PROJECT 25 TEKTALK

AN INTRODUCTION TO P25 (APCO25, PROJECT 25), THE VE9ARC REPEATER, AND THE RXP25 NETWORK.

Presented by Rod (VE9HV)

WHAT IS P25?

• Project 25 (P25 or APCO-25) is a suite of standards for interoperable Land Mobile Radio (LMR) systems designed primarily for public safety users. The standards allow analog conventional, digital conventional, digital trunked, or mixed-mode systems. P25 was originally developed for public safety users in the United States but has gained acceptance for public safety, security, public service, and some commercial applications worldwide. P25 radios are a replacement for analog (typically FM) radios, adding the ability to transfer data as well as voice for more natural implementations of encryption and text messaging. P25 radios are commonly implemented by dispatch organizations, such as police, fire, ambulance and emergency rescue service, using vehicle-mounted radios combined with repeaters and handheld walkie-talkie use. Although not as popular as DMR, P25 has a lot of amateur uses and can take advantage of Advanced encryption keys like AES256*.
• The audio quality is also very clear making it an excellent choice for mission critical applications.

* Make sure to read and follow the regulations concerning using encryption on the amateur bands.

• P25 standards use the proprietary Improved Multi-Band Excitation (IMBE) and Advanced Multi-Band Excitation (AMBE+2) voice codecs which were designed by Digital Voice Systems, Inc. to encode/decode the analog audio signals. It is rumored that the licensing cost for the voice-codecs that are used in P25 standard devices is the main reason that the cost of P25 compatible devices is so high.
• P25 may be used in "talk around" mode without any intervening equipment between two radios, in conventional mode where two radios communicate through a repeater or base station without trunking or in a trunked mode where traffic is automatically assigned to one or more voice channels by a Repeater or Base Station. Example: If you’re out of reach from the repeater and have your radio on DVRS mode; setting the “TA after no DVRS comms”will automatically put you in simplex mode to still reach nearby users.

P25 PHASES

• P25-compliant technology has been deployed over two main phases with future phases yet to be finalized.
• For Amateur Radio, we use Phase 1 where one user occupies the entire frequency channel at a time. VE9ARC runs on a “pseudo-trunking” setup where talkgroups can be activated on-demand either via the Motorola DVRS feature or keying up a certain talkgroup. If one talkgroup is in use by other users and you try keying up, you will get “bonked” (denied access) until the frequency is free. A true trunking system would just send you on another separate frequency for seamless communication with multiple active talkgroups.

PHASE I

• Phase I radio systems operate in 12.5 kHz digital mode using a single user per channel access method. Phase 1 radios use Continuous 4 level FM (C4FM) modulation; a special type of 4FSK modulation for digital transmissions at 4,800 baud and 2 bits per symbol, yielding 9,600 bits per second total channel throughput. Of this 9,600, 4,400 is voice data generated by the IMBE codec, 2,800 is forward error correction, and 2,400 is signaling and other control functions. Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature phase shift keying" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal deviation at symbol time as C4FM. Phase I uses the IMBE voice codec.

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• These systems involve standardized service and facility specifications, ensuring that any manufacturers' compliant subscriber radio has access to the services described in such specifications. Abilities include backward compatibility and interoperability with other systems, across system boundaries, and regardless of system infrastructure. In addition, the P25 suite of standards provides an open interface to the radio frequency (RF) subsystem to facilitate interlinking of different vendors' systems.

PHASE II

• To improve spectrum use, P25 Phase II was developed for trunking systems using a 2-slot TDMA scheme and is now required for all trunking systems in the 700 MHz band.^[18] Phase II uses the AMBE+2 voice codec to reduce the needed bitrate so that one voice channel will only require 6,000 bits per second (including error correction and signaling). Phase I is not backwards compatible with Phase 1 (due to the TDMA operation), although multi-mode TDMA radios and systems are capable of operating in Phase I mode when required, if enabled. A subscriber radio cannot use TDMA transmission without a synchronization source; therefore direct radio to radio communication resorts to conventional FDMA digital operation. Multi-band subscriber radios can also operate on narrow-band FM as a lowest common denominator between almost any two way radios. This makes analog narrow-band FM the de facto "interoperability" mode for some time.
• Originally, the implementation of Phase II was planned to split the 12.5 kHz channel into two 6.25 kHz slots, or Frequency-Division Multiple Access (FDMA). However it proved more advantageous to use existing 12.5 kHz frequency allocations in Time Division Multiple Access (TDMA) mode for a number of reasons. It allowed subscriber radios to save battery life by only transmitting half the time which also yields the ability for the subscriber radio to listen and respond to system requests between transmissions.
• Phase II is what is known as 6.25 kHz "bandwidth equivalent" which satisfies an FCC requirement for voice transmissions to occupy less bandwidth. Voice traffic on a Phase II system transmits with the full 12.5 kHz per frequency allocation, as a Phase 1 system does, however it does so at a faster data rate of 12 kbit/s allowing two simultaneous voice transmissions. As such subscriber radios also transmit with the full 12.5 kHz, but in an on/off repeating fashion resulting in half the transmission and thus an equivalent of 6.25 kHz per each radio. This is accomplished using the AMBE voice coder that uses half the rate of the Phase 1 IMBE voice coders.

CONVENTIONAL IMPLEMENTATION

• P25 systems do not have to resort to using in band signaling such as Continuous Tone-Coded Squelch System (CTCSS) tone or Digital-Coded Squelch (DCS) codes for access control. Instead they use what is called a Network Access Code (NAC) which is included outside of the digital voice frame. This is a 12-bit code that prefixes every packet of data sent, including those carrying voice transmissions.
• The NAC is a feature similar to CTCSS or DCS for analog radios. That is, radios can be programmed to only pass audio when receiving the correct NAC. NACs are programmed as a three-hexadecimal-digit code that is transmitted along with the digital signal being transmitted.
• Since the NAC is a three-hexadecimal-digit number (12 bits), there are 4,096 possible NACs for programming, far more than all analog methods combined.
• Three of the possible NACs have special functions:
• 0x293 ($293) – the default NAC
• 0xf7e ($F7E) – a receiver set for this NAC will pass audio on any decoded signal received
• 0xf7f ($F7F) – a repeater receiver set for this NAC will allow all incoming decoded signals and the repeater transmitter will retransmit the received NAC.

INTRODUCTTION OF THE VE9ARC REPEATER

VE9ARC DETAILS

• Frequency: 147.195+
• Modes: Analog FM, APCO25
• Network Access Code (NAC): $293 *for P25 use
• PL tones: 141.3Hz both incoming and outgoing to filter out digital noise when P25 traffic is heard. The repeater does not output the PL tone while transmitting P25 thus having an input PL tone on your radio will make it so the squelch does not open for digital traffic when using the analog side of the repeater.
• Hardware: Motorola Quantar 125Watt VHF P25 repeater.Output set to 60W to compensate for cavity loss but also not to overshoot its listening performance. No use of being able to listen from excessively far away while not being able to key it up.
• 8 cavity duplexer (3 notch + 1 bandpass per leg).
• Andrew LDF4-50 Heliax coaxial feedline.
• W3AXL DVM-V24 translator board.
• Sinclair single folded dipole antenna at approximately 87ft on the Stilesville tower aimed towards downtown Moncton.
• Dell Mini-PC running dvmhost under Debian Linux.
• VE9ARC is internet linked to the RXP25 network but can also be used as local only.
• Motorola Astro DIU3000 Analog-Digital bridge connected to a Motorola CDM1250 UHF link radio.

DVRS MODE �(DIGITAL VEHICULAR REPEATER SYSTEM)

• For the VE9ARC repeater, it is highly encouraged to use DVRS mode if you have the capability.
• When you have DVRS mode enabled the system will know if there is someone on the talkgroup therefore if the desired talkgroup is on-demand, as soon as you go to that talkgroup, it will “associate” and keep the talkgroup active for as long as the radio responds back when it gets pinged (or inactivity timeout). When you turn off the radio or switch talkgroups, it lets the system know to drop the previous talkgroup.
• If you radio is not capable of DVRS mode you will have to key up the selected talkgroup to enable it. Once enabled, it will stay active for 12 hours and drop if there’s no activity. Not having DVRS also means you will have to make a conscious effort to leave pauses between transmissions as someone not running DVRS might “step on” someone using DVRS. The reason for this is when using DVRS, upon pressing PTT, a transmission lasting a fraction of a second requests a channel grant from the repeater and if the frequency is clear, will give you the famous Talk Permit Tone. If the frequency is in use, it will “bonk” you and not permit you to talk. Not having your radio on DVRS mode does not go through this check. Please use it if you have it!

CURRENT TALKGROUPS

• 1: Local only. With this talkgroup, the traffic will not go through the network and will remain operable during internet outages.
• 5500: Canada English. This talkgroup is set to always on (static) for this repeater and is the most used talkgroup on the network.
• 9990: Parrot. This talkgroup allows you to test whether or not you’re making it into the repeater, how good (or bad) your audio settings are, and whether or not you’re in a fringe zone. It simply repeats what you say.�5550: Canada French�5558: Technicians�5000: System General�5501: CANTAC1�5502: CANTAC2�5503: CANTAC3�5551: FTAC1�5552: FTAC2�5553: FTAC3�5700: Intercom�3150: Vermont�3136: New York

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COMING SOON!

• DIU3000 Analog to Digital bridge on its own talkgroup.�This will allow me to link to any local UHF analog repeater on its own talkgroup as to not interfere with the digital only talkgroups. This will allow P25 users to cross from P25 to analog. The new talkgroup (to be determined) will be static while the others will be on-demand. The hardware is all ready to go, I am just waiting on the network admins to give me the configs for the new setup.
• This network is maintained by one or two individuals with full-time jobs. They maintain this on their spare time. If issues arise, please be patient as it’s out of my direct control.

WHAT KIND OF RADIO WILL I NEED?

SHORT ANSWER: WHATEVER YOU CAN AFFORD

• P25 radios are not “brand locked” like Yaesu’s version of C4FM/Fusion, meaning that you do not need a specific brand of radio to use P25. Motorola is the preferred radio however tons of manufacturers make P25 radios. Kenwood, Tait, EF Johnson, Harris, Bendix-King (BK RADIO), etc…
• P25 radios can vary widely in prices depending on its features, brand, condition, surplus availability, etc. I personally got an XTS5000 which is an older but extremely capable radio for about 150$ on Marketplace. They usually sell for around 300-500$ on ebay. The trick is to keep looking. If you find prices are too expensive today, check again tomorrow if new listings popped up on eBay. Some sellers have pallets of radios and sell for a great price. Don’t be afraid to make offers as well! Older radios will have a cheaper price but does not mean anything quality-wise. For amateur use, if it can do VHF P25, you’re good to go.
• There are no “cheap Baofeng equivalents” for P25; just good deals on quality equipment. The last thing we want is cheap radios with bad BER and bad audio on an otherwise robust network. All P25 capable radios are certified (FCC, ISED, etc) to be compliant with maximum allowable spurious emissions. Quality comes at a price.

CAN I USE IT ON A HOTSPOT?

• Yes!, but you cannot cross-mode from P25 to any other mode unless the reflector on the other side is already cross-moded (some talkgroups will say whether or not they are linked to another mode/talkgroup).
• For hotspots, DVRS mode cannot easily be used therefore you will have to set it up as conventional channels (need to key up to join a talkgroup).

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CAN I REACH VE9ARC THROUGH A HOTSPOT?

• Not as of today. My repeater is not linked to any reflectors accessible through pi-star, Droidstar, or WPSD. There is a possibility of eventually having access to the RXP25 network via reflectors.
• VE9ARC repeater is on a private network called RXP25 which is fairly new and is growing. There are several repeaters from different provinces and states already..
• As more and more users get interested in P25, there will naturally be more traffic IF you actually throw out your call. You’d be surprised how many people are just sitting and listening and also won’t throw their call. I’ve seen myself throw my call, get someone respond, and all of a sudden, we we’re 4 or 5 people on a subject ragchewing.
• The idea behind the DIU3000 Digital-Analog is to encourage traffic for the Moncton area by bringing it up for nets and such. It will most likely be parked on VE9SHM. The talkgroup will still be accessible through any other RXP25 repeater.

THROW OUT YOUR CALL ONCE IN A WHILE WOULD YA!

ARE P25 RADIOS HARD TO PROGRAM?

• It is certainly not like programing an analog radio. Programming software for P25 radios can be a learning curve however that is part of the hobby. Learning new things. P25 is not designed for “appliance operators” who just want to take it out of the box, punch in a frequency and operate (ex: Fusion).
• There is help available (Google, ChatGPT, forums, etc.) to get your radio programmed.
• P25 radio programming is somewhat comparable to DMR difficulty-wise but not quite. If you managed to learn DMR programming, you can also learn P25.

QUESTIONS?