P25 enjoys an almost unique capability in its ability to operate as a conventional, trunked, or simulcast system. As public safety agencies have already discovered, it is fairly straightforward from an engineering point of view to convert a P25 conventional network to trunking, while retaining the investment in the original network. Simulcast operation can be added to all or part of an existing trunked or conventional P25 system. Each mode of operation has its own strengths and tradeoffs which can be summarized as follows.
P25 Conventional Operation
In a conventional system each group of users is permanently allocated a particular communications channel which they can select on a radio using a selector knob or button. After manually selecting the appropriate channel for talking to a given group, a user simply presses the push-to-talk (PTT) button on the radio or microphone to begin talking. A scanning operation alerts the called parties to the incoming call on that channel. It is a simple, fast, and reliable mode of operation. It is also the simplest, most cost-effective network solution.
The chief disadvantage of this simple solution is that it does not use communications channels particularly efficiently. It can happen, especially if there are more than four channels on a radio site, that one of the channels becomes overloaded (and callers must wait), while the other channels are underused. This is similar to a bank in which the teller tasked with handling personal checks is overloaded, while the teller reserved for handling business checks is idle.
In a conventional system there is no indication that a channel is busy (other than discovering that a call cannot be made), nor when it becomes free. A radio user can only try again at random.
P25 Trunked Operation
If an agency has more than four channels at a site, it can use trunking to spread the load more evenly. The basic idea of trunking is to use a computer (a ‘trunking controller’) to control use of the communications channels. This is much like a bank opening up all of its tellers to handle any sort of transaction and then using one of its officers to direct each customer in a queue to the next available teller. The computer takes the role of this officer and, via its own reserved communications channel (called the ‘control channel’) which is in contact with both queued radio callers and with all the other communications channels, can direct the next caller to the next free channel. It can even alert a queued caller when a channel is freed up. In this way, an agency can get the best possible use out of all of its communications channels.
There are other advantages of trunking which arise from the presence of a controller. Radio users can be authenticated to see if they are allowed on a site. Group calls are more efficient because the only sites that participate are those where the radios are active (i.e. have ‘registered’). Priority calls are possible because the controller can promote prospective callers to the head of the queue. And for connected radio networks, a radio user can roam from one network to another while still communicating with his home dispatcher or his group.
The network intelligence associated with trunking is not without costs, however:
- it is a more complicated and more expensive design,
- it introduces another point of failure namely, the trunking controller,
- it uses an extra communications channel/frequency to support the control channel,
- it adds extra time to set up a call – conventional calls don’t have this overhead,
- the radios behave slightly different versus operation in a conventional system.
These costs need to be balanced against the powerful gains provided by trunking, such as:
- more users can be supported with the same frequencies because the channels are used more efficiently with idle time automatically minimized,
- calls can be queued, so no caller is left guessing when communication is possible,
- controlled access of end user devices to the network,
- calls can be prioritized, and
- there is a wide-area multi-network capability comparable to cellular networks (which, in most respects, are consumer-grade trunked radio systems).
P25 Simulcast Operation
When an agency must communicate over a large geographical region but has only one or a few frequencies at its disposal, then simulcast operation is an ideal solution. The idea is to use its few frequencies to create an all-informed broadcast-style network. To simplify matters, imagine a single frequency system that operates county-wide. A radio user on a P25 conventional simulcast can move anywhere within the coverage of the radio network and still stay in touch. There is no need to do anything special, such as manually selecting a channel, since all of the radio transmitters are working on the same frequency. This has the roaming advantage of trunking, but is simpler and faster since it does not have the overheads of trunking setup or scanning. It can even have a side benefit in providing better coverage inside buildings where the radio user is in the reach of two different transmitters.
Analog radio system users have been running wide area public safety simulcast networks for many years. P25 digital gives them a familiar type of communications environment, but with superior digital clarity and extra services (such as encryption, or the ability to disable stolen or lost radios). The technically difficult challenge of simulcast, whether analog or digital, is how to manage interference for a radio user who is in an overlap zone, where his radio can receive the same call coming from two different transmitters. Since the call from a nearer site can arrive sooner, it can interfere with the same call arriving just slightly later from a more distant site. Careful control of the frequency and timing of transmitters using extra equipment can virtually eliminate interference.
While highly economic and less complex than P25 trunking, digital simulcast comes with some additional costs and limitations over P25 conventional systems:
- more equipment is needed to make it work well,
- the network design and configuration is critical to reduce or eliminate overlap zones and to achieve the expected coverage,
- finally, P25 digital simulcast loses one advantage of P25 conventional namely, the ability to interoperate with analog conventional – P25 simulcast is all-digital.
But it also picks up an unusual advantage elsewhere. With P25 it is possible to simulcast trunked digital channels as well as conventional digital. This ability to combine different operational modes is another aspect of the design flexibility inherent in P25 technology. While public safety users generally design networks that use a single P25 operational mode, whether it be conventional or trunked or conventional simulcast, a few organizations have found reasons to deploy mixed systems that combine all three modes. Accommodating requirements such as these is an indication of the freedom of design that is possible with P25.
For a summary of the advantages and limitations of the three P25 operational modes refer to the table below.
|Features||Conventional||Trunked||Conventional Simulcast||Trunked Simulcast|
|Network Access||Uncontrolled. Radios must only have the right access code number programmed into them.||Controlled. Radios must have the right access code number programmed into them & be known to the network.||Uncontrolled. Radios must only have the right access code number programmed into them.||Controlled. Radios must have the right access code number programmed into them & be known to the network.|
|Dual (analog/ digital P25) Mode||Yes||Yes||No||No|
|Channel Efficiency||More efficient if less than 4 channels||More efficient if more than 4 channels||More efficient if less than 4 channels||More efficient if more than 4 channels|
|Spectrum Efficiency||Low||Medium. But higher channel efficiency means fewer channels required.||High||High|
|Call Setup Time||Fastest. Begin talking immediately.||Medium. Network check & call set up. Then wait for go-ahead beeps.||Fast. Slightly longer than conventional.||Medium +. Slightly longer than trunking.|
|Roaming||Can be designed to allow radios to move around network but not seamlessly.||Near seamless||Seamless||Seamless|
If you want the fastest, most cost-effective network and can support all of your teams with 4 channels or less. Choose conventional if you don’t mind the limitations of occasional congestion on some channels while others go idle, as well as little access control.
If you need more than 4 channels and must use all of them to the maximum, you need greater access control and high availability (fallbacks in case of equipment failure), and greater operational flexibility. Choose trunked if you don’t mind the cost of additional controller equipment, licenses, and the need to set aside one channel as a control channel, as well as the longer call setup time incurred.
Learn more about P25 radio network system types in our complimentary whitepaper Navigating the P25 Maze: which system is right for me?