This is the third and final installment of the APCO Project 25 system listing.
Project 25 is a suite of standards spanning more than 30 documents, specifying in detail how each piece of radio equipment should operate. Such a thorough and complete set of standards makes it much easier for different departments and agencies to work together (planners use the term interoperate) and allows radio system owners to purchase equipment from competing vendors with some guarantee that everything will work together properly.
The core of P-25 revolves around a standard called the Common Air Interface (CAI). The CAI specifies the layout and meaning of each digital message that is sent and received by a radio. The Phase I CAI has a number of requirements, the most relevant of which to a scanner listener is the use of the Improved Multi-Band Excitation (IMBE) voice encoder-decoder (vocoder) to carry voice signals digitally. (Phase II is a future standard that will operation in narrow 6.25 kHz channels.)
Some standards in the P-25 suite are mandatory, but many are not. Radio system owners may pick and choose from the optional standards, depending on what features they want to offer their users, their budget, and the product choices from the equipment vendors. The P-25 standard for trunking, for instance, is optional.
So, in any system you might run into, there are a number of possibilities.
Voice traffic on the system may be all analog, all digital, or a mixture of both. Analog traffic can be heard with any scanner that can tune to the proper frequency. Digital traffic used to be completely out of reach, but now with the new Uniden scanner, listeners can hear unencrypted IMBE voice traffic carried on systems that use the P-25 CAI.
There are other digital voice systems in operation, most notably older Motorola systems using a proprietary vocoder called VSELP (Vector-Sum-Excited Linear Prediction) and newer EDACS installations using Pro-Voice and AEGIS. The new Uniden scanners will not decode the audio from such systems. EDACS Pro-Voice systems happen to use the IMBE vocoder but the transmission formats are not compatible with the P-25 IMBE implementation.
A radio system will either be conventional or trunked. If it is conventional, then the traffic on any particular radio frequency will always be from the same group of radio users. In other words, specific frequencies will be assigned to specific users in a conventional system.
If a system with P-25 digital audio is trunked, it will almost always use one of two formats. The first format is the actual P-25 standard for trunking, which uses a 9600-baud control channel. These are common on the "pure P-25" systems, like the state of Michigan. The second format is a proprietary Motorola protocol running at 3600 baud, which is typically found on systems that have a mix of analog and digital radios.
Part of the confusion about this point may stem from the fact that Motorola uses the marketing term "ASTRO" for all of their digital products, regardless of the actual type of voice coding and trunking method.
Below are three tables that indicate which types of systems a listener will be able to monitor with a particular type of scanner. The column going down the left side indicates the types of voice traffic and the rows across specify the type of control channel. A "pure P-25" system, for example, uses P-25 IMBE voice and the P-25 9600 baud control channel, so the scanner results will be in the lower right-hand corner. A Motorola Type 1 system has analog only audio and a 3600-baud control channel, so results on that kind of system would be found in the top row, center column.
The first table indicates that a non-trunking scanner, as long as it can tune to the proper frequency, will be able to monitor the audio only from analog sources. Even if a system uses trunking, a non-trunking scanner will be able to pick up the audio on a particular frequency -- but it may be difficult to follow the action since the scanner will not automatically retune during a trunked conversation. However, on lightly loaded trunked systems or during relatively quiet periods this method can work fairly well. I've found that even a ten-year-old Bearcat BC200XLT is useful in such situations.
Table 1: Success with an analog non-trunk-tracking scanner
The second table shows that with a trunk-tracking scanner like the Bearcat BC245XLT or Radio Shack PRO-92, the results are identical to the non-trunking scanner except that systems with a 3600-baud control channel are now automatically followed.
Table 2: Success with an analog trunk-tracking scanner
The third table shows the results for a digital scanner like the Uniden BC250D or 785D with the BCi25D APCO-25 card. Now the listener will have all the benefits of an analog trunk-tracking scanner, plus can hear any (unencrypted) P-25 IMBE traffic, regardless of which trunking method, if any, is in use.
Table 3: Success with a digital APCO-25 trunk-tracking scanner
I welcome updates, corrections and additional information via electronic mail at dan @ signalharbor.com.
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