Understanding and Optimizing MDU Optical Cabling Systems

Transcription

1 Understanding and Optimizing MDU Optical Cabling Systems FTTH Conference 2008 Paper T-404G John George, Director, Systems and Applications Engineering

2 Introduction Understanding and Optimizing MDU Optical Cabling Systems FTTH Conference 2008 John George, OFS Deployments of fiber to Multiple Dwelling Units (MDUs) have severely lagged those to single family homes. While over 12M residences have been passed by FTTH in the US as of June 2008, MDUs comprise only about 6% of this total, despite representing about 30% of US residences. Fiber to each unit within MDUs (FTT-MDU) and even inside homes is presenting a new set of challenges for service provider deployments. Barriers have included difficulties gaining video franchises, exclusive MDU owner agreements with non-ftth service providers, and the high cost to build fiber networks within buildings, particularly existing buildings. The franchise and building access issues are being addressed by the FCC and Congress. New technologies such as plug and play indoor cabling systems, spooled distribution systems, and ultra-bend insensitive optical cables, can tilt the FTT MDU business case toward go by enabling faster, simpler, and lower labor deployments. Why Fiber in the MDU and Residence? Metallic media such as twisted copper pair, coaxial cable, or telephone service wire have traditionally been used to support voice, video and data applications in MDUs. Unfortunately, services providers bringing fiber to MDU buildings and homes increasingly find the existing indoor metallic cable needs expensive conditioning or replacement to support the desired services. This is understandable given that most metallic media inside MDU buildings or homes was never intended to support today s video and internet services reaching tens of megabits per second (Mbps) per subscriber, let alone the 100 Mbps to 1,000 Mbps services projected to be in demand in the next 5 to 10 years. While costly copper remediation measures might suffice for today s services, providers might be facing a metallic meltdown as projected bandwidth increases burn through the approaching limits of MDU and in home copper based cabling systems. Fortunately there is a solution to support the in-building bandwidth deluge. Fiber to each unit within MDUs (FTT-Unit) and even inside homes is fast becoming the preferred approach to providing leading edge video, voice, and internet services to subscribers that can keep pace with video driven date rates growing at about 40% per year. As one example Verizon recently announced a plan to offer its FIOS TV service throughout the 5 Burroughs in New York City that includes fiber to each unit within MDUs. With this large step towards a global migration to fiber inside the 700 million residential buildings and homes, understanding and optimizing the deployment of fiber inside residential buildings can help fiber investments generate greater returns.

3 The What and How to for Fiber in the MDU There are five basic subsystems in an FTTH optical cabling system: The Central Office or Head End (CO/HE) electronics, routers, and switches are connected through cross connect and/or interconnect patch panels and fusion splicing to the OSP cabling system. The feeder cable (F1) from the CO/HE is linked to the fiber distribution point (FDP), and from the FDP the distribution cabling is routed to a distribution terminal, from which a drop cable connects to the Optical Network Terminal (ONT) in each subscriber s residence. For a PON architecture, the FDP may consist of a fiber distribution hub (FDH) cabinet containing connectorized optical splitters serving typically 288 residences, or an enclosure housing fusion spliced optical splitters serving 8 to 64 subscribers. From the splitters to the residences there is typically a single fiber serving each subscriber within the distribution cabling or (F2) system. In an active Ethernet architecture, electronic Ethernet switches, instead of splitters, serve in place of the FDP. The FDP can be located in the outside plant between the CO/HE and MDU building(s), inside one or more of the MDU buildings, and in some cases it might be economically located in the CO/HE itself if the distance to the MDU is relatively short. The fiber will enter the MDU through the entrance facility (EF), typically in the basement or a lower floor of the building, The FDH may also be larger external unit serving multiple MDUs, in which case the EF is simply a splice enclosure providing the transition from outside plant to inside cabling... In an active Ethernet system a powered electronic switch will reside in place of the FDH. From the EF the fibers serving the units will typically be routed in a common pathway known as a vertical riser backbone or horizontal backbone. Fibers in the backbone will typically terminate in a number of fiber distribution terminals (FDTs) each serving typically 4 to 24 units. From the FDT a drop cable will to routed into each unit and terminated in the Optical Networking Terminal (ONT), a powered optical device that transmits and receives the optical signals for the subscriber. Sounds simple enough, but this simple sounding system can be slow and expensive to deploy unless one understands and selects a selects the right solution for a given installation. To simplify the analysis the optical cabling system options for three types of deployments will be considered: Vertical MDUs Garden Style MDUs, and In-residence optical drop cabling. Vertical MDUs Vertical MDUs (V-MDUS) can be defined as buildings housing residents on a typical range of 4 to 40 floors, with typically 4 to 12 residences per floor. Delivering fiber to each unit can be very slow and expensive, reaching hundreds of dollars per unit just for installation labor in the older buildings comprising the majority of V-MDUs. That cost can submarine a business case given that large service providers typically plan for about $800 per home passed and $800 per home connected for complete FTTH installations, including labor, electronics, and the optical cabling system.

4 What drives the high deployment cost is that V-MDUs typically have very restricted pathways and spaces for routing optical cabling systems, and require up to four fiber splices inside the building to reach each subscriber. Shrinking apparatus and cabling, while speeding and simplifying the installation, can help both sides of the business case equation. The benefits of lower installed cost are obvious, but less recognized are the benefits of a higher velocity deployment: A potentially greater return on investment enabled by reaching customers and revenue quicker and before the competition. Traditionnel V- MDU FTTH Installations For vertical buildings, designers have traditionally specified a dedicated riser cable for each floor, as shown in Figure 1. This approach requires a huge number of fiber terminations and management at each cable end, using fusion splicing or directly mounting connectors. Even with pre-terminated riser cables on one end, significant fiber terminations and management is still required. For example, if we assume a labor cost of $25 for each termination, and 2 terminations for each subscriber, the fiber termination labor cost alone reaches $50 per unit, and doubles to $100 if the drop to each unit is field terminated. In addition, the volume of space needed to support these cables can be substantial, even with the size advantages provided by fiber. As a result, many buildings require expensive and disruptive boring or construction of new pathways to house multiple backbone optical cables, new spaces to enable traditional fiber management, and new pathways to route drop cables to each unit. Terminals & Drops Ceiling Living Floor Many cables and floors passed OSP Closure FDH or Splice Terminal Figure 1: Traditional architecture for V-MDU FTTH deployment 5 floor building, 15 floors passed by cable with labor intensive splicing

5 Given that the traditional architecture shown in Figure 1 is expensive to deploy, what other options are available for V-MDUs? One approach provides a completely pre-engineered and preterminated riser cable solution: A single high fiber count trunk cable with factory installed access points could be installed in the MDU riser shaft. Such solutions have been used with mixed results for single-family residential FTTH applications. The obvious downsides to such an approach include: the amount and accuracy of up-front engineering surveys of each building; the potential complications posed by the condition and capacity of the riser space in a typical highrise, and finally the logistical and costly inventory management Spooled Plug and Play System for MDU FTTH A new MDU optical cabling system can be quickly deployed within the confines of typical riser space and to each unit by leveraging new technologies in optical fiber, cable, and connectivity. The Spooled Plug and Play System (SPP), as shown in the schematic in Figure 2, can dramatically increase the velocity and ease of MDU deployments within a small footprint. SPP systems are shipped with factory installed connectors and adapters inside the terminal, which are connected to a factory installed multifiber tether terminated with a multi-fiber connector. SPP systems utilize new spooling technology to store the tether cable slack coaxially around a mandrel on the back of the FDT. The tether is wrapped around the FDT mandrel prior to shipment and can be unspooled in the field to the custom length required for each MDU terminal, and plugged into the backbone combiner which aggregates to 60 fibers from five FDTs, or plugged directly into the FDH or entry splice box. Additionally, SPP systems employ bend optimized fibers to enable compact cables that install easily and quickly while conforming into building pathways. The Spooled Plug and Play system offers four key benefits to the installation compared to a conventional field spliced system: 1. Installation velocity up to six times faster enabled by: a. Plug and play elements b. Smaller cable diameters to reduce pathway creation time c. Over 50% less backbone cable pulling distance. d. Bend Optimized fiber to conform to the building 2. Installed cost up to 66% lower primarily enabled by lower labor hours. 3. FDT space reduction up to 50% by elimination of a conventional slack storage area. 4. Reduced inventory by using 4 basic elements that flexibly adapt to most MDU buildings.

6 Spooled Plug Distribution and Plug System System I ll i Ceiling Drop Assembly Floor Terminals Combiner OSP FDH Entry or FDH Splice Terminal Figure 2: Spooled Plug and Play architecture for MDU FTTH 5 floor building only 7 floors passed by riser cable, Plug and Play The SPP system is comprised of four flexible elements: an indoor fiber distribution hub (FDH) housing compact PLC splitters; a Combiner assembly, consisting of a riser cable pre-terminated in a small interface box; terminal assemblies with pre-terminated tether cables; and ultra bend insensitive drop cable assemblies. The SPP system is insensitive to the number of floors in a building, the distance between floors, and the locations of terminals on each floor. As shown in Figure 2, SPP architecture may be repeated for each set of five floors, with a combiner and five individual terminals. For the lower floors or in a low riser building, the fiber drop terminal tether can be pulled to connect (or splice) directly into the entry FDH. In an 8 story building, one combiner/riser assembly would be deployed serving the top 5 floors with a drop terminal on each floor, while the bottom 3 floors could be served by fiber drop terminal tethers pulled to connect directly into the entry FDH. For high rise buildings, this 5 floor per combiner architecture would simply be repeated. The elements of the SPP optical cabling system are described below.

7 Entry FDH with Pluggable Backbone Connections An example of an Indoor FDH shown in Figure 3 accommodates up to four 1x32 splitter modules to service up to 144 living units. Typically, this FDH would be placed in the basement of the MDU, and would be connected to the OSP network through a fusion-spliced, 4-fiber feeder cable. Backbone terminations can be either spliced, or plugged in with multi-fiber connectors to feed the backbone. Figure 3: Indoor Fiber Distribution Hub Combiner Spooled Riser Cable Assembly The next element is a simplified riser assembly, in which a compact ribbon riser cable (about 8 mm in diameter) is terminated with multiple 12-fiber MPO connectors in a small interface housing known as a Combiner (Figure 4). Typically a 60-fiber Combiner unit will be installed on the third floor of a MDU, and every fifth floor above that (3, 8, 13, etc). Integration of cable spooling with the combiner housing allows rapid, direct spool-off of cable into the riser and down to the FDH in the basement. This direct pay-off feature eliminates the time typically required to figure 8 riser cable inside hallways, where construction workers or tenants could interfere. The spool-off capability enables hands-free installation by allowing the unit to be temporarily mounted on a re-usable fixture, allowing a single technician to install the system in the riser without monitoring the housing and cable. After the riser cable has been run to the basement, it may be terminated at the FDH by fusion splicing, or by using plug and play MPO connectors. The small size of the combiner also enables technicians to work within confined closet spaces, staying out of the path of tenants and eliminating the need for larger dedicated telecommunications rooms. Once the riser cable has been installed, the spool on the Combiner

8 unit may be removed and discarded, followed by permanent installation of the Combiner on the wall of a closet. Figure 4: Combiner Assembly with Spooling Tool Fiber Distribution Terminal with Spooled Tether The system utilizes a terminal shown in Figure 5 that is similar to the combiner, and intended to be installed at each floor or area within the MDU. Five 12-fiber terminals, placed throughout the building, can be fed from a 60-fiber combiner. Typically terminals are installed on each of the two floors above and below the combiner, with one terminal installed on the same floor as the combiner. Each terminal includes a 50-foot tether cable, wound on an integrated cable hub. One end of the tether cable is terminated with a factory-installed MPO connector; the other end is terminated into 12 individual single-fiber adapters under a lockable cover. Terminals also take advantage of an integrated spooling technique similar to the combiner riser installation, yet takes advantage of the spool for slack storage. Once the tether cable is spooled off to the required length, it is terminated at the Combiner by simply plugging in the MPO connector. Any cable slack may then be taken back up on the terminal spool, enabling a very flexible and compact location of the drop terminal on each floor. Figure 5: Terminal with 12 fiber tether that can be un-spooled to length required to reach the FDH or Combiner. Twelve single-fiber adapters are protected by the hinged dust cover.

9 Bend Optimized Optical Fiber for MDU Cabling Systems The International Telecommunications Union (ITU) standard currently specifies 2 grades of reduced bending loss optical fiber. The first, ITU G.657A, specifies a singlemode fiber that is fully compliant to the ITU-G.652D standard singlemode specification, and offers less than 0.75 db of loss for a single turn at a 10 mm radius measured at the 1550 nm wavelength often used for long haul telecommunications, CATV, and FTTX video transmission. By meeting G.652D, fiber compliant to G.657A is intended to be fully compatible with the G.652D fiber typically used in most cables. The second ITU standard, G.657B, specifies a fiber which has lower bending loss than G.657A, at 0.5 db maximum for a single turn 7.5 mm radius at 1550 nm, yet need not comply with the standard singlemode specification of G.652D. Thus, G.657B compliant fibers may not be backward compatible and easily splice-able to G.652D fibers. Fortunately, there is a fiber available that is bend optimized for MDU backbone cabling systems and general connectivity use. It is fully G.652D and G.657A compliant, but also offers 66% lower bending loss than required by G.657A, and easier splicing than allowed for by G.657A. This optimized G.657A fiber, which can be referred to as O-G.657A, will support MDU backbone cabling needs without significant bending loss while enabling easy splicing and connector mounting. But although O-G.657A fiber is economically and performance optimized to meet most connectivity needs, the desire to rout drop cables without any fiber management has driven the need for a new class of fibers, known as Ultra-Bend Insensitive fiber, which is far beyond G.657B. Drop Cables for MDU and In-residence Applications The Ultra Bend Insensitive Fiber (U-BIF) Drop assemblies are the last yet critical fiber link to and even inside the subscriber s residence. These drop assemblies include robust and crush resistant cordage housing U-BIF, with high performance factory or field installed connectors on each end that connect the FDT to the ONT. The assemblies are capable of conforming to existing pathways to and within units, using easy attachment and routing methods, with minimal bending induced loss of the optical signal. The bending challenge for drop cables conforming to the building contours for routing is clearly evidenced in Figure 6 below. Corners Figure 6 Example of a tortured path in an MDU hallway.

10 A typical conventional singlemode fiber (SMF) would lose about db (90% to 99%) of a 1550 nm video signal around the 16 corners shown in Figure X, likely blocking services to those unlucky subscribers. The key to connecting customers through a maze of building corners is using an optimized U-BIF within the appropriate, protective cable structure. While fibers with holes or voids around the light carrying core region have traditionally offered very low bending loss, the need to plug holes in the endface to enable reliable connector mounting and end face cleaning adds cost and complexity to the system. New U-BIF technology solves this dilemma by using a fully solid glass construction that provides ultra low bending loss, splicing and connector mounting using standard procedures, and uniform optical properties along the length. U-BIF drop assemblies are usually factory connectorized on one or both ends, typically with SC- APC connectors. The accepted bending loss specification for U-BIF fiber is < 0.1 db of loss for a single fiber turn at 5 mm radius, measured at the bend sensitive wavelength of 1550 nm that is often used to transmit video services in FTTX systems. U-BIF drop cable assemblies are designed to be stapled and routed around dozens of corners without any significant signal loss, as shown figure Y. The 4.7 mm diameter U-BIF drop cables provide maximum protection for the fiber during installation and use and are suited for stapling, while the compact 3 mm diameter U-BIF cables are optimized for installations behind molding. The 4.7 mm diameter is available in riser, plenum, and indoor/outdoor versions compliant to the relevant cable standards, while the 3 mm is available in riser and plenum versions. U-BIF Technology 35 quarter turns, 10 staples Conventional SMF: 50 db Figure 7: Ultra Bend Insensitive Drop Assembly in Simulation Test Garden Style MDUs A Garden style MDU (GS-MDU) complex typically consists of a group of one to three story buildings and comprises the majority of existing installations in the US. An outdoor optical cable will typically feed the GS-MDU complex, terminating in an FDH or splice enclosure, from which point a multi-fiber cable is placed to each building. The GS-MDU is often served by an external FDH or Switch, or these may be inside one or more of the buildings. Each building usually houses 4 to 24 units on one to three floors. The optical cabling system for GS-MDUs is similar to the V-MDUs with the same available options of a traditional spliced system or a spooled plug and

11 play system. The differences are that the GS-MDU building using a PON architecture will typically be served by an OSP FDH, and is the SPP system is used the combiner assembly is often not needed due to the relatively small building size, in which case the U-BIF drop would plug directly to a splice box in the EF. Additionally, the backbone of a GS-MDU will often be horizontal rather than vertical, but the same basic cabling architectures as with the V-MDU will apply. Modeling Cost and Performance for Informed Decisions Cost Model A cost model for MDU installations has been developed based on feedback from experienced service providers deploying fiber to the MDU unit in large numbers, as shown in Figure 8. MDU Optical Backbone and Drop Relative Cost Fusion SPP and UBIF Drop Fusion SPP and UBIF Drop Drop Material Drop Labor FDH and BB Material FDH and BB Labor Difficult Path Existing Path / Greenfield Figure 8 MDU Cost Model Results for 10 Floor Building with 12 residences per floor. The model predicts a 33% installed cost advantage for the Spooled Plug and Play system, compared to a fusion spliced system for buildings having existing riser and horizontal pathways and spaces. In this case most of the savings result from the elimination of splicing labor and to some degree a reduction in installation time. The savings increase to 66% assuming a difficult pathway must be utilized, or a new pathway constructed. While the material cost is greater with the SPP system, these costs are more than offset by the labor savings. As a result of the reduced labor, the velocity of deployment is increased by almost 3 to 6 times compared to a conventional fusion spliced system, as shown in Figure 9.

12 MDU Optical Backbone and Drop Relative Deployment Velocity Fusion SPP and UBIF Drop Fusion SPP and UBIF Drop Faster Difficult Path Existing Path / Greenfield Figure 9 Deployment Velocity Comparison Optical Connection Loss and Reach Considerations A disadvantage of the SPP system is that it will have higher total optical insertion loss than a fusion spliced system. With 4 SC-A connections having a worst case loss of about 0.35 db per connection, versus 4 fusion splices at 0.1 db per splice, the fusion spliced system will have about 1 db of lower total insertion loss than the SPP system. This loss reduction translates to up to 3 KM of added reach at 1310 nm and up to 5 KM of added reach at 1550 nm. Conclusion A new architecture, the Spooled Plug and Play system, can help unlock the business case for fiber to each unit within MDUs. This solution can lower installation labor costs by up to 66%, reduce inventory requirements, and speed deployments by 6 times to help win customers faster. The system performance is enabled through the use of bend-optimized fiber throughout, without sacrificing performance, reliability, or compatibility with existing single-mode fiber. The compact riser cables reduce the number of floors passed by cable, reducing crowding in the riser space that may be already occupied in existing MDUs. The spooling features reduce the time required for handling cables and monitoring cable pull, and help reduce the number of technicians required for installation. The spooling features also provide a convenient, integrated, and compact means for handling cable slack. Use of factory-terminated connectors throughout the system can eliminate the time and expense of fusion splicing during installation of a fiber to the MDU network. When choosing between a fusion spliced and plug and play system, the added loss of the connections and the associated reach reduction should be considered. Using high quality, low loss connectors can minimize the reach impact. Finally, a new ultra-bend insensitive robust drop cable assembly protects the fiber routed to connect subscriber units to the drop terminal. This integrated solution of a Spooled Plug and Play system with bend optimized fiber can bring the bandwidth benefits of FTTH to MDU residents across the globe.