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TUCoPS :: Phreaking Cellular - Misc. :: freq1.txt

Cellular Frequencies, Channel Names, and Functions Explained




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Cellular Frequencies
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Cell phone frequencies start at 824.04 MHz and end at 893.7 MHz. 
That's 69.66 megahertz worth of radio frequency spectrum. Quite 
a chunk. By comparison, the AM broadcast band takes up only 1.17
megahertz of space. That band, however, provides only 107 
frequencies to broadcast on. Cellular may provide thousands of 
frequencies to carry conversations and data. This large number of 
frequencies and the large channel width required for each channel
account for the large amount of spectrum uaed. For example, 
AT&T's Advanced Mobile Phone Service or AMPS uses 832 channels 
that are 30 kHz wide. It's the most common system right now. AMPS,
though, has been replaced with NAMPS in crowded cell site areas.
NAMPS stands for Narrowband Advanced Mobile Service. It's a 
Motorola technology. It produces 2412 narrow channels. A NAMP's
channel is 10 kHz wide. AMPS, NAMPS and Hughes' ENAMPS are all 
FM based, analog systems. Digital systems like CDMA and TDMA provide
even more channels in the same space. CDMA, in particular, could
provide 20 times the number of frequencies that an AMPS system 
can. Let's back up a little before we drown in a sea of acronyms.

I mentioned that a typical cell channel is 30 kilohertz wide 
compared to the ten kHz allowed an AM radio station. How is it
possible, you might ask, that a one to three watt cellular phone
call can take up a path that is three times wider than a 50,000
watt broadcast station? Well, power does not necessarily relate to 
bandwidth. A high powered signal might take up lots of room or a 
high powered signal might be narrowly focused. A wider channel helps
with audio quality. An FM stereo station, for example, uses a 150 kHz
channel to provide the best quality sound. A 30 kHz channel for
cellular gives you great sound almost automatically, nearly on par
with the normal telephone network. That's what's impressive about
Motorola's NAMPS. The base station uses a special frequency control
circuit to keeps calls exactly on frequency. No wavering or moving 
off frequency to destroy a call's quality. Things should sound fine
with this narrow band if everything is working right.

I also mentioned that the cellular band runs from 824.04 MHz to 
893. 97 MHz. In particular, cell phones use the frequencies from 
824.04 MHz to 848.97 and the base stations operate on 869.04 MHz 
to 893.97 MHz. 45 MHz separates each transmit and receive frequency
within a cell. That keeps them from interfering with each other. 
Getting confusing? Let's look at the frequencies of a single cell 
for a single carrier. Maybe that will clear things up. For this 
example, let's assume that this is one of 21 cells in an AMPS system:

Cell#1 of 21 in Band A (The nonwireline carrier)

Channel 1 (333) Tx 879.990 Rx 834.990

Channel 2 (312) Tx 879.360 Rx 834.360

Channel 3 (291) Tx 878.730 Rx 833.730

Channel 4 (270) Tx 878.100 Rx 833.100

Channel 5 (249) Tx 877.470 Rx 832.470

Channel 6 (228) Tx 876.840 Rx 831.840

Channel 7 (207) Tx 876.210 Rx 831.210

Channel 8 (186) Tx 875.580 Rx 830.580 etc., etc.,

(Each cell has at least 15 frequencies or channels)

The cellular network assigns these frequency pairs carefully and in 
advance. The layout is confusing since the pattern is non-intuitive 
and because there are so many numbers involved. Don't get too caught
up with exact frequency assignments unless you want to go further. 
Channels 800 to 832 are not labeled as such. Cell channels go up to 
799 in AMPS and then stop. Believe it or not, the numbering begins 
again at 991 and then goes up to 1023. That gives us 832. Why offset 
at all? Cellular is not like CB radio. Citizen's band uses the same 
frequency to transmit and receive. A push to talk setup. Cellular 
provides full duplex communication like nearly all modern radios. 
It's more expensive since the mobile unit and the base station need 
the circuitry to transmit on one frequency while receiving on another.
But it's the only way that permits a normal, back and forth, talk
when you want to, conversation.

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Channel Names and Functions
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Okay, so what do we do we have? Three things: 1) Cell phones 
transmit on certain, dedicated frequencies, 2) base stations 
transmit on certain, dedicated frequencies and 3) a certain amount
of bandwidth separates these frequencies. Let's get even more 
specific. We call a cell site's transmitting frequency the forward
channel. A forward channel contains everything you hear since it 
is the cell site that transmits it. The cell phone's transmitting
 frequency, by comparison, is called the reverse channel. There's 
more. Certain channels carry only data. We call these control 
channels. They, too, have a forward and reverse frequency. This 
control channel is usually the first channel in each cell. It's 
responsible for call setup. Getting confusing? Let's go back to 
our friendly cell site for an example.

The first channel is always the control channel for each cell. 
You'll have 21 control channels if you have 21 cells. Calls get 
setup on these. A call gets going, in other words, on the control 
channel first. The MTSO then assigns a normal channel to carry the
conversation. The voice channels and the control channel may 
handle signaling during the actual conversation. A single call, 
therefore, involves two kinds of forward and reverse channels. 
One for voice and data and one for data only. Makes it hard to 
follow, doesn't it? But there are real benefits to figuring it 
out. A phone's ESN number, for example, is only transmitted on 
the reverse control channel. A person poaching ESNs need only 
monitor one of 21 frequencies. They don't have to look through 
the entire band. One last point at the risk of loosing everybody.
You'll hear about dedicated control channels, paging channels, 
and access channels. These are not different channels but different
uses of the control channel.


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