The Tale of Two SIMs

Like all large events, Beyond the City resembled a swan: the smooth appearance on the surface concealing frenetic activity beneath dealing with the inevitable challenges that come with so many delegates and exhibitors converging in a small space. Usually these involve finding enough power strips, or wine glasses, but Beyond the City had more unusual challenges. One of these was when both the BBC and Strathclyde University turned up wanting to use the same PLMN network code, requiring some last minute on-site SIM card programming, ePC and eNB reconfiguration to allow both our demonstrations to work.

All the user equipment in 5G RuralFirst requires a SIM card. While test SIMs can be obtained from the base station manufacturers, they are very expensive and can’t be customised – not an option for a project with hundreds of devices. Greig from University of Strathclyde, who had previously run a consultancy developing mobile firmware, was tasked with solving the problem. The first part was obtaining a supply of writable SIMs since these are normally sold in bulk to network operators.

Customers think of SIM cards as a means to hold their telephone number but the fact is, due to number portability and call forwarding, the telephone number may not be stored on the SIM at all. What is stored on the SIM is the IMSI, which identifies the mobile to the network, along with dozens of other parameters. All of these elements have to be correct for the mobile to work and connect to the 5G RuralFirst network. Some of the most important credentials are those related to security and the list of forbidden networks, which stops 5G RuralFirst devices trying to connect to public networks. Although we are designing a 5G network, SIMs also need parameters from 2G, 3G and 4G networks to work correctly, which makes writing them something of a dark art. The process is not helped by rather arcane software that expects you to know your OP from your OPc (and which it calls OPC!)

As part of the process, University of Strathclyde had to register with Ofcom to obtain a PLMN mobile network code making it the only UK university to hold one! In fact, we have three, so we can run multiple networks in the same location and test roaming and handover between networks, and demonstrate neutral hosting.

Over the course of the project, Greig has become Dr SIM, responsible for providing SIMs for all the different project partners who require them.

And notwithstanding two partners turning up with the same network code, the swan swam serenely on…

April 11, 2019 in 5G Access Technology