P 25 confusion

[AlphaFive]

I am just realizing that a P25 system does not have to neccesarily have to be a trunked system,, am I right?, National Parks seem to use a system for P 25 that is not trunked? I do need help with this one

[kennyblues]

P-25 is not just for Trunk systems, it is a digital mode, AM & FM are analog modes, P-25 can be used on a Trunked system or a single channel conventional channel.

[AlphaFive]

Thank you Sir, It's the first time I have posted anything, and you helped me out..

[jaspence]

Taking it a step further, it is not just for government either. Hams are using it in some areas with some of the older radios that have become available on their bands, mainly 2 meters and 440 MHz.

[kb2vxa]

Cutting through any confusion, P25 is the Apco Project 25 digital transmission mode, nothing more.

[MikeOxlong]

Originally Posted by AlphaFive

I am just realizing that a P25 system does not have to neccesarily have to be a trunked system,, am I right?, National Parks seem to use a system for P 25 that is not trunked? I do need help with this one

P25 is a standard for many things including voice and trunking formats.

A conventional channel can use P25 standard voice. Older trunking systems can also use a proprietary control channel while using P25 standard voice on the channels.

Newer trunking systems can use a P25 standard control channel and P25 standard voice m

[jeatock]

First:
Frequency: A specific point or defined range in the radio spectrum.
Channel: A single frequency or pair of frequencies with a name, squelch code and and other specific information.
Talkgroup: A virtual channel unique to trunking systems, and never a single frequency. All mobile/portable radios in a particular talkgroup will be automatically and remotely administered to be part of a conversation.
Note: Most receivers are pretty simple, and only do three things: 1) detect a signal on a particular frequency stronger than the background noise or squelch setting, 2) determine if the signal is getting stronger or weaker, and 3) if the center frequency of a carrier is changing follow it up or down. That's it. Making those shifts into something useful takes place in the components and software that are attached to the actual receiver.

Now, choose one from each section. Don't try and mix sections as each is a stand-alone item.

Section 1:
Pulse Modulation: Turning the transmitter completely on and off in a specific pattern to send a signal, like sending Morse code by blinking a flashlight on and off. Reliable, but slow.
AM Modulation: Content is transmitted by holding the frequency of the transmitter constant and shifting the power of the signal up and down (Amplitude Modulation). The flashlight is changed from bright to dim to send Morse Code. HF, CB and Aircraft use AM.
FM Modulation: Content is transmitted by holding the power of the transmitter constant and shifting the frequency up and down (Frequency Modulation). The flashlight stays on at a given brightness, but moves left and right to send infomation. 95% of all public safety and business radios use a form of FM.

Section 2:
Analog: Modulating the radio signal (see section 1) by semi-direct connection to the microphone. Only the amount of power (AM) or frequency (FM) change varies from type to type. Signaling or other functions are sent as clicks or tones before or after the voice content, or "underneath" it using audio tones in the 60 to 300 Hz range. Encryption takes place by inverting or otherwise mixing up the audio frequencies of the voice in a known pattern.
Digital: Take a short slice of voice from the microphone, sample and convert it to Ones and Zeros. Take a chunk of voice data and put it in a 'packet'. Non-voice data and other information is added somewhere into the packet's highly structured 'sentence' of Ones and Zeros. When the sentence is complete modulate the transmitter (see section 1) to send the Ones and Zeros of a packet in a long string, then start over and do it again. Encryption takes place by changing Ones to Zeros and Zeros to Ones in a known pattern. Formats (APCO P-25 Phase I, APCO P-25 Phase II, Turbo, NXDN, Tetra, Open Sky, Morse Code, etc.) are all different and are not compatible with each other.
There is no third option.

Note: The emission specified by the FCC defines the span above and below the center frequency of a channel that must contain the majority of the transmitter's radio energy. Wideband UHF energy could occupy 25 KHz of space, while Narrowband can only occupy 12.5 KHz. This is the difference between wideband, and the narrow emissions required after 12/31/2012. Since FM analog is sent by changing the radio frequency, current receivers cannot detect smaller frequency shifts and 10 KHz of bandwidth is the practical lower limit for analog radio. Digital radios can operate sucessfully with much smaller shifts in frequency, making them effecient with very narrow emissions.

Note: Most digital systems transmit their data by shifting and pausing the center of the transmitter's carrier frequency to defined offsets from the channel center frequency. Again, the offset distance and interpetation of the pauses changes from one digital format to another, making them incompatable.

Section 3:
Direct: All radios communicate directly, with nothing in the middle. This can be analog or digital. Base stations (and repeaters, if any) transmit on this channel
Repeater: A third radio that hears a mobile/portable on a different frequency then simultaneously 'repeats' or forwards it on to one, or to many others, on the direct frequency. Most repeaters are inactive unless someone is speaking, which is most of the time. Repeaters can be analog or digital.
Trunking: Multiple repeaters under the control of a device that assigns portable/mobile radios needing to be 'in' a conversation to a different repeater or channel for the duration of one conversation, or a part of it. One repeater is active all (or most) of the time with control information. When a mobile/portable radio is not receiving, it monitors a control channel for new instructions freeing the voice channels for other talkgroups. Mobile/Portables wishing to talk to their talkgroup first communicate with the controller for permission and repeater assignment. Trunking sites can have 3 repeaters, or a dozen. Trunking can be analog or digital.
Multi-Site Trunking: Trunking systems where one controller supervises repeaters at different physical locations, such as in a large regional or statewide system. As a mobile/portable radio moves, it 'registers' on the tower site with the best coverage for its current location so the controller can find it and tell it to temporarily change to a local repeater for a single communication. Technically, cell phone service is multi-site trunking on steroids. In Multi-Site Trunking, repeaters at many different locations may all become active with a single talkgroup and transmit the same message to many receivers. Again, this can be analog or digital.

Note: Since everyone doesn't talk at once trunking is more spectrum and cost efficient than a dedicated single purpose repeater for many user groups (dozens or hundreds). Trunking allows multiple user groups to share a smaller number of physical repeaters. Receivers (and scanners) need to be able to hear instructions from the 'trunking controller' to be able to follow any one conversation on a particular talkgroup. During breaks in a conversation, the controller may reassign the repeater to an active talkgroup and when the original conversation resumes put it on a different repeater.

Another note: A non-trunking scanner can monitor the repeaters in a trunking system, but will do so randomly. The scanner must hear the trunking controller data and be able to change channels just like the talkgroup's radios to hear a complete conversation without breaks. Scanning radio frequencies is pretty simple. Scanning talkgroups means that your scanner understands the trunking controller and switches to a repeater frequency containing your desired talkgroup's conversation and back again automatically. Scanners can be programmed to follow any talkgroup on a system, or only follow specific talkgroups.

Yet another note: In multi-site trunking systems, a talkgroup's conversation is only transmitted from sites where a mobile/portable belonging to that talkgroup is active and registered. For instance, while the Illinois State Police use the statewide trunking system, District 11 (E. St. Louis) traffic is only transmitted from District 2 (Chicago) towers if a District 11 radio is in District 2, turned on and registered, and requesting District 11 talkgroup traffic.

Section 4:
CS: Carrier squelch; a receiver that will 'open' when it hears any signal on a particular radio frequency.
CTCSS: A low frequency audio tone (67 to 255 Hz) transmitted continuously underneath an analog voice that qualifies the signal to a receiver. As an option, the receiver 'opens' only if it hears the correct tone. Manufacturers have adopted copyrighted names for their CTCSS systems, but they are all basically interchangeable. An analog repeater hears many 'foreign' or distant signals, but only repeats those with the right CTCSS. 67.0 and 156.7 are examples of CTCSS codes.
DCS: Similar to CTCSS, DCS uses a pulsed pattern at 134.4 Hz to accomplish the same thing as CTCSS. Again, Manufacturers have adopted copyrighted names for their DCS systems, but they are all basically interchangeable. D156N and D411N are examples of DCS codes.
Digital Squelch: A value or pattern incorporated into a digital radio packet to identify and qualify the sender. As an option, the receiver 'opens' only if it decodes the correct pattern or value. NAC's and RAN's are examples of digital squelch codes.

Note: Squelch codes do not prevent anyone from hearing a signal, but only prevent radios with squelch codes enabled from opening up on noise or unrelated signals.

As you can see, setting and enforcing standards is incredibly important, and the more open those standards are, the more agencies (and different vendor's equipment) can communicate.

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If this is too long and complex. just ignore it and move on. For the professional Geeks out there, this may be incomplete and somewhat inaccurate. For those who's interests are the basics of radio black magic, figuring out why your scanner won't pick something up, or what to buy, I hope this gives you the tools and points you in the right direction.

[W2NJS]

Originally Posted by jaspence

Taking it a step further, it is not just for government either. Hams are using it in some areas with some of the older radios that have become available on their bands, mainly 2 meters and 440 MHz.

That's not quite the case. In my area most of use the latest Motorola units, such as the APX7500 mobile and the APX 7000 or 6000 portables. And at last count we had about four P25 repeaters, plus a P25 remote receiver on our main UHF system.

[Fuzy_GSXR1000]

Wow that is very informative thank you!! I read and learn here!!!

[jeatock]

In a different thread, the posts were talking about using a 'real' trunking radio to monitor talkgroups on a multi-site system. Use great caution!

As I mentioned above, the trunking controller will only put a talkgroup's traffic on a specific regional/statewide site's repeaters if a mobile/portable is active on that talkgroup and registered on that site. This keeps sites from being 'saturated' and causing problems. Minnesota found this out the hard way during the W35 bridge collapse a few years back; too many curious users statewide tried to listen to collapse traffic and the entire system degraded to the point it pretty much stopped working. This has pushed system administrators to limit the number of talkgroups scanned, and beef up regional limitations and talkgroup priorities.

Public safety trunking system administrators (and most commercial providers) are very protective of their systems and take a dim view of unauthorized users trying to register radios on their system. If not correctly configured, during initialization the 'real' mobile/portable radio may transmit registration requests to the controller. Unauthorized transmissions, even if unintentional or unsuccessful, are in violation of FCC rules.

At best, the ESN of the radio and a general location will be logged. Don't think for a minute that the manufacturer and vendor won't provide the paper trail for the radio. At worst, the trunking controller will 'brick' the radio, and the next day while trying to unbrick the radio you will hear a knock on your door announcing delivery of an invitation to visit the judge and convince him not to fine you out of house and home.

Scanning receivers are a different story- they take what they can get and don't ask for more. As a firefighter, I like to monitor the statewide fire mutual aid talkgroup on my Uniden 396. I have to force it to a more distant tower site covering a larger city to hear that talkgroup reliably. The talkgroup is only active reliably in my home rural area when my own department's heavy rescue truck is out of the firehouse and the truck's statewide trunking radio is on. Since I'm most likely in the truck, the scanner back home is pretty useless.

[Thunder94]

Originally Posted by jeatock

FM Modulation: Content is transmitted by holding the power of the transmitter constant and shifting the frequency up and down (Frequency Modulation). The flashlight stays on at a given brightness, but moves left and right to send infomation. 95% of all public safety and business radios use a form of FM.

No, the flashlight would actually be changing color.

[rapidcharger]

Originally Posted by jaspence

Taking it a step further, it is not just for government either. Hams are using it in some areas with some of the older radios that have become available on their bands, mainly 2 meters and 440 MHz.

True but nearly all buyers of new p25 euqipment are governments. They're the only ones dumb enough and corrupt enough to spend $5,000+ on a handie talkie when a $500 handie talkie will do the job, do it better and do it more securely.

[mmckenna]

Originally Posted by rapidcharger

True but nearly all buyers of new p25 euqipment are governments. They're the only ones dumb enough and corrupt enough to spend $5,000+ on a handie talkie when a $500 handie talkie will do the job, do it better and do it more securely.

Where is the "like" button?