I am quite a newbie to retrocomputing, and I don't know if such a question should be asked here. But I hope maybe some of you could help me. I started reflecting on ways to send data without the internet and came up with the vague idea of using older cellular networks to slowly transfer code from one computer to another. Would it be possible to plug a computer (any type) to an old 2G (GSM) cellphone and act as a modem to receive raw binary code from another computer with a real 2G modem?
Sure, as long as these phones have a build in 2G modem interface - usually provided via a serial line and behaving like a hayes modem. So check the specs. There were also separate 'air' modems, essentially a phone in a box doing the same - or even PCMCIA cards.
Finding either might be less easy as all (especially the serial cables) were rather expensive and thus rare - after all, who had the money to transfer data over an expensive mobile phone connection back then?
Not as old, and maybe more phun are GSM modem modules which are still available today. They are essentially a GSM phone on a single carrier board, meant to be integrated into custom PCBs. The stuff used in renal bikes and alike.
Technology wise GSM was made to support modem connections at at least 9600 Bd looking like regular dial up from a user (computers) perspective.
Bottom line, read the specs and get it going.
Yes. GSM has support for both circuit-switched data (CSD) and packet-switched data (GPRS). As late as the original iPhone, GPRS was the main standard for mobile data with smartphones and similar devices. GPRS service remains available from many carriers, and most modern cell modems will fall back to GPRS if UMTS/LTE is not available.
The GSM standard defined a digital cellular network. Therefore, it has always been capable of doing data communications, computer-to-computer, from the very beginning. Nowadays, most public networks support 4G/LTE, and smartphones are advanced enough to serve as Wi-Fi hotspots. In the 2G time frame, you could do a similar function by connecting a serial cable from your PC laptop to a connector on the phone, putting the phone into a modem "mode", and then connecting to a terminal server upstream. Most likely, you would use PPP to form the conduit for TCP/IP networking and this would allow your serial modem connection to get you onto the Internet. It was quite similar to "dial-up" networking, but substituted the GSM cellular network for the landline. [Personal note: I was part of an R&D team that took this to the next level by inventing "Bluetooth Dial-Up Networking" implementations that eliminated the wire.]
Nowadays, I'm unsure the level of backwards compatibility available on public cellular networks. Maybe 2G and 2.5G (GPRS) connections still work for slow data transfer. This might vary from network to network and region to region. 4G normally falls back to 3G in areas of low coverage. I'm not sure if it falls back to 2G or not.
Likely, a better approach today for wireless networking with retro computers is to connect them to local Wi-Fi. There are very inexpensive modules, such as the ESP8266, which can bridge a retro computer's serial port, to a LAN in your house using W-Fi. It's a pretty common use-case for people wanting to recreate the old "BBS experience". Naturally, you can also use this approach to connect to Wi-Fi that in turn connects to 4G or 5G cellular through a smartphone or other "Hotspot" style device.