For the distance relays it is assumed that the failure rate of the relays will increase with 0.5% every year (so 3% à 3.5% à 4%, etc.). It will also become more difficult to repair the relays. Therefore it is assumed that the percentage of the relays that can be repaired will decrease with 5% every year (so 95% à 90% à 85%, etc.). With these assumptions it will take about 30 to 40 years before the complete population will have been replaced.