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Radio is analog, and a modem converts analog signals to digital signals and vice versa.
So is P2P sharing of digital files (like .exe, .7z, .pdf or any other file) possible using radio spectrum?
If yes then which tools do I need to do this?
Does WiFi work in frequency ranges other than 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz and 5.9 GHz?
All these frequency ranges have lower range of propagation, a lower frequency would be needed for greater propagation range.
Does WiFi work in MHz or in Hz range?
Radio is analog, and modem converts analog signals to digital signals and vice versa.
Everything is analog, yet you can do digital things with the physics that is given to us. As you've noticed, when transmitting something digital over an analog channel, you typically use a modulator, and to receive the same you'd need a demodulator, and modem is just a portmanteau of these two words.
So is P2P sharing of digital files (.exe, .7z, .pdf or any other file) possible using radio spectrum?
Yes, and with Wifi, Cellular networks, and many other techniques, I'm sure you're already aware of the feasibility.
If yes then which tools do I need? Does WiFi work in frequency ranges other than 2.4 GHz, 3.6 GHz, 4.9 GHz, 5 GHz and 5.9 GHz? All these frequency ranges have lower range of propagation, a lower frequency would be needed for greater propagation range. Does WiFi work in MHz or in Hz range?
Wifi, indeed, makes a few assumptions on the channel (ie. a microwave channel with 20 or 40 MHz width will behave differently than the much narrower, much lower, much noisier) channels you can get at shortwave, medium and longwave ranges – how could you have 20 MHz of bandwidth if your center frequency isn't even that large?
So, no, Wifi itself can't work at these frequencies. Also, Wifi has to assume a bounded flight-time of signals to work efficiently. To top that up, large-area covering WiFi networks are a bad ideaTM, mainly because of the hidden station problem.
There's other techniques, but as soon as you think about it: Low-frequency spectrum simply is sparse because of the low frequency. Thus, to get sufficient bandwidth, you move to higher frequencies, which is why Cellular networks operate at 800 MHz and up. There simply was no spectrum available below.
All these frequency ranges have lower range of propagation
That's an alternative fact. There's numerous long-range and space-ground links at frequencies much higher than that.
So, yes, while Free Space Path Loss might be higher for increasing frequency, antennas of a given size also get higher gain – and that makes all the microwave links that are the backhaul of a lot of the high-rate communication done today feasible.
P2P means Peer-to-Peer – I'm going to take that literally: It's not impossible to build a mesh network out of microwave links (in fact, that's what people do that operate cell phone towers over areas, and TV, towers on remote hilltops) and route stuff through that.
There's WiFi-based mesh networking – you might want to talk to your friendly Freifunk enthusiast about B.A.T.M.A.N.; but I still think that for long-distance mesh networks with links that are point-to-point, you'd probably want to do with a simple frequency division microwave link (ie. one center frequency for A->B and a different one for B->A) rather than a collision-avoidance based packet network (which has no advantages in a pure point-to-point, fixed terrestrial application).
You can rather easily build such a single link using (proper¹) SDRs that allow you to send arbitrary waveforms by calculating these in your PC, and do the inverse for reception. Using a network interface in your SDR application would allow you to plug together an IP network.
If you want to have omnidirectional rather than point-to-point links, you must share a single piece of spectrum across all users – which inherently divides the possible channel capacity by the number of users. You'll find, with a bit of consideration, that the easiest way to make the most of the spectrum that's available, is if there's a central node that everyone agrees on as controlling access timing – that way, for example, transmissions can be scheduled without the need for any "dead periods" in which stations have to listen to avoid colliding with others. If you do that, you'd be building some kind of TDMA base station, much akin to GSM / GPRS / EDGE (basically, in ascending order of flexibility in which time can be allocated to the stations) base stations. If you add the ability of base stations to communicate, and for user equipment to change between these, you've built a cellular network. Congrats!
If you don't need high rate (and, as Phil suggested) can live with half-a-century-old technology, the AX.25 packet radio technology might be an option – but certainly not for sharing any substantial files. AX.25 itself is horrible on many levels; it is especially bad-suited for networks, as there's no good network layer that is commonly used. APRS is often used atop of that to build what can best be described as a low-rate, sparse, very suboptimal, chaotic-good aligned, networking/relay system. It reduces the possible channel utilization by not having any awareness of routes, and thus would immediately break down if a couple of stations tried to continously use the channels at max rate for any substantial amount of time (or data). So, that's definitely not a candidate for file sharing.
WiFi has a channel width of 20 or 40 MHz, even up to 160 MHz with 802.11ac. Theoretically WiFi could operate at a lower frequency, though there would be a number of issues. For starters, a single channel would obliterate most or all of the HF spectrum.
That said, there are easily thousands of digital modulations that could be used to send data.
I'm not sure anyone is sending large computer files on HF over large distances though. With most of the HF spectrum able to fit within just one WiFi channel, there's just not much spectrum available there. The noise is higher, and the path losses are higher also, which limits the speed attainable at realistic power levels.
Within the amateur radio hobby, some people operate 300 baud AX.25 packet radio on HF, though it works quite horribly since it uses ancient technology and was not designed for this purpose. FreeDV comes to mind as a digital mode which was designed for HF and works quite well. DRM can also be found on the ham bands, as well as commercial broadcasting. To send files with these modulations you'd need to define an appropriate protocol.
There's a book that's available for free, which is very well-suited to answering your questions. It's called "Wireless Networking in the Developing World", and it's all about setting up wireless data networks across long distances inexpensively. Look for it at wndw.net.
Amodem can be used to transmit at arbitrary baudrates on any transceiver with a TRS port, 3.5mm or 2.5mm, or any other kind. Can take a while, but the data arrives. Includes a checksum (as does zip/rar/7z/etc archives). I've transmitted a few MB on UHF, wasn't a problem. You'll need a computer on either end, a lowly PI would probably suffice. It doesn't use much CPU. You can test the program with a 3.5mm TRS on your PC, output to input.
Hint: Start with lowest baudrate and go up from there.
https://github.com/romanz/amodem
Edit: Anything above 50KB, I'd chop up using a file splitter, since otherwise you'll be retransmitting a lot if using it at high range. 1200 baud can go 20km no problem if you have decent rx/tx antennas
We certainly did with the PBBS system we used at the Alaska National Guard building with the Alaska Division of Emergency Services. That station used my callsign with a "-1" appended to differentiate it from my personal systems I ran at home and in my truck. That station existed throughout the 90s.
It worked great and was quite busy at times. Basically it was a store-and-forward message system along with categorized forums like any common dial-up BBS system.
We also had a software library stored in .ZIP format but very clearly named to identify the programs as being very common to the time. We also offered the PBBS software itself so others could set up their own nodes if they wanted. Software downloads were done during off-peak hours by gentlemen's agreement.
It is worth mentioning that we never had any inappropriate communications there during the entirety of its existence.
We experimented with FIDOnet but never went operational with it. Had we done so we would have had a network of stations for message delivery similar to the way mail servers work on the Internet now. In the end, our PBBS had the coverage we needed without the need for additional software on the ham's end.
The only equipment required for hams was a radio with a cable interface to the TNC and an RS-232 port to any kind of computer. The TRS-80 Model 100 was very popular then.
Everything was plain text except the ZIPped software.
The PBBS was used heavily during disasters to distribute official daily sitreps and to provide a centralized way for participating hams that were only licensed for amateur frequencies. We wanted all the help we could get so we purposely kept it in-band instead of using MARS frequencies.
Equipment included a TAPR-2 TNC which I believe ran AX.25 protocol at 1200 bps on 145.010 simplex. Coverage was great based on location and the 1/2 wave antenna mounted at the same location as the State Troopers dispatch center.
Now that I am retired I find that I miss the good old days of radio BBS. ;-)
We, ARES, send files often via VHF packet and via several HF digital modes.
My favorite HF digital is based on the many modes of FLDigi, but set up the files (like ICS-213 form) using Winlink Express.
On VHF, my station is part of a packet network, running 24/7. www.midgadata.net. The net also supports DRATS on the same frequency; so we can use the DRATS software to send either packet or DRATS mode. Both have collision avoidance.
