With the increasing demands of new broad-band services, the construction of optical acess networks becomes one of key issues. Passive optical network (PON) technologies realize effective and economical optical access networks for these purposes. A novel protection switching scheme for PONs with ring plus tree topology is presented in this paper, which matches the fiber construction layout of access network and ensures high network surviability at low cost. The operating principles as well as protection swicthing methods that under certain circumstances are discussed in details. Reliable and
economical optical access networks, PONs, can be applied for broad-band applications with this protection switching scheme.