Roaming is the ability to use your own GSM phone number in another GSM network. If you live in Spain and travel to Sweden, for instance, you could bring your GSM phone with you to Sweden, since your home operator in Spain has a "roaming agreement" with at least one of the network operators in Sweden (actually all three). While in Sweden, not only can you make voice calls with your GSM phone, but you'll be reachable by anyone who dials your home GSM phone number. Best of all, if you have your laptop along, you have the capability for email, browsing the Internet, faxes, securely accessing your company's LAN/intranet, and other digital data features including Short Messaging Service.

A roaming agreement is a business agreement between two network operators to transfer items such as call charges and subscription information back and forth, as their subscribers roam into each others areas. When you return to Spain from Sweden, for example, your subsequent phone bills will include the calls you made while in Sweden, since this billing information will have been transferred back to your home operator.

How does this work? All GSM-enabled phones have a "smart card" inside called the Subscriber Identity Module (SIM). The SIM card is personalized to you and you alone. It identifies your account to the local network and provides authentication, which allows billing to the appropriate network.

Thanks to the efforts of organizations such as ETSI (European Telecommunications Standards Institute) and the GSM MoU Association, roaming has become very well established in Europe, and is being rapidly extended to create a global roaming infrastructure for GSM. Today, if you travel outside of western Europe, you're increasingly likely to be able to roam, as many new roaming agreements are in place.

GSM network frequencies determine whether your home cellular phone will work abroad

GSM networks presently operate in three different frequency ranges. Your phone and the SIM inside it will only work in an area which uses the same GSM frequency. These are:

• GSM 900 (also called GSM) - operates in the 900 MHz frequency range and is the most common in Europe and the world.
• GSM 1800 (also called PCN (Personal Communication Network), and DCS 1800) - operates in the 1800 MHz frequency range and is found in a rapidly-increasing number of countries including France, Germany, Switzerland, the UK, and Russia. A European Commission mandate requires European Union members to license at least one DCS 1800 operator before 1998.
• GSM 1900 (also called PCS (Personal Communication Services), PCS 1900, and DCS 1900) - the only frequency used in the United States and Canada for GSM. Note that the terms PCS is commonly used to refer to any digital cellular network operating in the1900 MHz frequency range, not just GSM.

  For specific country information, see the GSM Mou Association's Info Line.
  

Roaming to a country that uses the same frequency:
Mobile Station roaming

If you're traveling to a country or region that uses the same GSM frequency as your home network, you will be able to simply take your phone along and do MS-roaming. (The MS, or Mobile Station, is a fancy name for your cellular phone). In this situation, your phone and SIM (within the phone) should work fine.

There are differences between networks, so you'll still want to do a little research to ensure that your GSM phone and SIM (Subscriber Identity Module) card will work abroad with the same services as home, and that a roaming agreement is in place between your home network and a network abroad. The following example shows the steps one person took before departing for Sweden

1. First I went to the GSM MoU Association's Info Online web page, where I found information on roaming and data services for over 200 GSM operators worldwide.
2. From the list of countries, I selected Sweden to find out names and information about the operators there. For each of the three Swedish operators, I looked at the Roaming, Network, Service, and Coverage pages.
3. Within the Roaming pages, I was pleased to find the network I use listed, indicating that my operator in Spain had established roaming agreements with all three operators in Sweden.
4. To verify that I could use my own GSM phone with each network, I looked at each operator's Network page to verify that the network type was "GSM 900", the same as my home network frequency. I also checked the network status to verify that the network was "live" or actually operating.
5. Next, I checked the Service page for each Swedish operator to determine what data services each network supports. Again, I was pleased to see that each network supports both data and fax, in addition to SMS. This meant I would be able to use my GSM phone and laptop together to access my corporate email from abroad.
6. Lastly, I checked the Coverage page for each Swedish operator, to see if the region in Sweden I was visiting was covered by any or all of the networks ( I found that Sweden is well-covered by the networks.)

Roaming to a country that uses a different frequency:
SIM-roaming

However, if you travel to a country or region where the GSM networks operate at a different frequency from your own, you can no longer use your own phone (unless you own one of the new dual-band models).

What you CAN do if your home and travel destination frequencies are different is to transfer your SIM (Subscriber Identity Module) card to a phone of the correct frequency. This is called SIM-roaming. SIM-Roaming offers the advantage of letting you use your home phone number and being billed to your home account.

The drawback, of course, is that once you arrive in the travel destination, you must rent a phone from a local network operator so that you'll have a phone that uses the correct frequency. Then it's a simple matter of taking the SIM out of your home phone and inserting it into the rented phone (don't forget to take it back!). See the GSM Mou Association's Info Line for information by country on what network operators you can rent from.

Dual-band phones and other options

Achieving the long-term goal of global GSM roaming will most certainly create new market demands. We are already seeing the emergence of GSM 900/1800 and 900/1900 dual band phones, which will eventually eliminate the need for SIM-roaming. Also emerging will be SIM-less phone rentals at the airport, and new tariffing structures. As this happens, we will keep this site updated with the latest.