FTTH in MDUs (Multiple Dwelling Units)
When we normally talk about FTTH, we assume we are installing the fiber to a “home” where it terminates in a optical line terminal (OLT) and services (voice, data and video) are delivered inside the subscriber’s home. But since we may have detached single-family homes, row houses or units in a large building, the situations can be quite different, requiring different architectures and installation practices. We should add that office buildings are often similar to MDUs, with the exception that floor plans are generally more flexible and units are larger, but the concepts are similar.
Let’s assume we have fiber to the building, then what’s next? We must decide how to deliver broadband to each unit in the building and then inside the unit. What are the options for delivering services to each unit from the entry facility?
Options for connecting each unit in the building include using:
1. Currently installed phone lines using xDSL technology
2. Currently installed CATV or satellite coax using cable modem or MOCA (Multimedia Over Coax Alliance) technology
3. Installing wireless access points at appropriate points in the building connected by Ethernet as is usually done in hotels
4. Installing new category-rated UTP cable to each unit if it is within the 100m distance limit and use Ethernet
5. Installing new coax cable to each unit and use cable modem or MOCA
6. Installing fiber to each unit and mounting the ONT (Optical Network Terminal) at or inside the unit
Options 1 and 2 eliminate the need to install new cabling but assume the current cables are in good enough condition to carry the signal bandwidth required. Options 3, 4 and 5 require installing new cables and furthermore, option 3 assumes adequate bandwidth over the wireless for typical users. It would probably be overwhelmed by video users.
And options 1 through 5 require considerable investment in electronics, space to locate them and quality uninterruptible power at the building entrance facility. The actual network architecture is influenced by the choice of electronics. ONTs are available for single users or multiple users, allowing one to distribute ONTs in a building, for example to serve all the units on one floor with copper cables. However, these multi-user ONTs are going to divide up available bandwidth among the number of units served, perhaps not a problem if the system if offering Gb/s services to the ONT, but potentially a large problem, even today but certainly in the future, if the bandwidth allocated to multiple users is much lower.
With option 6, we would generally assume a GPON or EPON system, although a point to point (P2P) system can be used. In the case of a P2P system, fiber to the unit would entail either a switch in the MDU building itself or a large fiber count cable back to the central office or nearest switch.
Assuming a GPON or EPON network, option 6, installing fiber to every unit, has several variations that can be used and all have one big advantage: no matter how big the building and how many units, the size of the entrance facility is minimized and no power will be required except at each individual ONT at the unit. The options start with where to place the PON splitters to optimize the cabling and installation then what kinds of cabling and hardware are needed to simplify the installation.
Options for connecting units with fiber include these architectures:
1. PON splitters can be outside the building in a service provider facility and large fiber count cables brought into the building, then broken out in premises drop cables to units. This architecture also supports a P2P (point to point, not PON) system.
2. PON splitters can be located in the entrance facility of building, minimizing the fiber count into the building, then drop cables run from that point to each unit.
3. PON splitters can be cascaded from an initial PON splitter in the entrance facility to individual splitters on each floor or area of the building, supporting units on that floor or area.
4. Theoretically, one could have a OLT (optical line terminal) installed in the building connecting to splitters distributed throughout the building or buildings. Sonce these units support thousands of users, dedicating a unit to one building would probably not be done except in large complexes.
The actual architecture will be influenced by the design of the MDU building and where and how it is convenient to install components for the FTTH systems. Component cost may need to be compromised to facilitate installation and reduce cost there.
MDUs come in many varieties, of course, including rows of attached units, low-rise MDUs with only a few levels of units and high-rise MDUs. The first two are more horizontally distributed while high-rise buildings can have both many vertical levels and small to large horizontal distribution depending on the height and size of the building and the size of the units.
While some older units will allow cables to be installed on the exterior of the building, that is probably not going to be allowed on more modern buildings nor on high-rise buildings. However, most buildings will have facilities for cabling even if they are so old that they only had electrical and phone services originally.
Like any FTTH system, a “greenfield” installation offers much more flexibility for designing a building that simplifies cable and hardware installation. Plans can be made to include cable conduit and/or cable trays and facilities for other network hardware. But most large MDUs have provision for cabling for services like phones and CATV, perhaps even Internet if built more recently, that offer good options for FTTH fiber installation.
The PON splitter can be located in the building entrance facility and drop cables run to each unit. This mimics most phone wiring and pathways may be available to run drop cables. It uses more fiber and/or cables but does not require mounting as much hardware around the building nor splicing and/or termination.
One reasonable option is to use cascaded splitters. The first splitter can be located in the entrance facility with multiple fibers going out to the separate floors where a splitter is installed to serve the floor. Alternatively, the first splitter can be placed on one of the served floors. If one has 8 units per floor, a total of 4 floors can be handled on 32 split ratio system with a 4 way splitter feeding 8 way splitters on each floor. Likewise, one could use a first splitter of 8 ways to serve 4 floors. Or a 16 way first splitter would serve 4 floors. The best option probably depends on the building and how cabling would be installed.
Recent developments on distribution and drop components make MDU installations easier. Perhaps the biggest development was bend-insensitive fibers that allow the manufacture of drop cables in extremely small sizes that can be run along wall or ceiling junctions, around corners, placed inside baseboard or molding and even made with an adhesive surface that can be stuck directly on walls. Bend-insensitive fibers also allow the manufacture of small cables that allow opening at any location to break out one or more fibers for termination at that point and allow the whole cable to continue to another location.
Small boxes or closures are available that contain couplers and patch panels allowing drop cables to be terminated with prepolished/splice connectors, either fusion- or mechanical-spliced, to complete the connections.
Like any fiber or cabling installation, the actual project will be unique but be able to incorporate ideas that worked well in prior projects. If the building project is in the design stage itself, knowledgeable fiber optic designers can provide feedback that will make the installation easier, neater and much less expensive.
The most important part of the design of a project in an existing buildingis a “walk through” to familiarize yourself with the building. Inspect for entrance facilities, cabling pathways and locations for equipment on every floor. Look at several units to see where it is feasible to enter the unit and place equipment. Having a familiarity with the building itself will make choosing a design much easier.
Another issue, of course, is the take rate for FTTH connections., which can affect planning as well as the ultimate cost. On older buildings, units may already have CATV or satellite connections and not be interested in FTTH, so one cannot assume a 100% take rate. The building owner can survey those living in the units to determine the take rate for planning purposes, but one also has to assume some number of future additions in doing the design. New construction may be easier, as the developer/builder may decide to make FTTH a selling feature and provide it to all the units.
- Technical Information on FTTX From The FOA Online Reference Guide:
Testing FTTH Networks
- FTTH Architectures, MDUs (Multiple Dwelling Units)
- FTTH PON Protocols
- FTTH Installation
- FTTx Online Tutorial
- Here's links for more information on FTTx
- Training & Certification
- FOA Certification Overview
FOA FTTx Certification Requirements
FOA-Approved Training Programs
Table of Contents: The FOA Reference Guide To Fiber Optics