers game. How much is the system
actually loosing, compared to how much prevention would cost?
Preventive measures may cost millions of dollars to put in place
at each MTSO. Still, as the years go along, cooperation among
carriers is getting better and the number of easily cloned analog
phones in use are declining. Roaming is now easier than a few
AMPS carries on. As a backup for digital
cellular, including some dual mode PCS phones, and as a primary
system in some rural areas. See "Continues" below:
and Digital Systems compared
The most commonly used digital cellular system in America
is IS-136, colloquially known as D-AMPS or digital AMPS. (Concentrate
on the industry name, not the marketing terms like D-AMPS.) It
was formerly known as IS-54, and is an evolutionary step up from
that technology. This system is all digital, unlike the analog
AMPS. IS-136 uses a multiplexing technique called TDMA or time
division multiple access. The TDMA based IS-136 uses puts three
calls into the same 30kz channel space that AMPS uses to carry
one call. It does this by digitally slicing and dicing parts
of each conversation into a single data stream, like filling
up one boxcar after another with freight. We'll see how that
works in a bit.
TDMA is a transmission technique
or access technology, while IS-136 or GSM are operating systems.
In the same way AMPS is also an operating system, using a different
access technology, FDMA, or frequency division multiple access.
See the difference? Let's clear this up.
To access means to use, make available, or take control. In
a communication system like the analog based Advanced Mobile
Phone Service, we access that system by using frequency division
multiple access or FDMA. Frequency division means calls are placed
or divided by frequency, that is, one call goes on one frequency,
say, 100 MHz, and another call goes on another, say, 200 MHz.
Multiple access means the cell site can handle many calls at
once. You can also put digital signals on many frequencies, of
course, and that would still be FDMA. But AMPS traffic is analog.
(Access technology, although a current wireless phrase, is,
to me, an open and formless term. Transmission, the process of
transmitting, of conveying intelligence from one point to another,
is a long settled, traditional way to express how signals are
sent along. I'll use the terms here interchangeably.)
Time division multiple access or TDMA handles multiple and
simultaneous calls by dividing them in time, not by frequency.
This is purely digital transmission. Voice traffic is digitized
and portions of many calls are put into a single bit stream,
one sample at a time. We'll see with IS-136 that three calls
are placed on a single radio channel, one after another. Note
how TDMA is the access technology and IS-136 is the operating
Another access method is code division multiple access or
CDMA. The cellular system that uses it, IS-95, tags each and
every part of multiple conversations with a specific digital
code. That code lets the operating system reassemble the jumbled
calls at the base station. Again, CDMA is the transmission method
and IS-95 is the operating system.
All IS-136 phones handle analog traffic as well as digital,
a great feature since you can travel to rural areas that don't
have digital service and still make a call. The beauty of phones
with an AMPS backup mode is they default to analog. As long as
your carrier maintains analog channels you can get through. And
this applies as well as the previouly mentioned IS-95, a cellular
system using CDMA or code division multiple access. Your phone
still operates in analog if it can't get a CDMA channel. But
I am getting ahead of myself. Back to time division multiple
TDMA's chief benefit to carriers or cellular operators comes
from increasing call capacity -- a channel can carry three conversations
instead of just one. But, you say, so could NAMPS, the now dead
analog system we looked at briefly. What's the big deal? NAMPS
had the same fading problems as AMPS, lacked the error correction
that digital systems provided and wasn't sophisticated enough
to handle encryption or advanced services. Things such as calling
number identification, extension phone service and messaging.
In addition, you can't monitor a TDMA conversation as easily
as an analog call. So, there are other reasons than call capacity
to move to a different technology. Many people ascribe benefits
to TDMA because it is a digital system. Yes and no.
Please see the
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2 see also "Cellular Lite: A Less Filling Blend of Technology
& Industry News" Nuts and Volts Magazine (March
1993) (back to text)
[MIN] Crowe, David "Why
MINs Are Phone Numbers and Why They Shouldn't Be" Cellular
Networking Perspectives (December, 1994) http:/www.cnp-wireless.com
[Continues] AMPS isn't dead
yet, despite the digital cellular methods this article explores.
Besides acting as a backup or default operating system for digital
cellular, including some dual mode PCS phones, analog based Advanced
Mobile Phone Service continues as a primary operating system,
bringing much needed basic wireless communications to many rural
parts of the world.
I got an e-mail in late 2000 (11/12/2000) from a reader who
lives in Marathon,
Ontario, Canada, on the tip of the North Shore of Lake Superior.
As he refers to the Lake, "The world's greatest inland sea!"
He reports, "We just got cell service here in Marathon.
It is a simple analogue system. There is absolutely no competition
for wireless service. Two dealers in town sell the phones. In
the absence of competition there are no offers of free phones;
the cheapest mobiles sell for (and old analogue ones to boot!)
$399.00 Canadian . . ." And you thought you paid too much
More recently I got an e-mail from a reader living in Wheatland,
Wyoming. He, too, has only analog cellular (AMPS) to use. [back to text]
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