Several topologies are being used for the optical fiber cable network that connects CO to CP. These topologies are: Star, Ring (or Bus), and Tree topologies.
Star topologies are having separate connection for each subscriber. In the ring topologies the subscribers are sharing the single optical line. Tree type connections are have individual connections from CP to Aggregation Nodes, which share connection to the CO. There are Passive and Active types of networks of Tree and Ring topology. In Passive Optical Network, passive equipment is being used that does not require any power. Multiplexing is being deployed on the physical layer through passive optical combiners/splitters and optical filters. Active Optical Network is being powered and is multiplexing on high layers (Ethernet or IP). In Tree topology, Active Network is being deployed with regeneration of a shared TDM link. While it creates a risk of ONUs malfunctioning, it can be avoided in Passive network by using passive Optical ADD/Drop Multiplexers. Note that it requires optics deployment that might be expensive.
If comparing P2P to PON, it is clear that P2P requires much more fiber, especially compared to Ring PONs. While deploying fiber solution in the field, increasing labor costs and decreasing fiber prices are making installation costs exceed the material costs.
Tree topologies network design must note the costs of additional multiplexer/demultiplexer or power splitter and their installation costs.
In the Ring topology there is a risk of cable breaks or CP equipment malfunctioning leading to service interruptions, due the lack of redundancy. On the other hand, topology is minimizing the costs of fiber cable. Deployment of passive Optical Add/Drop Multiplexers or protected bidirectional CWDM rings might solve some of the issues.