Access Makes the Parts Grow StrongerAndrew Woodfin Corning Optical Fiber, Corning Incorporated, One Riverfront Plaza, Corning, NY 14831
[email protected] Fiber-to-the- home (FTTH) access networks are obviously off and running. With more and more deployments in the US and around the world, we learn lessons on an almost daily basis about what is working well and where problems are popping up. By sharing their experiences, both good and bad, network operators enable vendors to evolve products to improve system performance, increase design flexibility, and reduce network cost. There is room for improvement everywhere in the network, ranging from installation techniques, passive hardware, optoelectronics, to the optical fiber itself. This paper will touch on a selection of these opportunities. Due to its significance in the total value of a comprehensive triple-play broadband service, implications of delivering video services over FTTH are discussed, and associated infrastructure advancements to improve design flexibility are presented. Additionally, dramatic improvements in network installation allowed by a number of advancements in passive outside plant connectivity are reviewed. Radio Frequency (RF) Video Overlay The beauty of FTTH is the inherent capacity and flexibility to support a wide variety of services. This is most evident in the context of video offerings. While copper-based lowbandwidth access technologies such as the various VDSL and ADSL variants are fundamentally limited to baseband switched digital video delivered over IP protocol (IPTV), and hybrid fiber coax (HFC) architectures are best suited to RF video transmission, FTTH can support either technology. This flexibility is a significant competitive advantage, and some wisely note that the unique ability to elegantly combine the two technologies puts FTTH in a class by itself[1,2]. Both technologies have their advantages and disadvantages. A recent poll conducted by USTA indicates that network operators planning video services over FTTH are split nearly 50/50 in their choice to deploy either RF or IPTV. Clearly, RF is the more mature technology, as it has been (and continues to be) employed in Cable TV networks for nearly 60 years. Still, there are some key infrastructure limitations which can pose challenges to rolling out RF video services over FTTH. Video Delivery Architecture Options The RF video overlay in a BPON architecture can be implemented in a variety of different configurations. This fact is frequently overlooked in simplistic analyses of the limitations and requirements placed on video transport and distribution in FTTH. Three common implementations are illustrated in Figure 1. There are several commonalities between all three cases. Each complies with the requirements prescribed in the relevant ITU-T BPON and GPON recommendations (G.983.3 and G.984.1) for the “Enhancement Band” of wavelengths, supporting video service transmission between 1550 nm and 1560 nm, although specific requirements and limitations on video transmission truly are given and is thus typically the most common configuration addressed in basic overviews of FTTH video service. due to the relatively high received power requirements at the ONT (typically -6 to -3 dBm) coupled with the high passive losses seen in the PON Optical Distribution Network (ODN) between the Optical Line Terminal (OLT) and subscriber premises. and optical erbium doped fiber amplifiers (EDFAs) are housed in the same location as the OLT. aggregated video content may be transmitted digitally from a remote location and converted to RF at the collocated HE.mandated maximum distance of 20 km. (2) collocated HE/CO with remote OLT . Optical amplifiers and video/data WDM are not shown. cabled fiber. such as RF modulators. In this arrangement. Therefore.located Optical Network Terminal (ONT). and can be much longer than the PON ODN (orange bar). However. the RF video signal is only transported over the PON ODN. Here. large-diameter satellite dishes. optical splitters. Headend OLT Central Office 1xN ONT (1) ODN Video path OLT 1xN ONT (2) ODN Video path OLT 1xN ONT (3) ODN Video path Figure 1: Three configurations for RF video overlay transport and distribution (1) collocated HE/CO with collocated OLT and video Tx. and (sometimes) direct optical feeds from local broadcasters. each case requires that RF video signals are amplified in the optical domain. As a result. considering the impact of connectors. it is not necessarily the most common configuration. broadcast video signals are received with a combination of aerial off-air antennae. optical video transmitters. Configuration 1 in Figure 1 is easily the simplest to understand and most trivial to analyze. and (3) regional HE with RF transport. covering a standards. Alternately. along with allocated margins in the loss budget for aging and repairs. All terrestrial signal generation equipme nt. Optical losses encountered in this portion of the network are still quite significant. However. amplified optical launch . Length of the RF video signal path (blue bar) clearly varies depending on configuration. the terrestrial video signal (regardless of signal format) originates from the same location as the PON OLT and primary voice/data network functions from a collocated video headend (HE). there are also significant differences in the three cases. Additionally. splices.only a cursory review in the recommendations. Common throughout each of the configurations is the potential limitation of stimulated Brillouin scattering (SBS). Video signals in this case will typically see the longest transmission distances of all the configurations. This design somewhat resembles modern consolidated Cable TV networks. ODN characteristics such as loss budget may be similar to configuration 1. the total distance covered by the optical RF video signal can significantly exceed the total length of any individual ODN. design margins. SBS limits the maximum amount of optical power that can be launched into a length of optical fiber by establishing a threshold. Here. EDFAs will typically be placed at intervals of 20 to 40 km. Configuration 3 is commonly used in large (>10. In most cases. The impact of SBS on analog video signal quality is clearly illustrated in Figure 2.000 HHP) FTTH network deployments. Overall video signal distances in such configurations can exceed 30-40 km. with optical video signa ls originating at a local video headend collocated with the local central office. generating an acoustic wave which backscatters a portion of the signal back onto itself. choice of standard or specialized optics and electronics. in order to meet received power requirements. a regional video headend is centrally located to serve a number of separate central offices. SBS occurs when an optical signal excites the molecular structure of an optical fiber. These power levels are obviously dependent on distance. as illustrated. with a single broadcast headend serving multiple local hub facilities over “trunk” or “supertrunk” lines.power requirements for RF video can range from 14 to more than 20 dBm. but are distributed through the field in remote terminals (RT). operators can accommodate CO-to-ONT distances which otherwise would exceed the maximum PON ODN lengths determined by logical limitations and as defined in standards. This will obviously reduce the power received at the end of the network. Additionally.mode optical fiber infrastructure can present limitations in each of the RF video overlay network configurations discussed above. RF Video Transport Optical Limitations A standard single. well in excess of the maximum allowable ODN distance. Therefore. This configuration is sometimes found in reasonably-sized (several thousand HHP) rural FTTH deployments with relatively low subscriber density (perhaps 5-20 homes per mile). The headend in this case is sometimes referred to as a video hub office (VHO). sometimes approaching 60-80 km or more between headend and ONT. Optical Line Terminal (OLT) optics and electronics are not centrally located at the HE/CO. a portion of the signal is essentially reflected backwards. Configuration 2 in Figure 1 is quite similar to configuration 1. and overall configuration of the PON ODN. In this case. Therefore. however. . but it will also impart severe noise and distortions onto the transmitted signal. optical amplifiers will be located at both the HE/CO and at each RT. where video signals will be multiplexed with OLT signals onto a single fiber for each PON. above which RF video signals (both analog and digital) can be severely degraded. By remotely locating OLTs. where the introduction of both noise and distortion is evident. In simplified terms. 652. By incorporating fibers with enhanced SBS threshold. these techniques have their limits. situations such as those in configuration 1 can typically support optical power levels about 1. has allowed the design of ITU-T G. and the industry largely has learned to live within the limitations of SBS. SBS threshold actually increases with shorter distances. however. network operators are seeing more flexibility in network architecture. Transmitter used was conventional CATV-grade and rated for 17 dBm SBS threshold over 50 km of standard single-mode fiber. The SBS threshold of a system employing standard off.5 dB higher over a 20 km distance. assuming that the capability of single. and design budgets and margins. However. configurations 2 and 3 commonly see little improvement in SBS threshold since video signal distances commonly exceed the maximum 20 km encountered by PON optics. Installation and Connectivity With any broadband access network technology. All wireline architectures must deal with trenching and aerial installation of cables. through various electronic techniques. as well as .the-shelf video transmitters and standard single. video transmission equipment choice. No digital QAM loading and no PON 1310/1490nm wavelengths were present.mode fiber. The same holds true for FTTH. and has been an issue in video transport networks.a b Figure 2: Comparison of RF analog video signals with optical launch power (a) below the system’s SBS threshold. mitigated the effect significantly by increasing the fundamental SBS threshold of video transmitters through various applications of high frequency modulation.mode fibers with reduced water peak attenuation) is commonly considered to be 16-18 dBm over a distance of 50 km.mode fiber is static. Both examples are from CH78 monitored under a 78 channel analog load over 20km of single -mode fiber with loss simulated to represent presence of 1x32 optical splitter.Dcompliant single. and (b) above the system’s SBS threshold. for nearly 10 years since the advent of linearized 1550 nm transmitters and low-noise optical amplifiers. which would otherwise induce additional signal deterioration. and as a result.mode fiber (encompassing all standard single. the cost of network construction dominates. Transmitter manufacturers have. especially in overbuild scenarios. such as supertrunk links built by CATV operators. Recent improvements in the fundamental understanding of SBS behavior in optical fibers[4]. The effect of SBS has been identified for quite some time[3]. Still.mode fibers with significantly increased SBS threshold levels[5] while maintaining backward compatibility with legacy standard single. improved fiber management. To date. however. Aesthetics of an overall installation must also be taken into account. which has significant implications on installation and customer satisfaction. All this leads to a new design standard with extremely beneficial implications on the physical size of local convergence point cabinets. but it is not uncommon for a single cabinet to effectively support up to 432 or 864 customer ports. a significant issue still remains around the overall physical size of these network elements. and a crane is commonly employed as the larger units can weigh several hundred pounds. but very capable options do exist. The LCP is arguably the most critical element in the PON outside plant. Freight and storage costs are reduced as well. as well as a rugged enclosure for the passive optical splitters which form the basis of the PON architecture. Most significantly. offer easy access for the service technician and adapt to support increasing subscriber take rates and bandwidth.mode fiber while maintaining the same outstanding optical performance and mechanical reliability (< 1 ppm failure probability) of conventional single-mode fibers. With size reduction comes dramatic weight reduction. and no need for a crane. The density of connections in an LCP will vary depending on network density and operator design choices. network operators would much rather have subscribers be overwhelmed by the speed of the network entering the home than to be overwhelmed by the size of the box in the ir front yard! A well-designed cabinet can make very efficient use of space with newly designed splitter modules. Typical installation of an LCP requires several construction technicians.establishing cross-connect functionality in the field and making the eventual connection directly to the subscriber premises. . The standards-compliant fiber can be bent in tighter configurations than today's standard single. The dimensions of the LCP can now approach those of a common Cable TV pedestal. neighborhood residents will likely not even notice the presence of these devices. Reduced-Size Local Convergence Point A significant element of a FTTH PON-based network is the cross-connect functionality residing in a local convergence point (LCP) splitter cabinet. The cabinet must be craft-friendly with intuitive fiber routing. and jumper cables with reduced outer diameter combined with a standard single mode fiber with improved bend performance. Current offerings in the market can vary significantly in their ability to meet these design goals. as illustrated in Figure 3. as it is the point of basic connectivity between feeder and distribution segments of the network. Freight charges and allocated warehouse space are also directly dependent on the physical unit size. The importance of design considerations in an LCP cannot be overstated. allowing the possibility of installation with fewer technicians. replacing them with completely dry water-swellable tapes and yarns. .free cable designs can meet all relevant industry standards and testing requirements (e. with standard cable configurations continues to introduce significant time and labor costs. but offer dramatic improvements in craft-friendliness. in both loose tube and ribbon configurations. or gel. Gel. While this gel serves to water-block the cable. Preparation time.free loose tube and ribbon cable configurations are illustrated in Figure 4. fusion splicing equipment and techniques have evolved to a point where the actual time and effort required to perform and protect the actual splice is minimal. These gel. Source: Corning Cable Systems Gel-Free Optical Fiber Cables Although splice time still needs to be considered for deployment cost and productivity in the feeder and distribution section of FTTH networks.. Standard cables. but a dramatic reduction in size and weight is realized. not to mention stained clothing and contaminated equipment that result when opening a cable for access.g. to prevent water incursion into the cable. associated labor costs and productivity. the gel is universally reviled by installers for the tedious cleaning required to prepare fibers for splicing. however. Telcordia GR-20-CORE) just as conventional cable designs do.Figure 3: Comparison of a current generation LCP (left) and size-reduced cabinet (right). Port density and usability of the two devices are the same. Recent advancements in “gelfree” cable designs have eliminated the filling compounds. are filled with an oil-based flooding compound. Installation time and cost are significantly reduced. termination of FTTH optical drop cables at the subscriber premises introduces a completely new paradigm to most network operators.a b Figure 4: Cutaway views of (a) gel.free ribbon cable. rather than fusion splicing to a bare fiber pigtail. as well as increase the time required to activate new service. A pre-connectorized Optical Network Terminal is shown in Figure 5. . and (b) gel. Extending the skills of a fiber splicing technician to every household in a network deployment can significantly impact installation cost.free loose tube cable. The advent of pre-connectorized drop cables. Source: Corning Cable Systems Pre-Connectorized Optical Fiber Drop Cables Finally. dramatically alters this scenario. This technology allows a standard installation technician to quickly deploy a drop cable and simply attach an error-proof ruggedized connector to a connector assembly at the ONT. and the ease of connecting and disconnecting the drop allows for rapid and simple network testing when required for fault isolation and service diagnostics. either all-dielectric or toneable. We have shown various areas where network flexibility and/or installation costs can limit the success of FTTH deployments. Source: Corning Cable Systems Conclusion Network limitations. . The connector is circled in red. designed to accept a preconnectorized optical drop cable. compliance with accepted industry standards. Paramount in any optimization of the underlying physical infrastructure should be considerations of interoperability with legacy technologies and future increases in bandwidth. Boh Ruffin of Corning Incorporated for video signal data and experimental validation. and presented passive solutions to these issues which allow interoperability with the widest variety of electronics solutions and transparency to potential future network service upgrades. been largely unresolved and were simply tolerated. Acknowledgements The author thank s A. Other problems have. and David Meis and Mark Turner of Corning Cable Systems. to date. to varying degrees. and where appropriate. directly impact the bottom line of network operators looking to begin. but at the expense of increased active and/or passive equipment cost or reduced network design flexibility. or expand upon. FTTH network deployment.Figure 5: Pre-connectorized Optical Network Terminal (ONT). Some problems can indeed be addressed with existing options. .” Appl. Farmer. (July 2005). R. “Video in the FTTH triple-play package: Broadcast or IPTV Open this result in new window. 5338-5346 (July 2005). (July 2005). “NexCor™ Optical Fiber Product Information Sheet. J. Opt. 11.References 1. C. A.” Lightwave Magazine. Kobyakov et al. 3. 1972). 2. 2489-2494 (Nov. “Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering.” Proceedings of OFC (2005). “Design concept for optical fibers with enhanced SBS threshold. “RF/IP Hybrid Network for Video Delivery over FTTP. 4. Smith.” Corning Incorporated. 5. Knittle..” Optics Express 13. pp. pp.