International Roaming
You may remember a time before Covid when you travelled internationally, overseas, to another country, or abroad. You may have noticed, if you were rich enough, that your cell phone services may have worked differently when you were roaming compared to when you were at home. This is because voice, text and data services are engineered differently.
When you were overseas and wanted to make a voice call to someone back in NZ you needed to remember to put +64 in front of the phone number you had stored in your address book. This is because voice calling is routed relative to the network you are connected to. If you happen to be in Australia then to make a phone call you dialled the same way everyone one else in Australia would. The primary reason why voice calling aligned with the country you are in is that technically this was the method that would require the least changes to each telco’s network.
However, the other key reason for this is that in the very early days of international cellular roaming the technologies and systems available for inter-charging between the visited countries cellular provider and your home service provider were not very advanced. The billing relied on the international telco sending an itemised monthly invoice to the home network provider, which is why in the late 1990’s and early 2000’s your roaming charges could turn up several months after you returned home.
SMS/Text services work differently to the routing for voice calls. Within your SIM card is programmed the network address of the SMS system that all your text messages are processed.
No matter where you are in the world your Text messages are routed to this system, known as Short Message Service Centre (SMSC). The SMSC then reads the destination of your Text and then decides how to route it to the destination. Because all Text messages are routed by your home SMSC you are still able to send and receive messages from Short Codes. Short codes are 3 or 4 digit numbers you may receive bank alerts, apple and google 2factor authentication messages and all manner of other Application To Person (A2P) messages.
Data Services work a similar way to Text messages as all data services are routed back to your home network. Unlike Voice calls and Text messages which did not really have an option for how they were routed, the standards groups spend many hours, weeks, months, years… deciding on the best way to route Data services. On the one hand there was an argument to follow the voice method to shorten the route as much as possible between the phone and the internet. From a pure engineering perspective this seemed quite logical. I and a group of others spent quite a lot of time explaining that whilst this was an efficient approach the customer impact would be considerable.
Many countries have very specific Internet filtering rules. Some of these rules include blocking access to services such as Google Search and Apples Application Store. There are also many instances where service providers such as email and VPN providers prohibit access to their servers from outside of the country they provide service. By routing all data traffic back to the home network we explained that the variation in customer experience based on the country they are visiting would be minimal and thus reducing a large number of avoidable customers complaints.
What you might not appreciate is that before you can roam onto another network your home telco and the foreign telco will have configured and tested the technical parameters and signed commercial agreements with each other.
In the early days of international roaming we used to send people overseas to test each network we had signed a commercial agreement with. Then someone from finance approached an engineer and in a moment of weakness the engineer suggested that couriering SIM cards to the other operator and the engineers testing each other’s services, rather than travelling and testing their own, would be just as effective. Idiot.