About the effect of the applet on the iPhone in the LTE network

Forty years ago, when cell phones were an expensive wonder, the range of devices was very small. Today, one cannot even approximately estimate the diversity of devices operating over the radio channel. A large number of manufacturers, many models and the constant release of new and new devices from time to time lead to incorrect functioning of phones in the networks of different operators. And the latter have to make a lot of effort to test the work of various models in their frequency ranges.

Nevertheless, it is always impossible to foresee everything. Many operators often face the features of the work of certain devices on networks not only at the start-up stage, but also during operation, and Yota is no exception. Some time after the company's transition to the status of a federal mobile operator, we noticed the instability of the iPhone 6 and 6 Plus in the LTE network: a small proportion of the owners of these devices faced the fact that from time to time the smartphone lost the network and for a long time could not connect again. About how to fix this problem, we want to talk today.

What is an applet and what is it for?

Since Yota is a virtual mobile operator, or in a professional language, Full MVNO, we use MNO roaming contracts (in our case, MegaFon) worldwide. That is, in all countries of the roaming presence of MNO, Yota subscribers have the opportunity to use a full service.
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In addition to the obvious advantages, such a scheme of work entails following certain rules for using parameters of a company's MNO connection to networks of roaming partners. To implement this mechanism, on Yota SIM cards an applet is used that automatically switches the IMSI identifier from “home” to “roaming” if the subscriber enters the MegaFon roaming partner network and back when returning to the home network. Thus, for a third-party network, the Yota user at the hardware level is represented as a MegaFon user with all the attendant roaming conditions.

The same mechanism works tourist SIM-cards, which are sold at airports and train stations in many countries. They allow you to significantly save on calls and SMS while roaming, but, unlike Yota, it’s not very convenient to use them: you must first call a special number, you are called back, and then the connection with the subscriber you request is established.

IPhone Difficulty

Any mobile phone periodically sends various messages to the SIM card. Different models have their own characteristic properties, they can react differently to certain standard events. The peculiarity of the work of our applet on the iPhone was that the applet interpreted the initialization command of the phone as if the device was rebooting, and in response sent a command to search for the network. After finding the network, the iPhone again gave the initialization command, and again it received a command to search for the network. It turned out a vicious circle. After clarifying the reasons, protection against this event was made to the applet, as well as adjustments were made on monitoring in the home network and in roaming. By the way, the same problem with network search and initialization team was at one time on one of the LG Nexus models, but it was reproduced somewhat differently than on the iPhone.

It would seem that you can quickly update the applets - and the question is removed. But there is one caveat: the changes will affect not only iPhone owners, but in general all Yota customers. So, by changing the procedure for devices of one manufacturer, it is possible to disrupt the operation of phones from other companies. Therefore, before sending a new applet centrally, a procedure for testing it was conducted. To this end, a team of Yota specialists visited several countries with the necessary equipment, which, if necessary, could immediately detect possible problems with the performance of the new version. Different scenarios were checked. It's one thing when you fly a plane, the phone does not find the network for a long time, and then suddenly it turns out to be in roaming. And when, for example, you cross the border between your home network and roaming by car, there may be difficulties that do not occur in the case of air travel. All these scenarios had to be checked on the phones of different manufacturers.

At the same time, testing was carried out not only in other countries, but also in our network, and in the networks of other Russian operators, and as a result lasted about two months. When problem situations were discovered, it was necessary to reproduce them, eliminate the causes, test the new version of the applet, etc. In total, about 15 different models of smartphones participated in the testing, including those working under different operating systems. After the successful completion of the testing phase, before the New Year holidays, a centralized distribution of a new applet was launched, which did not require any additional actions from the users themselves, and this completely solved the problem .

This situation once again shows how much effort the mobile operators are making to support the smooth operation of various devices in their networks. It would seem that minor changes to the applet code are nothing complicated. But even small changes in one or another component require long and complex testing, including abroad. This is the only way to ensure high speed and stability of all services.