PDF of this tutorialA fiber-based system that uses a counter-rotating ring to distribute bandwidth around the ring is depicted in Figure 7. In this case, bandwidth is accessed at optical network units (ONUs) and ranges from POTS to tier-1 (T1) service. Because it uses a counter-rotating ring, only two fibers are required per ring (four fibers required, if both rings are in the same sheath). This minimizes the amount of outside-plant fiber.
Figure 7. Characteristics of a Fiber System for the Rural FSN
At the CO, the switch interface, the time-slot assigner, the multiplexing function, and the interface to fiber are all accomplished in a single terminal. On the field side, the fiber optics is converted to line cards and interfaced to drops in a single terminal. Because there is a minimum of conversions and a minimum of elements, this is a very simple network to troubleshoot. Rings are also naturally very easy to troubleshoot and to maintain. As bandwidth is everywhere on the ring, ONUs may be added only when and where required, allowing deployment of assets to match closely their associated revenues.
Because the signal is repeated at each ONU, the possible range with this architecture is enormous and makes it ideal for rural routes. Up to 30 ONUs can be distributed without exceeding delay and jitter requirements of the network. One of the best attributes of a ring configuration is that it allows route diversity and therefore is immune to cable cuts. Most importantly, this sort of system brings fiber further into the loop, positioning the network for future telecommunications systems.
The fiber system must accommodate the existing copper network. Besides the need to serve the subscribers' copper drops from the ONU, the fiber system must either allow copper extensions to remote terminals or copper ONUs, both from the CO and from ONUs. In the latter case, the fiber system replaces the need for repeated copper associated with T1 signals. A copper-fed ONU does not have enough bandwidth for broadcast video. Also, a copper network is not always available, so the system must be able to interface to microwave or radio systems as well.
