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Cellular 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].

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Cellular Basics Series

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

Sub-Menu

Cellular Basics Series I Introduction

II Cellular History

lII Cell and

SectorTerminology

IV Basic Theory and Operation

V Cellular frequency and channel discussion

VI. Channel Names and Functions

VII. AMPS Call Processing

A. Registration

B. Pages: Getting a Call C. The SAT, Dial Tone, and Blank and Burst

D. Origination -- Making a call

E. Precall Validation VIII. AMPS and Digital Systems compared IX. Code Division

Multiple Access -- IS-95

A. Before We Begin -- A Cellular Radio Review

B.Back to the CDMA Discussion

C. A Summary of CDMA -

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 2C 01.

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 allowed.

- Another transmission technique

The switch process this information along with other data. It first checks for a valid ESN/MIN

- Another transmission technique

D. A different way to share a channel

E. Synchronization F. What Every Radio System Must Consider G. CDMA Benefits

H. Call Processing -- A Few Details

X. Appendix

A. AMPS Call Processing Diagram

B. Land Mobile or IMTS

C. Early Bell System Overview of Amps D. Link to Professor R.C. Levine's .pdf file introducing cellular. (100 pages, 374K) Reserved Reserved

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

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 years ago.

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:

VIII. AMPS 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

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 system?

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 access.

TDMA's chief benefit to carriers or cellular

operators comes from increasing call capacity -- a channel can carry three conversations instead

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 next page -->

NOTES

[Programming]Thorn, ibid, 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 for cellular.

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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http://www.privateline.com : West Sacramento, California, USA. A Tom Farley production

Privateline.com: Cellular Telephone

Basics

The Web Privateline.com

SITE MENU HOME PAGE Old Home Page Advertise here Cell Phone Plans Cell Phone Basics Clip Art/Images Contact Me! Daily Notes Digital Basics Telecom History Links Miscellany Telecom News Website Docs Wired Telecom Wireless Pages Writers Sub-Menu

Cellular Basics Series I Introduction

II Cellular History lII Cell and

SectorTerminology IV Basic Theory and Operation

V Cellular frequency and

WiWCellular Telephone Basics

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(Page 8) Cellular Telephone Basics continued . . .

Advanced features depend on digital but conserving bandwidth does not. How's that? Three conversations get handled on a single frequency. Call capacity

increases. But is that a virtue of digital? No, it is a virtue of multiplexing. A digital signal does not

automatically mean less bandwidth, in fact, it means more. [See more bandwidth] Multiplexing means transmitting multiple conversations on the same frequency at once. In this case, small parts of three conversations get sent almost simultaneously. This was not the same with the old analog NAMPS, which split the frequency band into three discrete sub- frequencies of 10khz apiece. TDMA uses the whole frequency to transmit while NAMPS did not.

This is a good place to pause now that we are talking about digital. AMPS is a hybrid system, combing digital signaling on the setup channels and on the voice

channel when it uses blank and burst. Voice traffic, though, is analog. As well as tones to keep it on frequency and help it find a vacant channel. That's AMPS. But IS-136 is all digital. That's because it uses digital on its set-up channels, the same radio

frequencies that AMPS uses, and all digital signaling on the voice channel. TDMA, GSM, and CDMA cellular (IS- 95) are all digital. Let's look at some TDMA basics. But before we do, let me mention one thing.

Sponsor Sponsor Aslan Technologies Link to Aslan Sponsor Reserved Google Search channel discussion VI. Channel Names and

Wonderful information on IS-136 here. It's from a chapter in IS- 136 TDMA Technology, Economics, and Services, by Harte, Smith,

channel discussion VI. Channel Names and Functions

VII. AMPS Call Processing A. Registration

B. Pages: Getting a Call C. The SAT, Dial Tone, and Blank and Burst D. Origination -- Making a call

E. Precall Validation VIII. AMPS and Digital Systems compared IX. Code Division

Multiple Access -- IS-95 A. Before We Begin -- A Cellular Radio Review B.Back to the CDMA Discussion C. A Summary of CDMA - - Another transmission technique D. A different way to share a channel E. Synchronization F. What Every Radio System Must Consider G. CDMA Benefits H. Call Processing -- A Few Details

X. Appendix

A. AMPS Call Processing

Wonderful information on IS-136 here. It's from a chapter in IS- 136 TDMA Technology, Economics, and Services, by Harte, Smith,

and Jacobs (1.2mb, 62 pages in .pdf)

Book description and ordering information (external link to Amazon.com)

I wrote in passing about how increasing call capacity was the chief benefit of TDMA to cellular operators. But it is not necessarily of benefit to the caller, since most new digital routines play havoc with voice quality. An uncompressed, non-multiplexed, bandwidth hogging analog signal simply sounds better than its present day compressed, digital counterpart. As the August, 2000 Consumers Digest put it:

"Digital cellular service does have a couple of drawbacks, the most important of which is audio quality. Analog cellular phones sound worlds better. Many folks have commented on what we call the 'Flipper Effect." It refers to the sound of your voice taking on an 'underwater-like' quality with many digital phones. In poor signal areas or when cell sites are struggling with high call volume, digital phones will often lose full- duplex capability (the ability of both parties to talk simultaneously), and your voice may break up and sound garbled."

Getting back to our narrative, and to review, we see that going digital doesn't mean anything special. A

multiplexed digital signal is what is key. Each frequency gets divided into six repeating time slots or frames. Two slots in each frame get assigned for each call. An empty slot serves as a guard space. This may sound esoteric but it is not. Time division multiplexing is a proven technology. It's the basis for T1, still the backbone of digital transmission in this country. Using this method, a T1 line can carry 24 separate phone lines into your house or business with just an extra twisted pair.

Demultiplexing those conversations is no more difficult than adding the right circuit board to a personal

computer. TDMA is a little different than TDM but it does have a long history in satellite working.