Mobile Fundamentals · Cellular Networks · Lesson 4 of 4

Roaming & Carriers

How your phone works on someone else's network.

Someone else's towers

No carrier has towers everywhere. When you travel abroad, your phone connects to a different carrier's network in that country. This is called roaming. It can happen at home too, in areas your own carrier doesn't cover.

Roaming works because carriers sign agreements with each other. When your phone tries to join a foreign network, that visited network reads your SIM's identity, sees which home carrier issued it, and checks with them. Your home carrier confirms you're a real customer who is allowed to roam, and then you're let on. The two carriers settle the bill between them later, and your carrier charges you according to your plan.

Here's an everyday example. You land in Paris with a US phone and switch it on. Soon the status bar shows a French network name, and you may get a welcome text listing roaming prices.

Roaming also leaves a trail. Your home carrier learns which foreign network you joined and when, and the visited network keeps its own records. That can help an investigation show which country a phone was in.

Where's this SIM from?

Every SIM holds an IMSI, a number of up to 15 digits that identifies the subscriber to the network. It's a different number from your phone number. The IMSI is built in a fixed order.

• The first 3 digits are the Mobile Country Code. The US uses 310 to 316, and the UK uses 234 and 235.
• Next is the Mobile Network Code, 2 or 3 digits that identify the carrier within that country.
• The rest identifies the individual subscriber.

When your phone tries to join a network, the network reads the start of the IMSI to work out which carrier issued the SIM and where to send the request to check you. That's how a network abroad knows who your home carrier is.

The same codes help in investigations. An IMSI starting with 234 tells an analyst the SIM was issued in the UK, and the network code narrows it to a carrier. The ITU assigns the country codes, each country assigns its own network codes, and public lists match codes to carriers.