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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)


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WiWCellular Telephone Basics

Pages in This Article (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)

Wired article/ A future for cell phones on planes -->

Why can't I use my cellular phone on an airliner?

Cell phones on airliners interfere with the terrestrial cellular telephone network. Interference with a plane's avionics or navigation system is poorly documented and a secondary problem. Except to the Federal Aviation Administration, which usually favors no risks and consequently prohibits cell phone use on-board. Why allow the possibility of a cell phone interfering if you can simply ban them? But let's get back to the problem we can demonstrate.

Cell phones transmit in nearly straight lines. From an airplane a cell phone can connect to nearly any cell site in view below, causing much turmoil, especially with a jet moving 500 miles an hour, passing by one cell after another far more quickly than the systems were designed for. Here's something from the cellular basics article to clear this up:

Mark van der Hoek describes two people, a businessman using his cell phone in the city, and a hiker on top of a mountain overlooking the city. The businessman's call is going well. But now the hiker decides to use his phone to tell his friends he has climbed the summit.

From the climber's position he can see all of the city and consequently the entire area under cellular coverage. Since radio waves travel in nearly a straight line at high frequencies, it's possible his call could be taken by nearly any cell. Like the one the businessman is now using. This is not what radio engineers plan on, since the nearest cell site usually handles a call, in fact, Mark points out they don't want people using cell phones on an airplane "Knock it off, turkey! Can't you see you're confusing the poor cell sites?"

It may be true that airlines profit so greatly from their own in-plane phone services (like the GTE Airfone) that they do not want people using their own mobiles. It may also be true that cell phones do cause interference with an airplane's electronics. Read the _Wired_ article below. But remember, proving either is difficult compared to showing the real problem they present to the terrestrial cellular telephone network. At the least, though, it seems reasonable that cell phones should be allowed to work while a plane is on the ground.

Wired logo

Wired.com (external link)

Before you read this article, remember two points. 1) A ban on all radio frequency transmitters existed long before services like Airfone were installed. 2) An Airfone antenna is mounted outside of a plane's fuselage, causing the least amount of RF pollution to interior mounted avionics, whereas cell phones and their antennas are inside a plane's cabin, in a position more likely to cause interference.

Copyright 2001 Wired, All rights reserved. This article appears pending permission.

February 15, 2001

If We Can Fly, Why Can't We Talk? by Elisa Batista

The world is going mobile everywhere except in the air.

A Saudi Arabian army captain received 70 lashes earlier this month for using his mobile phone during an airplane's takeoff.

British oil worker Neil Whitehouse spent a year in jail for refusing to shut off his cell phone during a 1998 British Airways flight from Spain.

See also: Is Phone Interference Phony? Few Options For Yakkin' Flyers Can Cell Phones Crash Planes? Are Airborne E-Devices a Danger? Unwired News: The Next Generation

Swiss investigators believe that mobile phone interference may have helped cause last year's crash of Crossair flight LX498, which went down shortly after takeoff from the Zurich airport, killing all 10 passengers on board.

A Slovenian flight on the way to Sarajevo made an emergency landing last month after the cockpit fire alarm went off. Investigators say a cell phone left turned on in the luggage compartment triggered the erroneous warning.

To the frustration -- if not incredulity -- of airplane passengers, whose only option to communicate with someone on the ground is airplane seat-installed phones, the aviation industry touted these incidents as more proof that cell phone use in flight is dangerous.

And that belief only reinforces the industry's resolve to keep permanent a ban on using the devices during flights.

"Beyond a shadow of a doubt, (handheld devices) can interfere under very precise circumstances," said John Sheehan, who headed an RTCA study showing that portable electronic devices could interfere with a plane's navigation and communication systems.

"But it's a rare occurrence."

Rules and regulations are increasingly at odds with social, political and economic phenomena. On one hand, there are passengers who would like to see all portable electronic devices banned because they find them annoying -- even the ticking away at a laptop computer's keyboard, said U.S. Rep. John Duncan, Jr. (R-Tenn.). The use of laptop computers is generally allowed for the duration of flight and airplane-seat installed phones can be used any time.

"It's sort of like smoking," Duncan said in a July hearing on whether PEDs really pose a safety hazard to passengers. "When people ask, 'Do you mind if I smoke,' most people are too polite to tell them that they are, even though they hope secretly that they will not smoke. And in the same way, people really find people next to them, or near them, using laptop computers to be an annoying nuisance, too."

Because more people than ever before own cell phones (and are using them everywhere they go), and there are more flights -- and capacity flights -- than ever before, there are also more people wanting to use their cell phones during flights than ever before.

But they can't.

What's more, many of the reasons are unclear, especially since many airlines have FAA-approved, seat-installed cell phones of their own. It costs about $3 a minute to make an in-flight call in the United States; a 20-minute call costing $60 doesn't exactly make company accountants jump for joy.

"I question (the prohibition of cell phones in flight) because they have a telephone if you pay for it," said Larry Murphy, vice president of sales and marketing for Flying Food Group.

Besides, Murphy says, "In private jets you can use your own phone."

Then why are cell phones and other wireless devices not allowed during flights? This question is a growing concern because of the increase of business-purpose flights, when many passengers face pressures to maintain constant contact with the ground.

Both the airline industry and the Federal Communications Commission ban the use of cell phones aboard commercial flights. But they do it for different reasons, reasons which are contradictory and scientifically unsubstantiated, critics say.

Safety is the main concern, which Federal Aviation Administration officials say is reason enough for the ban. And there is plenty of anecdotal evidence, they argue, to strongly suggest that wireless devices can interfere with aircraft instruments.

The FAA used the findings of the RTCA, an independent aeronautics adviser, to justify the ban, although it leaves enforcement up to the airlines. The RTCA's three studies, published in 1963, 1988 and 1996, say handheld devices (excluding cell phones) should be banned during "critical phases of flight," which the airlines have interpreted as takeoffs and landings.

The studies don't include "intentional transmitting devices" such as cell phones and two-way pagers, because the organization did not receive the devices from the cell phone industry, planes from the aviation industry and funding to conduct the study. The RTCA works on a "volunteer basis so we had to rely on these people for the free use" of their equipment, Sheehan said.

The FAA recommendation doesn't extend to private jets, which have different rules.

The FCC has its own cell-phone ban, but it has nothing to do with airplane safety. The FCC says signals emitted by phones in the air could occupy multiple cell towers on the ground and cause interference with calls on the ground. This interference might even allow analog cell phone users to listen to others' conversations on the ground.

However, no study has been conducted to prove this. What's more, the ban does not extend to SprintPCS and AT&T wireless phones because of an FCC "oversight," according to a former FCC engineer.

SprintPCS and AT&T wireless phones use a different frequency than other cell phones. The oversight might imply that a user of either phone could use them in flight, but most, if not all, airlines adhere to FAA guidelines and prohibit all mobile phones anyway.

"You try to write the rules so that they cover everything," said Dale Hatfield, a former FCC engineer who is now telecommunications program director at the University of Colorado in Boulder. "Since the FAA has its own rules, there's not a lot of pressure to fix that."

Airlines generally abide by the FAA's recommendation, but what they don't tell passengers is that no agency -- not even the RTCA -- has come up with definitive evidence of portable electronic devices interfering with a plane's instruments.

Here's one possible explanation: Cell phones and other handhelds operate on different frequencies than onboard instruments. "The issue with cell phones has less to do with interfering with the airplane equipment," said Tim Brown, an engineering professor at the University of Colorado in Boulder. "It's conceivable that a cell phone may have enough energy spilling into an adjacent band, which could cause a problem."

Brown surveyed a chart detailing the various frequencies of portable electronic devices and airplane equipment and said, "Again, nothing is exactly adjacent to them."

Still, the FAA and FCC say they don't plan on changing the rules, although enforcing them has been a thorn in the side of airlines because passengers often forget to turn off their phones or else are refusing to comply with the policy.

According to the National Aeronautics and Space Administration, the second-leading cause of "air rage" results from passengers being told by in-flight attendants to turn off their PEDs. Whitehouse, the British oil worker who was jailed last year, was using his cell phone.

NASA, which maintains a database of flight problems anonymously reported by pilots, found that 15 percent of air rage incidents are attributed to the prohibition of PEDs, second only to alcohol (43 percent).

The FAA oversees two U.S. flights every second and moves approximately 1.5 million passengers a day. There are over 110 million cell phone subscribers in the United States, according to the Cellular Telecommunications and Internet Association.

"It's becoming hard to control," Forrester Research analyst Galen Schreck said. "Think about all the stuff in your purse that is wireless: my cell phone, my pager, my Palm, which has a wireless connection. Your laptop might have something built into it."

Neither the FAA nor the FCC has plans to provide passengers with alternative ways to communicate with someone on the ground, or implement a mechanism that would detect illegal -– and even harmful PED use. They advise passengers to use plane seat-installed phones.

Passengers can receive incoming calls on the phones by activating them with a PIN number and seat number every time they fly.

Airlines pocket about 15 percent of the profits racked by these phones, according to Sheehan. Neither GTE (now Verizon Communications) nor AT&T, which shares a duopoly on the phones, would say how much money they make off them. But an October 1999 Wall Street Journal article estimated the units' annual revenues at $150 million.

The FAA denies it implemented its policy based on economic incentives. FAA engineers say the phones are "exhaustively tested" –- making them more expensive to maintain -– to be compatible with onboard equipment.

Unlike other wireless phones, the signals of airplane-installed phones are shielded and controlled. Their calls go to a receiver in the plane's belly and then down to one of 135 ground base stations in North America, according to GTE Airfone, which is part of Verizon Communications.

Calls made 200 miles beyond the U.S. coastline run on a satellite system, where the calls are routed to a satellite station rather than a radio base station, the company said. Foreign carriers receive a share of the profits generated by international phone calls made on the phones, thus making those calls more expensive.

The cell-phone industry says it has no way of lifting its own ban because it is physically impossible to construct cell-phone towers to accommodate signals traveling 600 miles per hour at 33,000 feet in the air.

"I'm afraid it's simply a matter of physics that phone use in airplanes interferes with other signals," Cellular Telecommunications and Internet Association spokesman Travis Larson said.

None of this may appease today's busy, frantic traveler. "With today's technology, I'm sure they have a way around this," said Flying Food Group's Murphy, furiously pecking away at his laptop at Oakland International Airport.

^top of page^

Wired.com (external link)

Pages in This Article (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)

Appendix: Early Bell System overview of IMTS and cellular // Appendix: Call processing diagram // 


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