Ten 9s Availability Internet
University College London, Computer Science
The PSTN operators like to talk about “five 9s availability” - in practise, at any given day, 12000 US citizens cannot use the phone. In practice, a telephone exchange is defined as broken when ᡪ% of its lines are unusable: given this, the MTBF for an exchange is about 100 years. SJ4 is contracted to provide 99.75% availability per institute, with an MTTR of 20 minutes.