I couldn't access Google today.

I was on Swisscom's network using 4G data and here's the kicker though: I was using a China Telecom SIM card.

I'm assuming that I'm not behind the GFW in Switzerland, because, well, how could I be? I'm also assuming that SIM cards cannot automatically black/white-list certain webpages.

So...

Swisscom is censoring data to comply with Chinese telecom company's policies?

Any idea what's going on here?

  • 3
    Isn't this a technical question beyond the scope of travel? You're basically asking how data is routed in UMTS networks. The answer is the Gp interface sends roaming data back to the home network before reaching the Internet. – user71659 Aug 21 at 15:24
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    @user71659 Whether the Great Firewall is applied while roaming abroad seems very much like a travel question. – gerrit Aug 21 at 15:32
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    @user71659 Leaving the country make it travel related. – gerrit Aug 21 at 16:27
  • 1
    "I'm also assuming that SIM cards cannot automatically black/white-list certain webpages." - Chine Unicom blocks the access to LINE, even with VPN (at least in 2017). – Blaszard Aug 22 at 7:49
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    If you're after a non-GFW internet connection, connecting the phone to WiFi should work – craq Aug 23 at 4:15
up vote 54 down vote accepted

My experience when I use data roaming is that the local telco tunnels IP traffic to my home provider, and then it enters the global internet at my provider's premises.

So sites like whatismyip.com geolocate me to Denmark, even when I'm using data roaming in, say, England.

It would stand to reason that the same thing happens when you're roaming with a Chinese SIM in Switzerland -- so your traffic does actually pass through the Great Firewall.

  • 2
    This has been my experience, too, both with Canadian and U.S. providers, so I think it's a pretty typical thing for all mobile networks to do. – Jim MacKenzie Aug 21 at 17:09
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    Which is also the reason why roaming SIM cards are often much slower, even if you're connected to the local LTE network. Its a limitation on the part of the GSM roaming protocol rather than a wilful choice on part of the operators themselves. – JonathanReez Aug 21 at 19:28
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    And it's also why roaming foreign SIMs in China could bypass the GFW until they (mostly) blocked this loophole a few years ago. – jpatokal Aug 22 at 6:05
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    Yes, if you use a Chinese SIM you will experience the GFW. The opposite is also true. For example, if I use my Hong Kong SIM in China I will NOT be blocked by the GFW. – Rutger Huijsmans Aug 22 at 7:26

Data connections from Chinese SIM cards are routed through China. I confirmed this by my own experience and it has been common knowledge among Chinese travellers for quite some time now. A few years ago this apparently wasn't the case because I remember reading news articles about the change.

Likewise (though not confirmed) using a foreign SIM card in China should give you unrestricted access.

  • 1
    Do you by chance have some sources that confirm that this wasn't the case some years ago? Including them would probably increase the usefulness of the answer. – Trilarion Aug 23 at 7:41
  • @Trilarion i have been trying to find any reference without luck, all i found that the routing was already talked about in 2014 so it is older than what i thought was when i had read that article. or maybe it was always like this and i misread it. – eMBee Aug 23 at 15:35

I've been involved in IT and Cell Phone sales for 15 years, and I can confirm that not only would you have this strange behavior, but specifically why.

The phone, regardless of whether it's GSM, LTE, EVDO, CDMA, etc. always has a few basic settings to tell it how to communicate with the network. One of these settings is the internet gateway setting. Whether you've got a SIM card or a US-style phone where the settings are on NVRAM in the device, this setting is always present somewhere. The gateway IP is specific to your carrier, and ALL traffic from the phone that isn't considered "local" (inside the netmask specified by another setting in the same place - this would almost never apply excepting possibly some server managed by your mobile provider or a very strange coincidence) go through that gateway to get out to the internet. It's not a VPN or tunnel or anything fancy like that - it's the standard IP protocol itself that causes this. That does mean that you could sidestep it by sidestepping IP, but to my knowledge no one uses ARP packets to communicate directly via MAC address, so that's highly theoretical and pretty pointless.

The reason I'm answering this question when Henning Makholm already covered it pretty well is because it has a potentially dangerous unintended consequence - the traffic is not separately encrypted. That's probably fine if you're in a liberal country and the account is in an equally liberal country, as ISPs usually aren't in the business of arresting people or stealing credit card numbers. If you're coming from or visiting China (or another country known to engage in legal action for what they deem illicit internet activity), you're at risk - even if you're from outside the country in question. The traffic will pass through the public internet in whatever encryption state the destination server requires, through who knows how many different switches and ISPs, until it reaches your gateway, and will then proceed on to its destination from there.

In other words, your traffic is routed first to your mobile provider, then on to the internet at large. It's sent along in whatever encrypted state the destination requests. If it travels through a country that takes offense to your data, and any of that data (even the DNS name or IP in some cases - beware!) can be interpreted (is plaintext or weakly encrypted), you are potentially in serious trouble. So you could theoretically use a foreign SIM to get around the Great Firewall. You could also bake rat poison brownies. Neither are a good idea. Be careful when traveling abroad, and keep those internet searches nice and clean just to be safe.

  • Interesting. Is there a way to change these gateway settings? – Trilarion Aug 23 at 7:40
  • "or another country known to engage in legal action for what they deem illicit internet activity" goes for most countries in the world, for example when downloading child pornography. It's just a question of a different threshold. – pipe Aug 23 at 7:50
  • There's no good way to change the Gateway, no. Technically you could specify any valid gateway you wanted, but the cell phone towers can easily block that and likely do. Clarifying the "illicit internet activity", I was trying to be intentionally vague to avoid anyone reading this from getting in trouble. I was referring to a certain day that the Chinese government doesn't want anyone mentioning. There have been reports of people disappearing for saying the day, the name, or details regarding it. That day might as well not exist, and if you mention it, you won't either. – d33733t Nov 8 at 6:01

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