The most prominent competing architectures in the optical access arena are the point-to-point (P2P) optical access network and the passive optical network (PON). Which network should you choose?
July 1, 2002
Communications service providers must always have their eyes on the future. They know that virtually the only path to their continued strength and growth is to provide customers with more services (voice, video and data), better services (higher quality from an ultra-reliable plant), and, very soon, new services (i.e., video on demand, streaming digital video and interactive gaming).
Today’s service providers want certain service and technology considerations to be addressed as they decide on the best systems for their needs. If they are going to dig trenches and expend the large resources required to build new infrastructures, they must do things right the first time.
Service providers generally have four goals when seeking a new system. The system should:
1.deliver a full suite of services: voice, high-speed data, and video
2.minimize impact on customer premise equipment and wiring
3.provide a total system solution
4.cost effectively evolve to satisfy future demands
The key to meeting these challenges is an outside plant that delivers ultra-wide bandwidth to every customer at a reasonable cost. Each day, more and more carriers decide to extend optical fiber to each customer’s premise. This is commonly called Fiber-to-the-Home (FTTH), although businesses, large and small, are served as well.
The demand for huge bandwidth, coupled with advances in optics leading to greatly reduced costs, have made FTTH an attractive option for service providers to “future-proof” their networks. But in planning their state-of-the-art networks, providers find themselves with conflicting information about the best solution for their needs.
The most prominent competing architectures in the optical access arena are the point-to-point (P2P) optical access network, based on Ethernet, and the passive optical network (PON).
THE P2P AND THE PON
A P2P network is based on the switched Ethernet enterprise network architecture, in which a dedicated fiber or fiber pair extends from a central Ethernet switch to each desktop. For a residential/business service provider, each desktop is replaced by a subscriber’s premise, but the principle of a dedicated fiber to each user remains the same.
A PON allows for the integrated delivery of voice, data, and video, as well as other services, from the service provider’s central office or head-end location to the subscriber premise. A PON is defined as an optical network in which there are no active devices between a provider’s central electronics and the subscribers’ premises, and which uses optical splitting devices to manage light, reducing the feeder fiber count and sharing optical transmitter (laser) ports.
The PON shares a small number of feeder fibers (typically one to four) across a set of subscribers by use of passive optical splitters, usually located close to the subscriber’s premises. The feeder fibers can extend to a radius of between 10 and 20 kilometres. In a basic configuration, the PON splitters are placed at a fiber junction, or Local Convergence Point (LCP), splitting traffic from one fiber into four, and then four fibers into eight. An LCP therefore serves 32 subscribers.
There are a number of options that may be used, depending upon the actual “take rate” for the service provider. A typical optical access deployment consists of a central facility with content capacity for several hundred thousand subscribers. The central facility is connected to nodes, each of which aggregates traffic from 30,000 to 50,000 subscribers. The node is the point where the optical access network is terminated and is the focus for examining the difference between a P2P network and a PON.
Figure 1 shows a simplified 10-km distribution in both the PON configuration (top) and P2P configuration (bottom). The feeder length in both cases is 9.5 km long. In a PON, a single 622 mbps feeder is shared among 32 customers using a 1×32 passive splitter near the customer premise. Each customer premise has a 622 mbps optical transceiver, while the central office (CO) end of the feeder has one 622 mbps transceiver.
In the P2P configuration, the data rate is 100 mbps. Thirty-two individual feeder fibers are required, each 9.5 km long. Each customer premises has an optical transceiver, but in this case 32 transceivers are required at the CO. To meet the requirements of practical construction, P2P requires a splice in each fiber at about the 9.5-km point, connecting a single 0.5-km customer drop to each feeder fiber.
What are the cost tradeoffs that can be deduced from this example? P2P requires 31 more 9.5-km feeder fibers, at an average cost of $0.025 per foot of fiber, plus termination costs (which includes the splices). PON requires the splitters, at a cost of approximately $70 per port.
P2P uses more optical transceivers, but of lower power than the ones used in PON. The greater number of transceivers outweighs the cost saving inherent in lower power. Therefore, the transceiver cost per user is $200 for P2P and $110 for PON.
The overall system bandwidth is 3.2 Gbps for the P2P system, and 622 mbps for the PON system. This can be seen as a point in favour of the P2P, but it comes as at a considerable cost; the CO equipment for P2P must be capable of 3.2 gbps. The PON enjoys the cost saving of lower bandwidth in the CO, and this is, for the most part, imperceptible to subscribers because the PON network’s bandwidth is shareable. The P2P’s bandwidth is not shareable, and dedicates 100 MHz of bandwidth to each user.
Other, more subtle tradeoffs exist within the node and subscriber electronics. P2P Ethernet IP service-oriented with QoS at layer 3 while P2P is IP & TDM service-oriented with QoS at layer 2. Since P2P is not presented as an integrated system, separate components are required for value-added services, while the PON solution is available as an integrated system. This, in turn, means that the service provider will have to deal with separate management applications for the various components in P2P, versus an integrated EMS for the PON solution.
To many service providers the most important difference will be that the PON solution is “scalable”; it can be installed in stages, in step with customer acceptance. The feeder fiber can be laid first, with an initial build-out of splitters and customer drops, followed by additional stages of splitters and drops later, working from the same feeder and CO set.
In contrast, P2P, with a feeder for each customer, virtually demands that the entire network be laid from day one. Network design is a tug of war between the desire for the greatest capacity and the need to avoid “uncapitalized investment” — a plant in the ground that is not serving customers.
WHICH IS BETTER?
So, which system is ultimately better? As in many things in life, the answer is “it depends.” P2P is better suited to business bandwidth and densities, while PON is better suited to a residential model, which also includes many small- and medium-sized businesses. Most service providers find that their customer population is heavily weighted on the residential side. Yet, a mix of business and residential users can make very efficient use of a shared bandwidth system like a PON, since business usage peaks during the day and residential usage peaks in the evening.
Some service providers have argued that the P2P system is superior because greater capacity to the end user is possible with dedicated fiber. However, all network architectures incorporate points of concentration. In the case of the PON, it happens to be on the optical access. Aside from that point, the system is designed so that there are no concentration points in the voice or data electronics in the node. In the case of the point-to-point Ethernet, there is no concentration in optical access, but typical Ethernet equipment will concentrate forty-eight 100 mbps interfaces onto a single 1000 mbps interface. Overall, the system concentration is about the same as the PON.
However, many service providers feel that the PON solution wins on the scalability issue, and in
the fact that a complete, integrated solution does exist.
Ultimately, the bottom line is that for most service providers the choice is driven by the need to minimize risk and the ability to meet their own specific requirements.
Patrick J. Sims, RCDD, is Project Manager for Optical Solutions Inc. of Minneapolis MN, a leading developer and supplier to carriers of integrated systems for fiber to the home and the broader community.