P2000 is basically to receive a text. The C2000/TTR is encrypted, meant for Policy/Ambulance/Fire departments and apparently used by many countries in the world. It'll work
even when other networks are down. Apparently fully used since 2007.
I'm a first responder for my building, it used to be that the C2000 system (simply stated: "professional walkie walkies") would be difficult to use in my building (couldn't go through concrete). We'd lend them our walkie talkies (they work much better as we have a support transmitter within the building). Over the last few years they seem to have improved C2000, nowadays they don't need ours for most areas.
Compared to cellular, ptt networks are often vhf frequencies (<200 MHz) and higher power (>5 Watts), therefore coverage from a mountain top repeater is practically infinite compared to cellular, and limited only by terrain, requiring far fewer base stations and repeaters and backhaul for the same coverage.
I'm working on a state wide radio system for a power company, its a simulcast network, with 20 talkpaths. Your worse case bandwidth utilization is on the order of roughly 1.5 megabit a second per site (realistically, its much lower). In addition, if a site fails the other ones will still function, also, if the network core fails, the sites go into a failsoft mode, which allows single site communications to still occur.
FirstNet is based on LTE. Last time i looked into it, it was a private LTE network, with the ability to piggy back on public LTE when there's no private coverage (with a high priority). So you can forget about the simplicity of a basic PTT network. Here, it's MCPTT (Mission Critical Push-To-Talk), which is a layer on top of LTE leveraging the LTE multicast support (eMBMS) when applicable. So not exactly super simple.
There a protocol extensions for public safety on top of MCPTT. To better handle out of coverage situation (D2D or ProSe, which is device-to-device communication with relay support too), to hook custom applications on the network side, etc. But the infra is LTE, so at least it leverages an existing technology. Still, with some additions on top to better support public safety use cases.
And the "obsolete" part is not very convincing. Yes, there's a lot of hype about 5G but LTE will be long lived and perfectly fine for such an application for a long time. I can't comment on the overall efficiency of the FirstNet program, but the idea of leveraging LTE instead of rolling a custom tech or keeping using old and inefficient ones seems sound to me.
This question is a bit out my domain, but I believe the answer boils down to being able to re-use one antenna for send and receive with push-to-talk, but for full-duplex communication you either need another antenna, or more complex signal processing.
Strictly speaking you can use half as much bandwidth by having TX and RX share it, i.e. half-duplex versus full-duplex. It can also somewhat simplify transciever designs in a number of ways. It is also a fairly straightforward model: if you're on the channel and in range of the transmitter or a relay (and maybe have the right preshared key), then you will hear and be able to transmit on that channel.
The FirstNet is/should be unrelated, as far as I'm aware. I think it's supposed to be for data applications like image and document sharing.
