Comment by forgotusername6 12 hours ago
I was surprised when a SIM I purchased on Amazon was not only able to connect in China but was also able to bypass the great firewall. I wonder how these travel sims get round the government regulations.
They bypass the firewall precisely because they're roaming SIMs. Their internet connection goes through the home operator.
I imagine they simply don't allow selling such SIMs in China. It would be extremely easy to track and flag any that were e.g. used for longer than a few weeks.
It's because the government regulations only apply to Chinese citizens. My first trip to China was back in the '00s, and I went for work. I was also surprised to find that my home SIM worked just fine there without any interference from the Great Firewall.
Roaming works somewhat unintuitively from what you'd expect. You do indeed connect to the local mobile network, but all of your data traffic is tunneled back to your home wireless provider's PoP. I realized this once I checked what websites I was visiting saw as my public IP address, and it was an address from a network in Texas!
So China's Great Firewall can't actually inspect or block your traffic while you're traveling, and using roaming on your home mobile network's SIM. It's all sent over the equivalent of a VPN to your home soil before going out to the public internet. This iswhy latency can be pretty bad while roaming.
When you ROAM, you traffic abroad is routed to your home country ( for security reasons among other things) and then off to the internet from there. You can check that your public IP, when roaming, is an IP from your cellco.....unsure if there are any changes with 5G though.
You are not bypassing any firewall as your traffic is actually happening at home. If you access local sites, traffic is coming from home.
Not home country, home PLMN(~=carrier). IIRC there were changes in 4G/LTE that lets the GW be at visiting carriers.
I'm suspecting that that post-4G architecture is just formalization of actual commercial deployment. Latency for roaming data was long inconsistent with the 3G diagrams, and exorbitant roaming fees that would be consistent with the diagrams also started rapidly subsiding from late 3G era.
It's how data roaming works in general -- it's tunneled through to the SIM's home provider. Conversely, a Chinese SIM roaming overseas is still subject to the Great Firewall.