Choose the Right Topology for Your Fiber-to-the-Home Network

Transcription

1 Research Brief Choose the Right Topology for Your Fiber-to-the-Home Network Abstract: Service providers' network architecture choices affect their capital costs, operating expenditure and revenue. Passive optical networks may be cheaper to install, but point-to-point systems are a better long-term option. By Peter Kjeldsen and Jouni Forsman Recommendations Service providers need to analyze carefully the long-term business implications when choosing between passive optical network (PON) and point-to-point (PtP) fiber-to-the-home (FTTH) solutions. Service providers should distinguish between FTTH equipment technologies and FTTH network topologies and consider them as two separate but mutually dependent investments. In terms of equipment, both PON and PtP solutions have their merits, and the question of which is superior is expected to be a "gray" area for at least five years. In terms of network topology, PtP architectures have significant advantages. They are more flexible and scalable, and therefore have economic lifetimes in excess of 20 years. Service providers that deploy PON networks in point-to-multipoint topologies to save initial capital expenditure must be aware of the limited flexibility and scalability associated with such topologies. They could reduce the economic life of the fiber part of such investments. If governments and regulators want to stimulate a move toward "open" networks they should regulate in favor of PtP FTTH architectures. But they should be neutral on equipment technologies. Publication Date: 20 November 2003

2 2 Choose the Right Topology for Your Fiber-to-the-Home Network Introduction There is little doubt among service providers that FTTH based on single-mode fibers offers the best fixed broadband access solution, measured in terms of bandwidth "agility." FTTH is starting to gain momentum, despite its high initial costs. But not all FTTH networks are created equal. This document sets out the principal differences between PON and PtP solutions, and compares them in terms of equipment technology and network topology. The solutions are analyzed from the perspective of service providers' business needs. The section titled "Government and Regulatory Aspects" addresses telecommunications policymakers. Fiber-to-the-Home Solutions Passive Optical Network Systems In networks based on PON technology, users are connected to a fiber "tree." This consists of optical fibers and passive optical splitters that together form a point-tomultipoint medium for broadband services. The medium is shared among all users that are connected to the fiber tree. Figure 1 illustrates such a network. Figure 1 Example of Passive Optical Network Architecture Passive splitters 1:8 Passive optical network CPE Passive optical network headend 1:4 1:8 User premises Carrier point of presence 1:8 1: CPE =customer premises equipment Source: Gartner Dataquest (November 2003) PON systems have two distinctive features: Passive splitters distribute downstream traffic and aggregate upstream traffic so that fewer fibers are required in the aggregated parts of the network (compared with PtP architecture). All users share the same headend equipment, so the total port count in a network is reduced (compared with PtP architecture). PON standards can be divided into those determined by the Full Service Access Network group and those promoted by the Ethernet in the First Mile Alliance.

3 3 The Full Service Access Network organization has initiated two sets of PON specifications that have been adopted as standards by the International Telecommunication Union (ITU). The first set defines a PON system based on asynchronous transfer mode (ATM), with capacity for 622 Mbps downstream and 155 Mbps upstream. This was adopted by the ITU as recommendation G.983.x in Systems based on this standard are referred to as ATM PONs (APONs) or broadband PONs (BPONs). The Full Service Access Network organization is working on a new set of specifications for Gigabit PONs (GPONs). The ITU has already published some of these as recommendation G.984.x. GPONs will have capacity for up to 2.4 Gbps in both directions. A common characteristic of these standards is the multiservice nature of PON systems based on them. The Ethernet in the First Mile Alliance promotes PON solutions centered on Ethernet standards. The ah task force of the Institute of Electrical and Electronics Engineers (IEEE) is defining standards for EPON solutions. As well as bidirectional data channels, most PON systems include a TV broadcast channel. Service providers can exploit this aspect of the technology to target the lucrative cable TV market. Point-to-Point Systems In PtP systems, each user has a dedicated fiber running from the home to the carrier's point of presence (POP). This is arguably the simplest type of FTTH architecture, because PtP equipment does not have to deal with the complexities associated with multiple users on the same communication medium. PtP equipment is essentially a fiber-based modem system. Equipment on the customer's premises communicates with a line card in aggregation equipment located at the carrier POP. While it is possible to implement PtP equipment as a simple fiber modem, it can also include advanced features such as TV broadcasts. In principle, anything that canbedonewithaponsystemcanalsobeimplementedinaptpsolution in some cases making PtP equipment look very similar to PON equipment. It is possible to use PON equipment in a PtP infrastructure by placing passive splitters at the carrier POP. It is not possible to use PtP equipment over a fiber tree intended for PON equipment. PtP solutions can be based on established standards like ITU-T recommendation G.985, as well as Fast Ethernet and Gigabit Ethernet from the IEEE ah task force. AnexampleofaPtPFTTHnetworkwith32usersisshowninFigure2.Inthis example, a 32-fiber cable runs from the carrier POP to a branching point, where it is split into four eight-fiber cables for distribution. These cables eventually split into single-fiber cables that run into individual homes. Although the cables may resemble a tree structure, individual fibers are entirely separate, in contrast to the point-to-multipoint topology usually used in PON systems. The fiber count in PtP may look expensive compared with the relatively few fibers in a PON network, but digging up the road is the main expense when deploying FTTH. Unless part of the infrastructure is based on leased dark fiber, the fiber count is not a major problem in terms of cost. Like PON systems, PtP FTTH networks are also passive networks so the PON term is somewhat misleading, as there are other types of passive optical network than the ones covered by the PON term.

4 4 Choose the Right Topology for Your Fiber-to-the-Home Network Figure 2 Example of Point-to-Point Architecture 32-fiber cable Eight-fiber cable aggregation CPE Carrier point of presence User premises CPE =customer premises equipment Source: Gartner Dataquest (November 2003) Evaluation of Equipment Technologies Performance PON systems offer attractive capacities when compared with nonfiber technologies. But the performance of a PON system is limited by its point-to-multipoint topology. In theory, a PON deployment could run out of bandwidth. PON bandwidth is a shared resource, which makes PON solutions more vulnerable to unpredictable bandwidth demands than PtP solutions. PtP solutions can deliver gigabits or even a terabit of data per second to each user. For practical purposes, they have infinite bandwidth potential in access networks. Capital Expenditure Service providers should consider that: PON equipment has the advantage of having established standards. This should add economies of scale to the supply chain relatively early on. PtP equipment is technically simpler and could ultimately become cheaper. A PON network has fewer ports than a corresponding PtP network. But the use of passive splitters adds to the network's budget for power loss and increases the power requirement of each transmitter in the network. Neither system has a clear advantage in terms of capital expenditure, and comparisons should be made on a case-by-case basis.

5 5 Operational Costs Service providers should consider that: The lower port count of PON systems can give them an advantage. Full Service Access Network and ITU PON systems have standardized operations and maintenance features. Bandwidth is a shared resource in PON systems. This makes the operation and troubleshooting of these networks and the introduction of new services more complex. A single headend failure affects all users in a PON network. In this area, too, neither system has a clear advantage, and comparisons should be made on a case-by-case basis. Evaluation of Network Topologies Supported Business Models If a service provider deploys a PON solution over a point-to-multipoint topology, other types of equipment will not work on the same infrastructure. This means that if other service providers want to gain access to customers served by this network, they will need to buy the capacity from the service provider that owns the network. Alternatively, if a service provider deploys either PON or PtP equipment over a PtP network topology, other operators can install other types of equipment over the samefiberinfrastructure(seefigure3).ponequipmentcanbeinstalledoveraptp network topology, but PtP equipment cannot be used over a point-to-multipoint topology. From a business model perspective, a PtP network topology clearly offers much more flexibility than a point-to-multipoint one. Figure 3 Example of Passive Optical Network and Point-to-Point Equipment Coexisting in a Point-to- Point Network Topology CPE Carrier A backbone aggregation CPE Carrier B backbone Passive optical Passive network optical headend network Multicarrier point of presence CPE =customer premises equipment Source: Gartner Dataquest (November 2003) Passive optical network CPE Passive optical network CPE User premises

6 6 Choose the Right Topology for Your Fiber-to-the-Home Network Performance A PtP network topology can be scaled up to terabit-per-second capacities per user, and individual users can be upgraded without affecting others on the same network. A point-to-multipoint network topology does not offer such unlimited bandwidth, and all users need to be upgraded simultaneously, which adds complexity to capacity upgrade processes. PtP network topology offers superior performance, but even a point-to-multipoint network offers a significant improvement over nonfiber technologies. Capital Expenditure The higher fiber count makes a PtP network generally more expensive than a pointto-multipoint one. But the cost of fiber cable is usually dwarfed by the digging costs associated with FTTH. If the entire fiber infrastructure needs to be established from scratch, digging costs should be identical for the two topologies. If existing fibers (owned or rented) are already in place for some parts of the network, there could be differences in the digging costs. Either way, it is important to include both fiber and digging costs when comparing capital expenditures for the two topologies. PtP architectures have significant advantages in terms of flexibility and scalability. A PtP topology extends the useful life of the fiber part of FTTH deployments to 20 years or more. Service providers that deploy PON networks in point-to-multipoint topologies to cut initial costs should be aware of the limited flexibility and scalability of such topologies. They risk shortening the economic life of the fiber in their investments. The longer economic life and the greater flexibility offered by a PtP topology can result in such networks having lower monthly costs than a point-to-multipoint network, although they need a higher initial investment of capital. Operational Costs The operational costs of both network topologies should be comparable, because both are passive network solutions. However, the greater number of ports and fibers in a PtP topology will probably make it slightly more expensive. Traditional telecommunications operators are used to PtP topologies in their copper-wire operations, whereas cable operators are more familiar with running shared network topologies. Government and Regulatory Aspects Both PON and PtP FTTH networks represent a major leap forward, but PtP systems scale to higher bandwidths and can support multiple networks over the same infrastructure. Governments and regulators should favor PtP topologies over pointto-multipoint topologies if they want different operators to share the cost and benefits of a fiber infrastructure. Governments and regulators must realize they have an opportunity to enact regulations before major investments are made.

7 7 Dataquest Perspective Service providers must realize that their choice of FTTH network architecture will affect their business opportunities in the long term. It will have an impact on their choice of business models and the performance and cost of their network. FTTH investments should be divided into two separate, but interdependent, parts equipment and network topology. By choosing a PtP network topology, service providers can separate fiber cable infrastructure considerations from equipment considerations, while ensuring virtually infinite bandwidth scalability. Service providers that choose the inferior, but less expensive, point-to-multipoint network topology must pay close attention to the limited flexibility and scalability associated with this topology and the implied risk of a reduced economic lifetime for their investments. In terms of equipment, both PON and PtP equipment have their merits. Choices must therefore be made on a case-by-case basis, with close attention paid to the services that the network is supposed to carry. Governments and regulators should favor PtP networks if they want to create openaccess networks, but should be neutral about the type of equipment that service providers choose to deploy over this type of infrastructure. Service providers' network and equipment decisions will depend on the services they intend to provide over the network. It is conceivable that they will deploy hybrid PtP and point-to-multipoint topologies, or in some cases use a combination of FTTH and other technologies, such as Very High-Speed Digital Subscriber Line (VDSL). In areas with little competition or low subscriber density, a PtP network may be unnecessary for a very long time. But in highly competitive urban areas, the higher bandwidth capability of a PtP topology may be needed in a much shorter time. Key Issue How will developments in access technology affect network evolution?

8 8 Choose the Right Topology for Your Fiber-to-the-Home Network This document has been published to the following Marketplace codes: TELC-WW-DP-0628 For More Information... In North America and Latin America: In Europe, the Middle East and Africa: In Asia/Pacific: In Japan: Worldwide via gartner.com: Gartner, Inc. and/or its Affiliates. All Rights Reserved. Reproduction of this publication in any form without prior written permission is forbidden. The information contained herein has been obtained from sources believed to be reliable. Gartner disclaims all warranties as to the accuracy, completeness or adequacy of such information. Gartner shall have no liability for errors, omissions or inadequacies in the information contained herein or for interpretations thereof. The reader assumes sole responsibility for the selection of these materials to achieve its intended results. The opinions expressed herein are subject to change without notice