There are two major types of fiber access network architectures: actively switched and passively split. Ethernet Switched Optical Networks (ESONs) contain an active (or powered) electronic element, a switch aggregator, between the switch or server in the central office (CO) or head-end and the network terminating device located at the premises. (Of course, that active aggregator device could reside in that Central Office or Head End adjacent to the switch for a “homerun solution.”) Outside of the CO or Head End, Passive Optical Networks (PONs) do not contain any electronics between those two points, although they do at the premise site.

The tradeoff is one additional powered element for every 48 subscribers (the number of premises served by each distribution element) versus a passive power splitter with an inherently lower failure rate but no ability to isolate faults, switch local traffic or provision narrow or uni-cast transmissions. In a PON the entire downstream bandwidth is transmitted to the power splitter, and a portion of the optical power is delivered to each subscriber. Since bandwidth in a passive system is not dedicated to each subscriber, each user shares the total capacity of the system. During times of peak usage, each user’s data rate may begin to fall – a phenomenon called “latency.” Latency is a killer for applications like video, gaming, and voice.

The terminating equipment at the premises in a PON contains additional electronic components designed to switch the optical transceivers on and off during the allocated time slots and select their content. In an active system only the content destined for a particular CPE is delivered to that subscriber. Even if a rogue premises device is installed in an active network, no content is delivered to it.

PON Systems generally require far less fiber than an ESON solution, and this is touted as the basis for a lower system cost. However, the primary cost component associated with the “fiber” is the construction associated with deploying the cable. The physical deployment of a multi-fiber cable intended for an ESON solution can be very similar to the deployment of an eight fiber cable for PON. (Trenching / Aerial deployment does not occur as a star emanating from a CO as is often depicted in logical block diagrams.)

ESONs can be initially provisioned to deliver 20 Mbps to each subscriber and later remotely upgraded to 100 Mbps. PONs must physically restrict the number of subscribers on a power splitter to achieve higher throughputs. If the total network capacity is exhausted, then the electronics at each end (CO and Premises) must be added or upgraded to a newer technology.