Summer vacation season has arrived, and Facebook feeds are being filled with photos of toes at the end of a towel pointed at a sandy beach.  Family vacation season has begun!

For some families this will mean heading to foreign countries to immerse themselves in different foods, cultures and languages.  But how will you stay in touch with your friends and family back home?  If you still have your domestic SIM in your phone when you land, you’ll receive a friendly message from your carrier (NOTE: these texts are over a year old, the numbers may be different today):

At $2.05/mb, your mobile data roaming could quickly balloon to ‘mortgage payment’ levels, so please sign up for a plan when prompted!

So, how does Data Roaming Work?

Let’s begin with how a basic data connection is established.  Your phone connects to a tower, and the data request passes through the carrier’s infrastructure to the internet.  Then the remote server (shown as a Facebook ‘cloud’) sends data back to the phone.

Facebook’s cloud servers are deployed all over the world, so the time the data takes to go from the carrier to Facebook is minimal.

This all changes when you are roaming

So how does roaming change this?  Imagine that you are taking your dream vacation to Paris, and want to upload a photo of the Eiffel Tower.  You might imagine that the connection to the French carrier would connect to a Facebook server in France:

However, this is not how it works.  Your home carrier needs to account for the data traffic passing over the network, so the actual data path looks like this:

Your home carrier (in the US) has to account for all of the traffic from your phone, so all of your data is sent under the Atlantic Ocean to the US, and therefore hits the Facebook servers in the USA.  Now, since Paris to New York is ~3600 miles, we can estimate that this adds about 58 ms for each round trip.

Ilya Grigorik’s book, “High Performance Browser Networking” (a must read! – and you have no excuse, it is free!) shows that latency can have a huge effect on network throughput:

Looking across the bars, 58ms of additional delay will add nearly a full second of load time to a webpage.

If you are traveling abroad this summer and you add international roaming to your data plan – you will notice that the data traffic is a bit slower, and this is one of the reasons why.