The broadband access market is heating up. Recent months have witnessed several new companies introduce active Ethernet products, while passive optical networking (PON) vendors continue to make noise about their solution to the last mile bottleneck. Verizon has famously chosen PON for its ambitious FTTH rollout. But beyond all the hype, what are the real technical differences between the various PON flavors and active Ethernet?
The rationale of using a passive optical infrastructure to deliver broadband services made clear sense in the mid 1980s when PON solutions were first introduced. Since prices for fiber-optic cable and optical transceivers were exorbitant, sharing those resources was the only practical way to deploy optical services cost-effectively. Today, the cost of fiber-optic cable and optical transceivers is a fraction of what it was, and great advances have been made to reduce the cost of installing, splicing, and terminating fiber. These cost reductions combined with the reach, scalability, and revenue generation potential of a switched, point-to-point optical solution make selecting standards-based Ethernet a very attractive option for PON.
The basic premise of all PON architectures is to share the optical feeder and a port on the central distribution unit known as the optical line terminal (OLT) among as many subscriber terminals, or optical network units (ONUs) as possible. Passive optical splitters terminate the optical feeder and provide optical connections to the ONU. This portion of a deployment is referred to as the optical distribution network (ODN). PON was created to allow the cost of the deployment to be shared between all subscribers the OLT can support. While this may have been desirable when the cost of the access infrastructure was very high, the tradeoffs and restrictions of a shared access infrastructure are not necessary today.
Two factors impede the reach of a PON deployment. The first is the total available optical power budget, which is a factor of the OLT laser port and the total loss budget, including the fiber feeder and splitters. The second is the risk of simultaneous transmission between the ONUs. Because ONUs share the optical feeder and OLT port, a sophisticated algorithm is required within all the devices to prevent more than one ONU from transmitting at the same time. If a simultaneous transmission occurs, the resulting traffic collisions would render most applications unusable.
By their nature, OLT ports are expensive because they are designed to push a broadband optical signal up to 20km or 30km with enough power to light up 32 or more ONUs. The cost of an OLT port doesn’t start to become economical until the OLT is serving approximately 26 ONUs, or approximately 81 percent of the total supported ONUs. Cost of a PON deployment can widely fluctuate because of the high price of the OLT, the large number of ONUs that are required to make the OLT deployment economical, and the limited geographic coverage of the ONU serving area.
At first, the cost to deploy the first PON subscribers spikes, mainly due to the fact that the OLT is expensive. The cost per subscriber for PON falls as the number of ONUs sharing an OLT increases. However, once the maximum number of ONUs an OLT supports is reached, a new investment in an OLT is required, and the cost per subscriber goes up dramatically. Active Ethernet cost predictions for the first subscribers are much lower than the first subscriber cost for PON. It also has a much narrower range as subscribers are added. Active Ethernet thus provides a linear and predictable cost of customer acquisition from the first few subscribers to many thousands of subscribers.
The cost per subscriber for PON is also greatly influenced by the location of the splitters in relation to the OLT. The further away the splitters are located from the OLT, the smaller the ONU serving radius becomes. This results in the best utilization of the fiber feeder plant, but requires a high density of potential customers in the ONU serving area to achieve the number of served customers necessary to make the OLT deployment economical. The nearer the splitters are located to the OLT, the larger the ONU serving radius becomes. For areas where potential customer density is lower, a larger ONU serving radius is able to address more potential customers. However, this results in the least efficient utilization of the fiber feeder plant.
While PON technologies can be deployed with lower split ratios resulting in more available bandwidth per subscriber, this effectively increases the cost-basis of the solution. For example, decreasing the split ratio of EPON from 1:32 to 1:16 increases the available bandwidth per subscriber to just over 60mbps, but more than doubles the cost-basis of the solution since the OLT port is now shared among fewer ONUs. PON vendors typically espouse the cost benefits of their systems using the maximum number of subscribers supported, but fail to disclose the resulting limited bandwidth per subscriber.
The geographic limitations of PON make it difficult to achieve the necessary number of ONUs for a cost-effective OLT deployment. Pre-planning the exact location of the splitters in relation to the OLT to maximize the number of subscribers per OLT is challenging. Many factors influence subscriber behavior and take-rates for services.
Because of these geographical limitations, PON deployments result in unpredictable customer acquisition costs that can yield significant amounts of stranded capital waiting for ONU serving areas to reach the optimum number of subscribers.
Capital expenses for active Ethernet are coincident with subscriber acquisition. This means deployed capital is immediately put to work generating revenue. And because subscribers can be added up to 120 km from the distribution or concentration site without any geographic restrictions or bandwidth implications, active Ethernet eliminates the need for pre-planning deployments.
Bandwidth per subscriber and revenue generation potential
Because all subscriber terminals share the OLT and the optical feeder, the available bandwidth per subscriber terminal is also shared. The total amount and range of bandwidth available to share depends on the specific type of PON technology and the split ratio deployed. Active Ethernet solutions support fully symmetrical 100mbps connections to each subscriber, far in excess of even the fastest PON implementations. The same active Ethernet system can also simultaneously support fully symmetrical 1gbps, or even 10 gbps subscriber connections. Only standard-based Ethernet solutions are capable of this scalability and are able to leverage the low cost of Ethernet components.
The additional bandwidth and significantly expanded coverage area of the active Ethernet systems, combined with the linear cost of customer acquisition, make selecting this technology over PON simple.
PON vendors assert that using passive splitters instead of active elements between the OLT and ONU, will realize cost savings for the network operator by eliminating the need to power and service active components, thus lowering the overall maintenance costs since fewer powered elements will require service. But it is important to note that the OLTs and ONUs, which are located in the field at the customer premises, are actually active components.
Even in the worst case, active Ethernet deployment, the total number of concentrator sites that contain active, powered components is just over 1 percent of the total number of sites. The significant benefits of a deployment far outweigh the additional maintenance costs due to active components in the field versus a similar passive deployment.
Passive deployments have only one path between the OLT and the ONU. If an OLT fails, or the fiber feeder between the OLT and the passive splitter is cut, all of the ONUs will be out of service. In addition to having mean time between failure ratings measured in years, active Ethernet systems can be deployed in any topology, including rings that can provide economical redundant paths to protect against fiber path or electronic equipment failures, thereby providing very reliable and resilient network deployments.
Active Ethernet promises to reshape the access market. Now that fiber-optic prices have eliminated the desire for passive components, more providers have the opportunity to turn to a fully standardized option – Ethernet technology – to provide connectivity in the first mile.
Barry Kantner is vice president of marketing and product management for World Wide Packets. He can be reached at barry.kantner@wwp.com.
