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A single cell in an analog system uses one-seventh of the available duplex voice channels. That is, each cell (of the seven on a hexagonal grid) is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:
- A cell-phone carrier typically gets 832 radio frequencies to use in a city.
- Each cell phone uses two frequencies per call — a duplex channel — so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels — more on this on the next page.)
- Therefore, each cell has about 56 voice channels available.
In other words, in any cell, 56 people can be talking on their cell phone at one time. With digital transmission methods, the number of available channels increases. For example, a TDMA-based digital system can carry three times as many calls as an analog system, so each cell has about 168 channels available (see this page for lots more information on TDMA, CDMA, GSM and other digital cell-phone techniques).
Testing for Radiation
Every cell-phone model has to be tested and meet FCC standards before it is allowed to be sold in the United States. Testing is primarily done by the manufacturers themselves, which creates some uncertainty about the testing procedures, according to a GAO report.
The exposure limit set by the FCC for cell phones is based on the overall heating effects of radio-frequency energy. The exposure limit was established by the FCC in 1996. A year later, the FCC published non-mandatory testing guidelines that helped manufacturers comply with exposure limits. The FCC still allows other testing techniques once an FCC review of the procedures is completed.
Radiation levels are tested based on the specific absorption rate (SAR), which is a way of measuring the amount of radio-frequency energy that is absorbed by the human body. In order to gain an FCC license, a phone’s maximum SAR level must be less than 1.6 watts per kilogram (W/kg). In 2000, the Cellular Telecommunications & Internet Association (CTIA) ordered cell-phone manufacturers to place labels on phones disclosing radiation levels.
Photo courtesy U.S. General Accounting Office
A probe attached to a mechanical arm is directed to take SAR measurements throughout a human-shaped mold. The mold is filled with a liquid mixture that simulates the electrical properties of human tissue.
Testing techniques vary somewhat, but are generally pretty standard. The GAO report “Research and Regulatory Efforts on Mobile Phone Health Issues,” published in May 2001, describes how SAR levels are checked. Here is what the report described:
- A mold shaped like a human head and torso is filled with a fluid mixture that is designed to simulate the electrical properties of human tissue.
- The cell phone under review is placed on the outside of the mold.
- A probe attached to a computer-controlled mechanical arm is inserted into the mixture at various locations.
- The phone is made to transmit a signal at full power while the probe is moved through the mixture.
During the test, the phone’s antenna is extended and retracted in order to check for any fluctuations in radiation that the phone might demonstrate in different configurations. The manufacturer is supposed to submit the highest SAR level measured during these tests to the FCC. Phones are required to test below 1.6 W/kg averaged over 1 gram of fluid.
To find the specific absorption rate of your phone, you can visit this FCC Web site. Your phone should have an FCC identification code. Type that code in the correct field and the site should offer information on your device.
Due to the lack of any industry-wide testing standard, the FCC must evaluate the individual procedures used by each manufacturer in certifying the SAR level of each new phone, according to the GAO report.
It’s still unclear as to whether cell phones actually cause any significant damage to the human body. Studies continue to contradict one another on the issue. Additional studies may shed some light on the true effects of cell-phone radiation, but will likely only confuse consumers even further. In the meantime, millions of cell-phone users take whatever risk may be involved in using the devices.
For more information on cell-phone radiation, cell phones and related topics, check out the links on the next page.
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