Passive Optical LAN: Determine If It's Suitable for Your LAN Infrastructure

Transcription

1 G Passive Optical LAN: Determine If It's Suitable for Your LAN Infrastructure Published: 9 February 2016 Analyst(s): Ian Keene, Mark Fabbi A passive optical LAN can be a lower-cost alternative to the traditional Cat 5/6 twisted-pair switched Ethernet architecture. This report will help CIOs and network architects in large enterprises to make an informed choice about whether to deploy, based on the technology's strengths and weaknesses. Key Challenges Change management: PoLAN is relatively new. Today's dominant physical LAN architecture is based on the 26-year-old Base-T standard and its close alignment with standard Cat 5/6 structured cabling systems. These standards are the basis of a LAN ecosystem with a large pool of installers, support staff and vendors eager to protect their $24 billion annual market. Different technology: PoLAN eliminates the need to upgrade to new, higher-priced versions of physical twisted-pair cabling as user bandwidth demands increase. However, the PoLAN architecture is different and introduces the concept of statistical multiplexing and shared network links, which LAN managers must learn to make an informed choice of network. Is it right for me? Many CIOs find it difficult to understand whether PoLAN is suitable for their LAN and can really lower costs, with claims and counterclaims by suppliers of the new and traditional architectures. Recommendations Understand your LAN requirements and then evaluate the strengths and weaknesses of available alternatives, particularly when embarking on a major LAN refresh or a "greenfield" LAN deployment. Determine if benefits in deploying a physical cable infrastructure that will not need upgrading, needs no switching closets and can offer capex savings outweigh the unfamiliarity of deploying a statistical multiplexed solution and the issues surrounding no PoE. Consider not only communication bandwidth needs, but also issues with power supply to remote units and the use of switching closets.

2 Beware of exaggerated PoLAN claims, which do not look at the real performance and cost when all of your requirements are taken into account. Choose solutions where the equipment vendor can provide cost-effective upgrades to 10 Gbps speeds and beyond. Table of Contents Introduction... 2 Analysis...3 Passive Optical LAN: Is It Suitable for Your LAN Infrastructure?... 3 Understanding the Architecture...3 Are Cable Reach Limitations an Issue?... 5 Determine Cost and Disruption of Any Future Cable Upgrades... 5 Evaluate Potential Capex and Space Savings... 6 Distributed Versus Centralized Switching Is It Advantageous for Your LAN?... 6 The Desire for Power over Ethernet... 6 Shared Bandwidth and Statistical Multiplexing A Concept That Should Be Understood... 7 Keep With Tradition, or Try Something New?... 7 The Hybrid Approach...8 Gartner Recommended Reading... 8 List of Tables Table 1. Evaluating Strengths, Weaknesses and Differences of Dominant IEEE xbase-t Solution Compared With PoLAN...5 List of Figures Figure 1. Passive Optical LAN Architecture... 4 Introduction Inquiries from Gartner clients on the subject of passive optical LANs (PoLANs) have increased, as more enterprises are assessing this alternative physical LAN architecture. The number of system integrators and telecom carriers offering the PoLAN solution has also increased, along with the number of equipment vendors providing enterprise equipment. Vendors Tellabs and Zhone Technologies have been joined by telecom equipment manufacturers Alcatel-Lucent (now Nokia) Page 2 of 10 Gartner, Inc. G

3 and Huawei. These vendors and their channels to market now have a worldwide reach to provide enterprise solutions. Interest from enterprise CIOs is fueled by the promise of lower capital expenditure (capex) and operating expenditure (opex), the elimination of switching closets, reduced power requirements, and the future-proofing of the physical infrastructure to prevent the need for upgrades to the next generation of twisted-pair cable. The traditional xbase-t architecture has multimode optical fiber riser cables from the data center leading to a local switching cabinet and from there Cat 5/6 twisted-pair cable to the end termination. PoLAN is a different concept and, as with any alternative technology, PoLAN has potential benefits and potential downsides. Analysis Passive Optical LAN: Is It Suitable for Your LAN Infrastructure? The following section describes how PoLAN works and highlights its strengths and weaknesses. This can help you make an informed choice of LAN architecture. Understanding the Architecture PoLAN has been developed from the fiber to the home (FTTH) standards set by the ITU. The ITU focuses on telecommunication carrier technologies, rather than technologies for the enterprise, which is typically covered by the IEEE. These ITU standards are the basis of fiber broadband access services and distribution products for carriers, which have high volumes (installed base of hundreds of millions of connections) and are highly price competitive. Equipment vendors, starting with Tellabs in 2007, have been attracted to the enterprise LAN market for their products because of the higher sales margins that they can realize while still providing a capex-competitive alternative solution to switched Cat 5/6 architectures. Figure 1 shows the PoLAN architecture. Gartner, Inc. G Page 3 of 10

4 Figure 1. Passive Optical LAN Architecture OLT = optical line termination ONT = optical network terminal Source: Gartner (February 2016) Essentially, PoLAN uses single-mode optical fiber throughout in place of multimode fiber in the risers and Cat 5/6 twisted-pair copper cable for the horizontal. This fiber cable has the capacity to support data rates of many hundreds of gigabits per second. In practice, the speed is limited by the performance of the electro-optical components at either end, typically 2.5 Gbps downstream and 1.2 Gbps upstream, although it is upgradable to faster speeds in some implementations. By using a fan-out design from the campus core to the end user, localized switching closets/cabinets are eliminated or reduced significantly in footprint. A single port in the campus core optical line termination equipment (aka centralized LAN switch) can use an optical splitter to serve up to 128 endpoints, although smaller ratio splits are recommended in many cases. The essential point to grasp here is that PoLAN is a shared bandwidth; it is shared among the number of end terminations supported by the optical splitter. The performance is governed by statistical multiplexing, which is a concept that hasn't been deployed in campus networks for two decades. However, the approach is similar to that of wireless access points, where bandwidth is shared among connected users. Table 1 compares the strengths, weaknesses and differences between an IEEE xbase-t solution and PoLAN. Page 4 of 10 Gartner, Inc. G

5 Table 1. Evaluating Strengths, Weaknesses and Differences of Dominant IEEE xbase-t Solution Compared With PoLAN xbase-t PoLAN 90-meter horizontal reach from the switching cabinet to the user termination. Cat twisted-pair cable upgrades may be needed for higher bandwidths. Cat 6 and above cable is expensive. Local switching cabinets are needed; space, power and access for maintenance can be issues. Distributed LAN switching architecture with features replicated on all ports. Power over Ethernet as standard. Nonblocking switch architecture at the access layer, with oversubscription in uplinks to the core network. 26-year-old physical design with plenty of technical support available. No effective distance limitation in a LAN environment. No cable upgrades or replacement of active electronics needed for higher bandwidths. Capex savings on cable and installation. Space savings. All switching located in the campus core, which can be remote from the LAN in a centralized location (up to 20 km). Features are centralized. Either power cable needs to be integrated with the fiber cable, or local power must be supplied to the end ONT terminal. Shared bandwidth that relies on statistical multiplexing. A relatively new technology that requires training and change. Source: Gartner (February 2016) Are Cable Reach Limitations an Issue? The use of Cat 5, 5e or 6 cable is limited to 90-meter horizontal cable runs. Conversely, single-mode fiber has no effective distance limitation, even at very high bandwidths. Determine if the relaxation of 90-meter cable run limits is an advantage. Determine Cost and Disruption of Any Future Cable Upgrades Enterprises have lived through at least one twisted-pair cable upgrade, maybe from Cat 3 to Cat 5, 5e and/or Cat 6. While Cat 5 e/6 can support 1 Gbps (1000 Mbps), some vendors promote the use of nonstandard Cat 6a and Cat 7 cables. Work is also underway on a new standard Nbase-T that will support 2/5/5 Gbps on Cat 5e/6 cabling; see "Don't Let the New WLAN Standard Break the Bank or Your Wired Network" for recommendations on 2.5/5 Gbps. Installation and upgrades of the physical copper cable plant are expensive; in existing locations, they are difficult to perform without significant disruption. The installation rules for installing Cat 6 and beyond cabling are restrictive and can increase capex. PoLAN single-mode cable can theoretically support terabits per second of bandwidth capacity. In practice, the bandwidth is limited by the electro-optical active equipment in the OLT centralized LAN Gartner, Inc. G Page 5 of 10

6 switch and the ONT end-user terminal. All PoLAN vendors support equipment that can deliver 2.4 Gbps downstream and 1.2 Gbps upstream. Most vendors support 10 Gbps upgrades, some support 10/2.4 Gbps, and some support 10 Gbps symmetrical. These upgrades follow the optoelectronic product developments for carrier broadband access equipment. Currently in the telco carrier market, 10 Gbps is two to four times the cost of 1 Gbps active equipment. A recent development is the use of standardized XGS-PON active equipment in carrier networks. This is a single-wavelength 10 Gbps version of a system capable of supporting four wavelengths to deliver a total of 40 Gbps line speed. Evaluate Potential Capex and Space Savings Single-mode fiber cable is smaller in diameter, easier to install, and of lower cost than Cat 6 and future twisted copper pair cable standards. The elimination of local switching closets can also save real estate, reduce power consumption and help centralize maintenance. All these features can result in lower capex costs, and in some cases, vendors claim lower opex costs too. Capex saving claims can be large; for example, Nokia (formerly Alcatel-Lucent) has models that the vendor claims offer capex savings between 34% and 56%, depending on the size of the installation. Other vendors claim various levels of savings, but pricing of both PoLAN and traditional solutions varies. Enterprises must compare PoLAN solutions to actual competitive proposals to get a better feel for possible savings, as the range of prices and discounts for traditional switched Ethernet solutions varies widely. The promise of capex and opex savings is the main reason enterprises look at PoLAN as a potential alternative physical infrastructure. However, Gartner has seen solutions proposed by system integrators in which the cost savings have been exaggerated, if the required performance is to be reached. Large numbers of ports at the optical splitter can reduce cost but may not provide sufficient bandwidth performance. The lack of copper pairs to supply DC power to the ONTs will keep costs down but may not meet the needs of the facilities management team. Work out your power supply and user bandwidth needs before comparing capex costs of PoLAN and xbase-t solutions. Distributed Versus Centralized Switching Is It Advantageous for Your LAN? In theory, local switching can keep traffic local and reduce the load on the LAN core but, because most traffic travels to the campus core switch regardless, this is not much of an advantage in practice. Operating and maintaining switching cabinets can be problematic. This has particularly been the case in the hotel industry, where PoLAN has had more traction than in most other vertical industries. The ability for PoLAN to support centralized switching in data centers up to 20 kilometers away has proved attractive for some distributed enterprises. Together with the extra security that optical fiber cable provides (it is extremely difficult to tap into a single-mode fiber cable without detection), PoLAN has become the chosen architecture for many government-run LANs. The Desire for Power over Ethernet Providing power to end terminations over Ethernet twisted-pair cables is in common use for powering devices such as WLAN access points and IP phones. Not having Power over Ethernet (PoE) is inconvenient. Obviously, optical fiber can't provide power, so either power units are needed next to the ONT or copper DC power wires need to be integrated into the PoLAN cable solution Page 6 of 10 Gartner, Inc. G

7 normally injected at the optical splitter junction. Using local power sockets and AC/DC conversion can cause issues with electrical safety (if under the floor) and likely lowers the overall reliability of the system. Network architects should consult with the facilities management team to assess power options. Integrating power wires into the optical fiber cable will increase costs. Make sure that the power supply solution can accommodate the needs of ac WLAN access points and other end devices that consume higher levels of power. Shared Bandwidth and Statistical Multiplexing A Concept That Should Be Understood Traditional switched LANs can be built to provide nonblocking bandwidth up to the capability of the twisted-pair cabling installed. However, while most solutions are nonblocking within a single wiring closet, there is typically an oversubscription rate of 10 times from the wiring closet to the building/ campus core. Oversubscription rates can vary from a low of two times to 20 times or higher. With PoLAN, while the fiber cabling does not limit bandwidth, the LAN architecture and OLT/ONT performance do. PoLAN uses optical splitters to share the OLT port bandwidth (typically 2.4 Gbps downstream and 1.2 Gbps upstream) among many ONTs. Endpoints share this bandwidth and work on the principle of statistical multiplexing/dynamic bandwidth allocation to optimize performance: The bandwidth sharing is adapted to the instantaneous traffic demands of the data streams that are transferred over each end device. It relies on not all endpoints demanding high bandwidth at any single point in time, so that peak data rates of typically 1 Gbps can be achieved. Minimum data rates, typically 100 Mbps, can also be set. While statistical multiplexing is common practice in both enterprise WANs and telco consumer broadband access networks, it is a relatively new concept for LAN architectures. In "How Cloud, Mobile and Video Will Increase Enterprise Bandwidth Needs Through 2017," Gartner concludes that the LAN capacity needed for a superior user will be circa 7 Mbps in 2017, while a standard user will require 0.5 Mbps. Therefore, using a fiber splitter with a ratio, common in PoLAN supplier proposals, should be satisfactory for most end users. However, make sure that there is enough extra fiber installed to reduce this split to a ratio of 1-to-64 or 1-to-32 as bandwidth demands increase in the future. Enterprises should assess end-user requirements in making the final decision. For some end devices, such as WLAN access points, bear in mind that they are shared among multiple users, and therefore lower-count fiber splitters will be needed for optimum performance. Take note that ac Wave 2 access points can theoretically support multiple gigabit per second bandwidths, and a direct fiber feed would be needed if the access point performance was to be maximized. However, in practice, such highbandwidth consumption is limited to specific location types and is not required in most enterprise environments; see "Don't Let the New WLAN Standard Break the Bank or Your Wired Network." Keep With Tradition, or Try Something New? The common LAN physical architecture is nearly 30 years old, with plenty of experienced support staff, installation engineers and a massive installed base of physical copper (Cat 5e/6) infrastructure. While optical fiber cable and PoLAN solutions have evolved into a cost-effective, easy to install solution, the familiarity and ubiquity of existing switch solutions are difficult to overcome. With each generation of Cat x twisted-pair cable increasing in cost and increasing restrictions on installation technique, it makes sense to consider alternatives. We find that most enterprises interested in Gartner, Inc. G Page 7 of 10

8 PoLAN solutions are going through a significant upgrade to the infrastructure or are moving into new locations with brand-new facilities. In this case, there is no copper cable plant to leverage, and enterprises can more readily evaluate alternative technologies. While experienced PoLAN personnel are not readily available, training can help with the change management and architectural requirements needed to design and support the PoLAN environment. For initial installation, make sure that the installation engineers and contractors used have a proven track record on PoLAN installation. The Hybrid Approach Some system integrators offer a hybrid architecture, in which PoLAN fiber is terminated in workgroup switches, typically 16 or 24 port switches locally powered with Cat 6 twisted pair distributed to the end terminations. While this can solve the power supply issue and reduce the number of ONTs needed, the downsides are that distributed switches will need to be maintained, eliminating a potential advantage of PoLAN, and installation costs may be higher. Other vendors offer ONTs with 12/16/24 ports of copper that more easily integrate into an existing copper plant. The ONT is deployed in the wiring closet and injects the required PoE. If taking either of these hybrid approaches, it is imperative to ensure the fiber splitter ratios are lower to take into account the split of endpoints at the workgroup switch. Acronym Key and Glossary Terms PoLAN passive optical LAN singlemode fiber OLT ONT very high bandwidth optical fiber, as opposed to the limited-bandwidth multimode fibers used in the risers of traditional LANs optical line termination; an ITU standards term to describe the central switch port, normally located in the campus core/data center. Each OLT can support multiple fiber ports. optical network terminal; an ITU standards term to describe the end termination. Usually a short Ethernet cable is used to attach the end device: WLAN access point, user PC, printer or other terminal devices. Gartner Recommended Reading Some documents may not be available as part of your current Gartner subscription. "How Cloud, Mobile and Video Will Increase Enterprise Bandwidth Needs Through 2017" "Don't Let the New WLAN Standard Break the Bank or Your Wired Network" Evidence Gartner has leveraged its coverage and analysis of both the carrier network infrastructure market and the enterprise LAN market. Briefings from and conversations with equipment vendors and Page 8 of 10 Gartner, Inc. G

9 system integrators are combined with knowledge gained from many enterprise Gartner client inquiries on this subject. Gartner, Inc. G Page 9 of 10

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