Cost of ownership – the SIM

Cost of ownership may not seem that obvious in something like airtime. A SIM is a SIM and one network very much like another.  So, the idea that it would be worth paying more for such a commodity product seems counter-intuitive.

But the networks do behave very differently. The connections themselves have different rules applied to them and even the physical SIMs themselves have different properties which can impact their Mean Time Between Failures (MTBF).

If we take that last point first. At Mobius we have been publishing the physical spec of the SIM that we have been supplying for nearly 6 years. The key parameters are the temperature range of the SIM, the cache size for network messages and the read write cycles of the on-board memory.

Temperature range may not be seem applicable. Obviously if it is extremely cold; for example, a payment terminal in a ski resort, or a tracking unit in a snow blower it will have a direct impact. The plastic substrate of the SIM will twist itself into a butterfly shape with such force that will destroy everything around it. The heat of an oil pipe monitoring system on the other hand hastens the aging of the electronic circuit shortening its useful life.

Most applications, even M2M applications will be in relatively benign environments if only for the sake of all the other electronics around the SIM. However, temperature range still plays a part because the smaller the temperature range of the SIM the more sensitive that SIM will be to normal temperature cycles, the faster it will age and the quicker it will need to be replaced.

Cache size is really just the onboard memory. If there are changes in the network some networks need to keep the SIM up to date with how to connect, what permissions it has, perhaps when access it is allowed. Sometimes this update is in the form of a text which can cause its own problems if your designer is unaware this can happen. These messages are then stored and since it doesn’t figure in most people’s thinking there is no mechanism for clearing the memory. Once the cache is full it can no longer update itself and the SIM is rejected when it attempts to connect to the network. It then needs to be replaced.

Lastly the read/write cycles of the on-board Electronically Erasable Programmable Read Only Memory (EEPROM) usually (and erroneously) called Flash. This kind of memory simply has a finite span. Each time your device connects essentially that SIM loses a life. Cheap SIMs can have a 30k read/write cycle. The ones that Mobius supply are around half a million.

None of these are really issues that impact on voice. If the SIM in your phone gets tired cold or forgetful the Operator will send you a new one out, next day, for free. Problem solved.

It’s when that unit is on a gantry above the M1 and this is the third time in two years that you have had to spend £5000 to swap it out that the quality of the SIM provided has to form part of your thinking, not about the ticket price but about the cost of ownership.

Written by

Douglas Gilmour

Douglas formed Mobius in 2003 after twenty years’ experience in the Semiconductor industry. He was driven by the idea that airtime could be better and more secure.