The Rev-B cards allow rings of up to 128 nodes. The Rev-B cards also incorporate a Stratum-3 oscillator for increased transmit frequency accuracy. The Rev-B cards are backward compatible and can interwork with Rev-A cards. They require an IOS software release level of 12.0(15)S or greater.This OC48/SRP-LR-SC-B= is a single-mode long-reach version.