楚雄：
這里有詳盡的解釋:Theringerinthetelephonepresentsaloadthatisanimpedancemadeupofaresistivepartandacapacitivepart.ThisimpedanceiscalledtheRingerEquivalenceNumberorREN. IntheUS,1REN=6930ohmsinserieswith8uF.Theresistivepartisfromthecoilintheringerandthecapacitivepartisfromtheblockingcapacitor.Iftwo1RENphonesarebothconnectedtothesameline,togethertheypresentaloadof2RENwhichis6930ohms/2and8uF*2or2REN=3465ohms+16uF. InEurope,1RENisdefineddifferentlyas1800ohms+1uF.Aswell,theringingfrequencyis25Hzinsteadof20Hz.USBellcorespecscallforthecentraloffice(CO)linecardtobeabletodrive5REN(=1386ohms+40uF)toatleast40Vrmsforringing.