There are several topologies suitable for the access network: tree, ring, or bus. A PON can also be deployed in redundant configuration as double ring or doubletree; or redundancy may be added only to a part of the PON, say the trunk of the tree. For the rest of this article, we will focus our attention on the tree topology; however, most of the conclusions made are equally relevant to other topologies
All transmissions in a PON are performed between Optical Line Terminal (OLT) and Optical Network Units (ONU). Therefore, in the downstream direction (from OLT to ONUs), a PON is a point-to-multipoint network, and in the upstream direction it is a multipoint-to-point network. The OLT resides in the local exchange (central office), connecting the optical access network to an IP, ATM, or SONET backbone. The ONU is located either at the curb (FTTC solution), or at the end-user location (FTTH, FTTB solutions), and provides broadband voice, data, and video services. In the downstream direction, a PON is a P2MP network, and in the upstream direction it is a MP2P network.
The advantages of using PONs in subscriber access networks are numerous.
1. PONs allow for long reach between central offices and customer premises, operating at distances over 20km.
2. PONs minimizes fiber deployment in both the local exchange office and local loop.
3. PONs provides higher bandwidth due to deeper fiber penetration, offering gigabit per second solutions.
4. Operating in the downstream as a broadcast network, PONs allow for video broadcasting as either IP video or analog video using a separate wavelength overlay.
5. PONs eliminate the necessity to install active multiplexer at splitting locations thus relieving network operators
6. Being optically transparent end to end PONs allow upgrades to higher bit rates or additional wavelengths.
One possible way of separating the channels is to use wavelength division multiplexing (WDM) in which each ONU operates at a different wavelength. While a simple solution, it remains cost prohibitive for an access network. A WDM solution would require either tunable receiver or a receiver array at the OLT to receive multiple channels. An even more serious problem for network operators would be wavelength-specific ONU inventory instead of having just one type of ONU, there would be multiple types of ONUs based on their laser wavelength .It would also be more problematic for an unqualified user to replace a defective ONU. Using tunable lasers in ONUs is too expensive at the current state of technology. For these reasons a WDM PON network is not an attractive solution in today's environment.