SIM (Subscriber Identity Module) cards are often a crucial part of successfully running a custom cellular network. While it is possible to allow “public” SIM cards (i.e. T-Mobile, Vodafone) to connect to a network the cards will by default try to connect to their home network first.
Each SIM card comes with a configured IMSI (International Mobile Subscriber Identity) which begins with a tuple of MCC (Mobile Country Code) and MNC (Mobile Network Code). Based on this tuple a phone is able to identify its home network. For T-Mobile Germany this would be 262 01 and Vodafone 262 02. Using custom SIM cards ensures that a) your network is the devices primary network and b) You don’t have to spoof a valid network and thus prevents potential trouble for both you and the network operator
The cards I work with come from Chinese company called Green Card (Shenzen) Co., Ltd. and can be found via Alibaba.com. The USIMs I have cost around $1 each (highly depending on the amount you want) and simply do the job.
I tend to work with the “triple-SIMs” which can be split into mini and nano format without needing to cut them.
When ordering the SIMs at Green Card you will be provided with a small Windows tool for programming the cards. Hardware-wise all you need is a chip card reader. My choice here is the SCR3310 (there will be many more readers doing the job).
- The “Same with LTE” button will copy the contents from the LTE/WCDMA side to the GSM side, the “Same with GSM” will copy the GSM settings to the LTE side.
- The batch programming function is slightly buggy. Make sure to read the cards prior trusting that they work
- The PLMN field contains a list of home networks (MCC/MNC)
- The FPLMN field is there for “forbidden” PLMN, as such you can place public networks here to make sure your devices won’t try to connect there.
- The SMSP field is the SMS Center. Some devices insist on this being set, others ignore it
- The MSISDN is basically there for a phone number connected to the SIM. This setting is only visible for the client device and not relevant for the network
- The SPN is the network name string you will be able to see on your device when it’s connected to the home network.