Cellular Telephony

By Brian Oblivion

The benefit of a mobile transceiver has been the wish of experimenters since the late 1800's. To have the ability to be reached by another man despite location, altitude, or depth has had high priority in communication technology throughout its history. Only until the late 1970's has this been available to the general public. That is when Bell Telephone (the late MaBell) introduced the Advanced Mobile Phone Service, AMPS for short.

Cellular phones today are used for a multitude of different jobs. They are used in just plain jibber-jabber, data transfer (I will go into this mode of cellular telephony in-depth later), corporate deals, surveillance, emergencies, and countless other applications. The advantages of cellular telephony to the user/speaker are obvious:

1. Difficulty of tracking the location of a transceiver (especially if the transceiver is on the move) makes it very difficult to locate.
   
   
2. Range of the unit within settled areas.
   
   
3. Scrambling techniques are feasible and can be made to provide moderate security for most transmissions.
   
   
4. The unit, with modification can be used as a bug, being called upon by the controlling party from anywhere on the globe.
   
   
5. With the right knowledge one can modify the cellular in both hardware and software to create a rather diversified machine that will scan, store and randomly change ESN's during each call there by making detection almost impossible.

I feel it will be of great importance for readers to understand the background of the Cellular phone system, mainly due to the fact that much of the pioneering systems are still in use today. The first use of a mobile radio came about in 1921 (remember prohibition?) by the Detroit police department. This system operated at 2Mhz. In 1940, frequencies between 30 and 40Mhz were made available also and soon became overcrowded. The trend of overcrowding continues today.

In 1946, the FCC declared a 'public correspondence system' called, or rather classified as "Domestic Public Land Mobile Radio Service" (DPLMRS) at 35 - 44 MHz band that ran along the highway between New York and Boston. Now the 35-44MHz band is used mainly by amateur radio hobbyist’s due to the band's susceptibility to skip-propagation.

These early mobile radio systems were all PTT (push-to-talk) systems that did not enjoy today’s duplex conversations. The first real mobile 'phone' system was the 'Improved Mobile Telephone Service' or the IMTS for short, in 1969. This system covered the spectrum from 150 - 450MHz, sported automatic channel selection for each call, eliminated PTT, and allowed the customers to do their own dialing. From 1969 to 1979 this was the mobile telephone service that served the public and business community, and it is still used today.

IMTS frequencies used (MHz):

VHF High frequencies are the most popular frequencies in the IMTS band. VHF low bands are used primarily in rural areas and those with hilly terrain. UHF bands are primarily used in cities where the VHF bands are overcrowded. Most large cities will find at least one station being used in their area.

ADVANCED MOBILE PHONE SYSTEM

Bell Telephone, made the next step for Mobile telephone in 1979 again (gee... where was the competition?), introducing the Advanced Mobile Phone Service. This service is the focus of this document, which has now taken over the mobile telephone industry as the standard. This system was brought to life by the new digital technologies of the 1970's. Those being large scale integrated custom circuits and microprocessors. Without these technologies, the system would not have been economically possible.

The basic elements of the cellular concept have to do with frequency reuse and cell splitting.

Frequency reuse refers to the use of radio channels on the same carrier frequency to cover different areas, which are separated by a significant distance. Cell splitting is the ability to split any cell into smaller cells if the traffic of that cell requires additional frequencies to handle all the area's calls. These two elements provide the network an opportunity to handle more simultaneous calls, decrease the transmitters/receivers output/input wattage/gain and a more universal signal quality.

When the system was first introduced, it was allocated 40MHz in the frequency spectrum, divided into 666 duplex radio channels providing about 96 channels per cell for the seven cluster frequency reuse pattern. Cell sites (base stations) are located in the cells, which make up the cellular network. These cells are usually represented by hexagons on maps or when developing new systems and layouts. The cell sites contain radio, control, voice frequency processing and maintenance equipment, as well as transmitting and receiving antennas. The cell sites are inter-connected by landline with the Mobile Telecommunications Switching Office (MTSO).

In recent years, the FCC has added 156 frequencies to the Cellular bandwidth. This provides 832 possible frequencies available to each subscriber per cell. All new cellular telephones are built to accommodate these new frequencies, but old cellular telephones still work on the system. How does a cell site know if the unit is old or new? Let me explain.

The STATION CLASS MARK (SCM) carries the task of identifying a cellular phone’s age. This Number is 4 bits long and broken down like this:

Bit 1:
0 for 666 channel usage (old)
1 for 832-channel usage (new)

Bit 2:
0 for a mobile unit (in vehicle)
1 for voice-activated transmit (for portables)

Bit 3-4:
Identify the power class of the unit

Cell Sites: How Cellular telephones get their name

Cell sites, as mentioned above are laid out in a hexagonal type grid. Each cell is part of a larger cell, which is made up of seven cells in the following fashion:

	   |---|      ||===||      |---|       |---|       |---|       |---
          /     \    //     \\    /     \     /     \     /     \     /
         |       |===||  2  ||===||     ||===||      |---|       |---|
          \     //    \     /     \\   //     \\    /     \     /     \
           |---||  7   |---|   3   ||==||  2   ||==||      |---|       |---|
          /     \\    /     \     //    \     /     \\      Due to the      \
         |      ||---|   1   |---||  7   |---|   3   ||--|  difficulty of    |
          \    //     \     /     \\    /     \     //    \ representing    /
           |--||   6   |---|   4   ||--|   1   |---||      |graphics with  |
          /    \\     /     \     //    \     /     \\    / ASCII characters\
         |      ||==||   5   ||==||  6   |---|   4   ||--|  I will only show |
          \     /    \\     //    \\    /     \     //    \ two of the cell /
           |---|      ||===||     ||===||  5   ||==||      |types I am trying-
          /     \     /     \     /     \\    //    \     / to convey.      \
         |       |---|       |---|       ||==||      |---|       |---|       |
          \     /     \     /     \     /     \     /     \     /     \     /
           |---|       |---|       |---|       |---|       |---|       |---|
	        

As you can see, each cell is a 1/7th of a larger cell. Where one (1) is the center cell and two (2) is the cell directly above the center. The other cells are number around the center cell in a clockwise fashion, ending with seven (7). The cell sites are equipped with three directional antennas with an RF beam width of 120 degrees providing 360-degree coverage for that cell. Note that all cells never share a common border. Cells, which are next to each other, are obviously never assigned the same frequencies. They will almost always differ by at least 60 KHz. This also demonstrates the idea behind cell splitting. One could imagine that the perimeter of one of the large cells was once one cell. Due to a traffic increase, the cell had to be sub-divided to provide more channels for the subscribers. Note that subdivisions must be made in factors of seven.

There are also Mobile Cell sites, which are usually used in the transactional period during the up scaling of a cell site due to increased traffic. Of course, this is just one of the many uses of this component. Imagine you are building a new complex in a very remote location. You could feasibly install a few mobile cellular cell sites to provide a telephone-like network for workers and executives. The most unique component would be the controller/ transceiver which provides the communications line between the cell site and the MTSO. In a remote location such a link could very easily be provided via satellite up/down link facilities.

Lets get into how the phones actually talk with each other. There are several ways and competitors have still not set an agreed upon standard.

Frequency Division Multiple Access (FDMA)

This is the traditional method of traffic handling. FDMA is a single channel per carrier analog method of transmitting signals. There has never been a definite set on the type of modulation to be used. There are no regulations requiring a party to use a single method of modulation. Narrow band FM, single sideband AM, digital, and spread-spectrum techniques have all been considered as a possible standard. But none have yet to be chosen.

FDMA works like this: Cell sites are constantly searching out free channels to start out the next call. As soon as a call finishes the channel is freed up and put on the list of free channels. Or, as a subscriber moves from one cell to another the new cell they are in will hopefully have an open channel to receive the current call in progress and carry it through its location. This process is called handoff, and will be discussed more in-depth further along.

Other proposed traffic handling schemes include Time-Division Multiple Access (TDMA), Code-Division Multiple Access (CDMA), and Time-Division/Frequency Division Multiple Access.

Time Division Multiple Access

With TDMA calls are simultaneously held on the same channels, but are multiplexed between pauses in the conversation. These pauses occur in the way people talk and think, and the telephone company also injects small delays on top of the conversation to accommodate other traffic on that channel. This increase in the length of the usual pause results in a longer amount of time spent on the call. Longer calls result in higher cost of the call.

Code Division Multiple Access

This system has been used in mobile military communications for the past 35 years. This system is digital and breaks up the digitized conversation into bundles, compressed, sent, then decompressed and converted back into analog. There are said increases of throughput of 20: 1 but CDMA is susceptible to interference, which will result in packet retransmission and delays. Of course error correction can help in data integrity, but will also result in a small delay in throughput.

Time-Division/Frequency Division Multiple Access

TD/FDMA is a relatively new system, which is an obvious hybrid of FDMA and TDMA. This system is mainly geared towards the increase of digital transmission over the cellular network. TD/FDMA make it possible to transmit signals from base to mobile without disturbing the conversation. With FDMA there are significant disturbances during handoff with prevent continual data transmission from site to site. TD/FDMA make it possible to transmit control signals by the same carrier as the data/voice thereby ridding extra channel usage for control.

Cellular Frequency Usage and channel allocation

There are 832 cellular phone channels, which are split into two separate bands. Band A consists of 416 channels for non-wire line services. Band B consists equally of 416 channels for wire line services. Each of these channels is split into two frequencies to provide duplex operation. The lower frequency is for the mobile unite while the other is for the cell site. 21 channels of each Band are dedicated to 'control' channels and the other 395 are voice channels. You will find that the channels are numbered from 1 to 1023, skipping channels 800 to 990.

I found these handy-dandy equations that can be used for calculating frequencies from channels and channels from frequencies.

N = Cellular Channel #
F = Cellular Frequency
B = 0 (mobile) or B = 1 (cell site)

CELLULAR FREQUENCIES from CHANNEL NUMBER:
F = 825.030 + B * 45 + ( N + 1 ) * .03
where: N = 1 to 799

F = 824.040 + B * 45 + ( N + 1 ) * .03
where: N = 991 to 1023

N = 991 + (F - 824.040 - B * 45) / .03
where: F <= 825.000 (mobile)
or F <= 870.000 (base)

Now that you have those frequencies, what to do with them? Well, for starters, one can very easily monitor the cellular frequencies with most hand/base scanners. Almost all scanners pre-1988 have some coverage of the 800 - 900 MHz band. All scanners can monitor the IMTS frequencies.

Remember that cellular phones operate on a full duplex channel. That means that one frequency is used for transmission and the other is used for receiving, each spaced exactly 30 KHz apart. Remember also that the base frequencies are 45MHz higher than the cellular phone frequencies. This can obviously make listening rather difficult. One way to listen to both parts of the conversation would be having two scanners programmed 45 MHz apart to capture the entire conversation.

The upper UHF frequency spectrum was 'appropriated' by the Cellular systems in the late 1970's. Televisions are still made to receive up to channel 83. This means that you can receive much of the cellular system on you UHF receiver. One television channel occupies 6Mhz of bandwidth. This was for video, sync, and audio transmission of the channel. A cellular channel only takes up 24 KHz plus 3KHz set up as a guard band for each audio signal. This means that 200 cellular channels can fit into one UHF television channel. If you have an old black and white television drop a variable cap in there to increase the sensitivity of the tuning. Some of the older sets have coarse and fine tuning knobs.

A variable resistor tunes some of the newer, smaller, portable television sets. This made modifications MUCH easier, for now all you have to do is drop in there a smaller value pot and tweak away. I have successfully done this on two televisions. Most users will find that those who don't live in a city will have a much better listening rate per call. In the city, the cells are so damn small that handoff is usually every other minute. Resulting in chopped conversations.

If you wanted to really get into it, I would suggest to obtain an old Television set with decent tuning controls and remove the RF section out of the set. You don't want all that hi-voltage circuitry lying around (fly back and those caps). UHF receivers in televisions down convert UHF frequencies to IF (intermediate frequencies) between 41 and 47 MHz. These output IF frequencies can then be run into a scanner set to pick-up between 41 - 47 MHz. Anyone who works with RF knows that it is MUCH easier to work with 40MHz signals than working with 800MHz signals (not to far away from GHz... mmmmmmm. Wave guides are just sooo much fun). JUST REMEMBER ONE THING!!!! Isolate the UHF receiver from your scanner by using a coupling capacitor (.01 - .1 microfarad (50V min.) will do nicely)!!!! You don't want any of those biasing voltages creeping into your scanners receiving AMPLIFIERS!!! Horrors. Also, don't forget to ground both the scanner and receiver.

Some systems transmit and receive the same cellular transmission on the base frequencies. There you can simply hang out on the base frequency and capture both sides of the conversation. The handoff rate is much higher in high traffic areas leading the listener to hear short or choppy conversations. At times you can listen in for 5 to 10 minutes per call, depending on how fast the caller is moving through the cell site.

You can spend hours just listening to cellular telephone conversations but I would like to mention that it is illegal to do so. Yes, it is illegal to monitor cellular telephone conversations. It just another one of those laws like removing tags off of furniture and pillows. It's illegal, but what the hell for? It’s also illegal to spit on the sidewalks here in Massachusetts, yet you can carry a shotgun on Sunday’s with you to mass (that’s still in the books. Obviously it was for the original settlers). At any rate, I just want you to understand that doing the following is in violation of the law.

Now back to the good stuff.

Conversation is not only what an avid listener will find on the cellular bands. One will also hear call/channel setup control data streams, dialing, and other control messages. At times, a cell site will send out a full request for all units in its cell to identify itself. The phone will then respond with the appropriate identification on the corresponding control channel.

Whenever a mobile unit is turned on, even when not placing a call, whenever there is power to the unit, it transmits its phone number and its 8-digit ID number. The same process is done when an idling phone passes from one cell to the other. This process is repeated for as long as there is power to the unit. This allows the MTSO to 'track' a mobile through the network. That is why it is not a good reason to use a mobile phone from one site. They do have ways of finding you. And it really is not that hard. Just a bit of RF Triangulation theory and you're found. However, when the power to the unit is shut off, as far as the MTSO cares, you never existed in that cell, of course unless your unit was flagged for some reason. MTSO's are basically just ESS systems designed for mobile applications. This will be explained later within this document.

It isn't feasible for the telephone companies to keep track of each customer on the network. Therefore the MTSO really doesn't know if you are authorized to use the network or not. When you purchase a cellular phone, the dealer gives the units phone ID number to the local BOC, as well as the number the BOC assigned to the customer. When the unit is fired up in a cell site its ID number and phone number is transmitted and checked. If the two numbers are registered under the same subscriber, then the cell site will allow the mobile to send and receive calls. If they don't match, then the cell will not allow the unit to send or receive calls. Hence, the most successful way of reactivating a cellular phone is to obtain an ID that is presently in use and modifying your rom/prom/eprom for your specific phone.

RF and AF Specifications:

Everything that you will see from here on out is specifically Industry/FCC standard. A certain level of compatibility has to be maintained for national intercommunications; therefore a common set of standards that apply to all Cellular telephones can be compiled and analyzed.

Transmitter Mobiles: audio transmission

• 3 kHz to 15 kHz and 6.1 kHz to 15 kHz
• 5.9 kHz to 6.1 kHz 35 dB attenuation
• Above 15 kHz, the attenuation becomes 28 dB
• All this is required after the modulation limiter and before the modulation stage

Transmitters Base Stations: audio transmission

• 3 kHz to 15 kHz
• Above 15 kHz, attenuation required 28 dB
• Attenuation after modulation limiter - no notch filter required

RF attenuation below carrier Transmitter: audio transmission

• 20 kHz to 40 kHz, use 26 dB
• 45 kHz to 2nd harmonic, the specification is 60 dB or 43 + 10 log of mean output power
• 12 kHz to 20 kHz, attenuation 117 log f/12
• 20 kHz to 2nd harmonic, there is a choice: 100 log F/100 or 60 dB or 43 log + 10 log of mean output power, whichever is less.

Wideband Data

• 20 kHz to 45 kHz, use 26 dB
• 45 kHz to 90 kHz, use 45 dB
• 90 kHz to 2nd harmonic, either 60 dB or 43 + 10 log mean output power
• all data streams are encoded so that NRZ (non-return-to-zero) binary ones and zeroes are now zero-to-one and one-to-zero transitions, respectively. Wideband data can then modulate the transmitter carrier by binary frequency shift keying (BFSK) and ones and zeroes into the modulator must now be equivalent to nominal peak frequency deviations of 8 kHz above and below the carrier frequency.

Supervisory Audio Tones

• Save as RF attenuation measurements

Signaling Tone

• Same as Wideband Data but must be 10 kHz +/- 1 Hz and produce a nominal frequency deviation of +/- 8 kHz

The previous information will assist any technophile to modify or even troubleshoot his/her cellular phone. Those are the working guidelines, as I stated previously.

UNIT IDENTIFICATION

Each mobile unit is identified by the following sets of numbers.

The first number is the Mobile Identification Number (MIN). This 34 bit binary number is derived from the unit’s telephone number, MIN1 is the last seven digits of the telephone number and MIN2 is the area code.

For demonstrative purposes, we'll encode 617-637-8687.

Here's how to derive the MIN2 from a standard area code. In this example, 617 is the area code. All you have to do is first convert to modulo 10 using the following function. A zero digit would be considered to have a value of 10.

100(first number) + 10(second) +1(third) - 111 = x

100(6) + 10(1) + 1(7) - 111 = 506

or you could just - 111 from the area code.)

Then convert it to a 10-bit binary number: 0111111010

To derive MIN1 from the phone number is equally as simple. First encode the next three digits, 637.

100(6) + 10(3) + 1(7) - 111 = 526

Converted to binary: 1000001110

The remainder of the number 8687 is processed further by taking the first digit, eight (8) and converting it directly to binary.

8 = 1000 (binary)

The last three digits are processed as the other two sets of three numbers were processed.

100(6) + 10(8) + 1(7) - 111 = 576

Converted to binary: 1001000000

So the completed MIN number would look like this:

            |--637---||8-||---687--||---617--|
            1000001110100010010000000111111010
            \________/\__/\________/\________/

A unit is also identifiable by its Electronic Serial Number or ESN. This number is Factory Preset and is usually stored in a ROM chip, which is soldered to the board. It may also be found in a 'computer on a chip', which are the new micro-controllers, which have ROM/RAM/microprocessor all in the same package. This type of setup usually has the ESN and the software to drive the unit all in the same chip. This makes is significantly harder to dump, modify and replace. But it is far from impossible.

The ESN is a 4-byte hex or 11-digit octal number. I have encountered mostly 11-digit octal numbers on the casing of most cellular phones. The first three digits represent the manufacturer and the remaining eight digits are the units ESN. I'll go more into the ESN later in the document.

The Station Class Mark (SCM) is also used for station identification by providing the station type and power output rating. This was already discussed in a previous section.

The System Identification (SID number is a number which represents the mobile's home system. This number is 15-bits long and a list of current nationwide SID's should either be a part of this file or it will be distrusted along with it.

PUTTING IT ALL TOGETHER - Signaling on the Control Channels

There are two types of continuous wideband data stream transmissions. One is the Forward Control Channel, which is sent from the land station to the mobile. The other is the Reverse Control Channel, which is sent from the mobile to the land station. Each data stream runs at a rate of 10 kilobit/sec, +/- 1 bit/sec rate. The formats for each of the channels follow.

Forward Control Channel

The forward control channel consists of three discrete information streams. They are called stream A, stream B and the busy-idle stream. All three streams are multiplexed together. Messages to mobile stations with the least significant bit of their MIN number equal to "0" are sent on stream A, and those with a "1" are sent on stream B.

The busy-idle stream contains busy-idle bits, which are used to indicate the status of the reverse control channel. If the busy-idle bit = "0" the reverse control channel is busy, if it equals "1" it is idle. The busy-idle bit is located at the beginning of each dotting sequence, word sync sequence, at the beginning of the first repeat of word A and after every 10 message bits thereafter.

Mobile stations achieve synchronization with the incoming data via a 10 bit dotting sequence (1010101010) and an 11-bit word sync sequence (11100010010). Each word contains 40 bits, including parity and is repeated 5 times, after which it is then referred to as a "block". For a multiword message, the second word block and subsequent word blocks are formed the same as the first word block including the dotting and sync sequences. A "word" is formed when the 28 content bits are encoded into a (40, 28; 5) BCH (Bose-Chaudhuri-Hocquenghem) code. The left-most bit shall be designated the most-significant bit.

The Generator polynomial for the (40, 28;5) BCH code is:

                       12    10    8    5    4    3    0
              G (X) =  X   + X   + X  + X  + X  + X  + X
               B

Each FOCC message can consist of one or more words. Messaging transmitted over the forward control channel is:

• Mobile station control message
• Overhead message
• control-filler message

Controller-filler messages may be inserted between messages and between word blocks of a multiword message.

Message Formats: Found on either stream A or B

MOBILE STATION CONTROL MESSAGE

The mobile station control message can consist of one, two, or four words.

Word 1 (abbreviated address word)

   +--------+-------+---------------------------------------+-----------+
   | T   t  |       |                                       |           |
   |  1   2 |  DCC  |    Mobile Identification Number 1     |     P     |
   |        |       |                      23-0             |           |
   +--------+-------+---------------------------------------+-----------+
 bits:  2       2                      24                         12
    

Word 2 (extended address word)

     +------+-----+-----------+------+--------+-------+----------+-----+
     | T  T |SCC =|           | RSVD | LOCAL  | CRDQ  |   ORDER  |     |
     |  1  2| 11  |  MIN2     | = 0  |        |       |          |     |
     |   =  +-----+     3-24  +------+-----+--+-------+----------|  P  |
     |  10  |SCC =|           |    VMAC    |       CHAN          |     |
     |      | 11  |           |            |                     |     |
     +------+-----+-----------+------------+---------------------+=----+
    

The Reverse Control Channel (RECC) is a wideband data stream sent from the mobile station to the land station. This data stream runs at a rate of 10 kilobit/sec, +/- 1 bit/sec rate. The format of the RECC data stream follows:

     +---------+------+-------+------------+-------------+-----------+-----
     | Dotting | Word | Coded | first word | Second word | Third word|
     |         | sync |  DCC  | repeated   |   repeated  |  repeated | ...
     |         |      |       | 5 times    |   5 times   |  5 times  |
     +---------+------+-------+------------+-------------+-----------+-----

        DCC = Digital Color Code            Dotting = 01010101...010101
      Received DCC    7-bit Code DCC     Word sync = 11100010010
          00            0000000
          01            0011111
          10            1100011
          11            1111100

All messages begin with the RECC seizure precursor with is composed of a 30 bit dotting sequence (1010...101), and 11-bit word sync sequence (11100010010), and the coded digital color code.

Each word contains 48 bits, including parity, and is repeated five times, after which it is referred to as a word block. A word is formed by encoding 36 content bits into a (48, 36) BCH code that has a distance of 5, (48 36; 5). The left most bit shall be designated the most-significant bit. The 36 most-significant bits of the 48-bit field shall be the content bits.

The generator polynomial for the code is the same for the (40,28;5) code used on the forward channel.

CONTROL CHANNELS (SETUP CHANNELS)

Each wire line and non-wire line service has 21 channels. These channels are used by the MTSO and the cell sites to directly communicate with the mobile unit. The first signal sent to initiate a call is the Supervisory Audio Tone (SAT). This can be thought of as the voltage used to close the loop on a land telephone.

SAT Tones with corresponding binary codes:
5970 Hz (00)
6000 Hz (01)
6030 HZ (10)

The mobile unit receives the SAT from the cell site and transponds it back (closing the loop). Tone recognition must take place within 250 milliseconds or the site interprets it as the mobile is out of range. If the SAT is returned, then a Signaling Tone is issued. This Tone is 10kHz and is present when the user is either being alerted (call initialization), being handed off, or disconnecting The Signaling tone is used only in mobile to land direction

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