This chart (PDF link) on the reliability of the Nortel BCM just isn’t true.  It lists a MTBF of 43.1 years with a an average chance of failure per year of 2.3%.  We’ve been using the Nortel BCM 400 for four and half years and we are now undergoing our second replacement.  Therefore I’d put the mean time between failures at a (generous) three years and an average chance of failure per year of 33%.  Plan for your situation accordingly.