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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 -- 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)



WiWCellular Telephone Basics
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Cell phone privacy

Q. Is it possible for someone to listen in on my cell phone call?

A. Possible but not probable. Today's digital cell phones are nearly impossible to monitor by the public, radio hobbyists, or any non-governmental investigative agency. Law enforcement and the wireless carrier itself can listen in but they would tap at the telephone switch level, not over the air. Just like with any ordinary wiretap. Nothing can be done about that. Let's not consider targets of the FBI or CIA. What do ordinary folks have to worry about?

Wireless to wireless calls

All digital phones like GSM/PCS types, ones that use a smart chip, are essentially impossible to monitor over the air. No over the air intercepts have been publicly recorded. Mobiles use a strong encryption scheme which encodes voice traffic flowing between the mobile and the cell site. Two people communicating with each other, each using a GSM/PCS phone, should have little to worry about. Older analog cellular telephones, though, can be easily monitored with a scanning radio or another cell phone, one that's been modified to do that. This brings up a problem: you both may have a digital phone, but they may not be operating in digital mode.

Some digital mobiles (non-GSM/PCS, ones that don't have a smart chip) default to an analog mode when a caller is out of a digital service area. So, if you are on an all digital phone in New York City but talking to your friend Frank in rural South Dakota, both sides of your conversation could be monitored from the South Dakota side. That's because Frank's mobile might be in analog mode. The person monitoring would have to be quite close, probably in the area of the nearest cell. Many mobiles signal when they default to analog but many don't.

If you're really worried use a GSM/PCS mobile and make sure the party you're talking to does as well. No, you won't have the best coverage across the states since the GSM/PCS networks aren't as built out. Failing that, if the other party doesn't have a smart chip enabled phone, make sure they are in digital mode with the mobile they are using. Until now we've been discussing wireless to wireless calls. But what if one party is on a land line?

Wireless to wireline

A far easier way to listen to your conversation is by tapping a landline phone from inside or outside the building you are in. At your firm's PBX, the telephone demarc point on an outside wall, or out on the street or up on a pole at a telephone company junction box. Any amateur can do this. In this case, if you are really paranoid, you can contact one of the companies that sell encryption units for wired and wireless phones. In this case you'd have a black box wired into your desk phone. Your people in the field would have smaller, matching units on their cell phones. Your call would then be encrypted from end to end. Something like the military has for secure lines, although with less strength in the encryption. But encryption equipment is expensive stuff and you'd need an extraordinary threat to justify getting it.

Privacy issues

Is privacy really a concern when using a mobile? I never worry about it. But I can see, from a liability and ethical point of view, that lawyers, accountants, and medical professionals might be very concerned about discussing matters over the radio. What happens if a client's privacy is violated? Companies trying to protect their secrets would also be at risk from stockholders if they failed to investigate this security issue. I think by taking the few steps above that you can avoid any problems.

One last thing since we've been discussing privacy: talk and toss phones, the pre-paid phones you get at a corner market or even at cellular dealers. They should be all digital, check first, and if you pay with cash there should be no records traced traced back to you, anymore than with a pre-paid telephone card. Even the spooks would have problems at first since they wouldn't have a telephone number to begin monitoring. If two people are both calling each other with this kind of phone then over the air privacy and record privacy will be virtually assured.

Spyware or vaporware? The website advertising this product says that it can intercept GSM calls but must be custom made to the cellular radio standards of your country and for your needs. Such equipment would be illegal to use in the United States. No price or maker is given for this device. I find it very odd. Perhaps if you express interest in this someone starts a file on you . . .
PGFDigitalCellularIntercepter.htm (external link)

Using a cell phone to eavesdrop

Q. If I call home I can listen to the conversations in the room where my answering machine is. Can my cell phone be used in the same way? So that I might never know my conversation is being monitored? Sites like these claim to modify cell phones to do so: (external link)

A. Theoretically possible but not likely. Such a device would be illegal to make or own in the United States. They ask that you send them a phone to retrofit. I suspect both your phone and money would disappear. That company in particular insist they are not thieves. Fair enough. But they do not say whether their equipment is FCC type approved or whether they would refund your money if your phone was seized by customs when sent back to the States.

The engineering and software mods needed would cost far more than anything they charge; all cell phones these days use surface mount components, microscopic wire connections, and software using strong encryption. Modifying an existing phone is possible, perhaps, by only the most advanced intelligence agencies.

Having written this, a very few cell phones do have a listening capability built in. GM's OnStar and Mercedes Benz' Tele-Aid systems, both cellular radio schemes, allow on-board monitoring in case of an emergency. One U.S. Appellate court, though, has disallowed the F.B.I. from listening in, even with a lower court wiretap order: (external link)

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