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This is a mistake, as different architectures allow more or less flexibility in topology and therefore have more or less path dependency in the choice of technology. The right method is to analyze the architectures and the topologies that can be supported by each architecture. In each potential "fiber-to-the-X," the main architectural choice is this: a full optical path to each home/apartment, or not?<p>In Fiber-to-the-Curb (aka VDSL), Fiber-to-the-Basement, or HFC (Hybrid Fiber/Coax) access networks, the length of copper wire or coax cable to the home is shortened, but not to zero. Somewhere before entering your home, the optical signal is transformed to an electrical signal, which is transported by an older medium. In the HFC network, the electrical signals are shared in the last segment, while in FttC or FttB, they're not. So these access networks effectively have the same architecture as before (telephony/DSL and cable), but with shorter distances for the old medium, allowing for higher capacity. The optoelectronic conversion of signals is positioned closer to the home.<p>In Fiber-to-the-Home access networks, a full optical path is built to the home. The optoelectronic conversion which defines the transmission technology is positioned in the home (or on the outside wall). The three main architectures are deep PON, staged PON, and homerun (or point-to-point). In deep PON (DPON), a single fiber is fed to a block of homes and split optically close to the homes with a passive splitter to as many fibers and homes as you design for, typically a multiple of two in the range of 32 or 64. Each endpoint receives the same light from the source, and sends light back through the same single fiber. Staged PON (SPON) is a variation where a much smaller number of homes (typically eight) are served in the same manner by a single fiber. In the aggregation point a 1:4 splitter is used to get to the 1:32 ratio. A homerun architecture (point-to-point) does not split light at all, at least between a home and the aggregation point. Each home gets one or more dedicated fibers all the way from the aggregation point.<p>If the networks at home are all made out of copper/coax (or wireless) anyway, does it matter where the optoelectronic conversion is positioned in the network, inside your home or a few hundred meters away? To start with, the assumption that copper and electrical signal networks will stay dominant in the home is questionable. Swisscom already has started to invest in optical plastic fiber networks that can be used inside the home, to accommodate the much higher capacity needed.<p>Second, a fully optical path gives so many options in transmission technology for such a low price that we have only just begun to explore the possibilities for home use. The universities and researchers have already pushed the envelope very far, and Moore’s law is helping us to get price levels down to a level where anyone can afford the technology. If you have a fully optical path, the only thing you need to change if the need arises is the equipment. So a fully optical path allows for a very fast and selective upgrade (compared with a slow and laborious physical network upgrade if you did not invest in a fully optical path). Imagine what a fix AT&T would be in if a service that would require a full optical path to the home suddenly would become popular. Verizon could respond quickly, but AT&T would not be able to follow.<aside aria-label="In Content advertisement"class=ad_wrapper><span class=ad_notice> Advertisement </span><div class="ad ad_instream"></div></aside><p>The best fiber possible does nothing except guide light of all possible wavelengths. Each wavelength you can separate from others is comparable to the complete “ether” available for all wireless communication. As long as you can modulate the light source as you wish and keep the signal intact through the fiber, you can do whatever you want with the light. This fact allows for many variations in technology over the same fiber.<h3>Fiber options</h3><p>So let’s look at the options and see what optical technologies are already available and practical for everyday use at the home. What are the path dependencies for each architecture?<p><strong> Active Ethernet </strong> is a technology for data transmission which requires a unique fiber or wavelengths (one down, one return), and it's very well known and stable. One of the technology's advantages is the distance you can cover or the speed of modulation you can achieve (or the reduction in light power you need), because there is no optical splitter that reduces the light strength between you and the receiver and shares the capacity with your neighbors.<p><strong> GPON </strong> is currently the most dominant FttH technology, since it's specifically designed for shared architectures. Again, specific wavelengths, one for down and one for the return channel, are used. Through clever time-division-multiplexing between users, a guaranteed capacity and a much higher peak capacity can be delivered. Currently this requires the same brand of equipment both at the aggregation point and the home.<p><a href=http://en.wikipedia.org/wiki/10G-EPON> <strong> 10GPON </strong> </a> is one of the more promising next steps in GPON, delivering up to four times as much capacity. However, the limits of the pulse width/distance/light-strength combination can become a limitation: if the splitter ratio of a Deep PON architecture becomes too high there is a risk that the number of photons per pulse is getting dangerously low after being split (bit error rates high). Increasing the light strength at the source is prohibited by safety laws so that is no option. In a Deep PON architecture you are stuck unless you want to run more fibers to the splitter. A Staged PON architecture does give the option of removing the 1:4 split at the aggregation point. A home run architecture has the option of putting splitters in the aggregation point at will.<p><strong> Linear TV </strong> ( <a href=http://en.wikipedia.org/wiki/Radio_over_Fiber> RF over Fiber </a> ) is a method for delivering radio frequency channels over fiber. A wavelength is modulated with these RF signals, broadcasted to each home, and converted to the same signal that an HFC cable network delivers. In fact, the fiber part of the cable network uses this technology to deliver all these channels to the coax segment.<p>Because coax has its limitations when it comes to transmitting high frequencies (over 1 GHz) over longer distances, the frequency band for linear TV is limited to the standard 50-900 MHz. The optical equipment and the fiber, however, do not have these limitations (5 GHz is easily broadcasted). Recently, <a href=http://www.bktel.com/bktel/gb/Fttx/fiberxsat.htm> equipment has been introduced </a> that superimposes multiple satellite TV signals in the frequency band higher than 1 GHz in the same optical wave. These signals are separated at the home and converted to multiple outputs that can be fed to standard satellite receivers, as if you had 4 satellite dishes. This technology is very useful in dense cities where homeowners hate all the individual dishes on top of the roof or the balcony.<p><strong> Wave division multiplex </strong> (WDM) involves sending multiple colors over one fiber, and the technology was first used to increase the capacity in core networks. One of its uses in FttH networks is to reach faraway homes in a shared architecture. A standard shared topology (PON) runs into problems, like not enough light, at larger distances, and requires a lower split ratio, more fiber, and a higher average costs per customer. WDM has a much larger optical power budget (15db more), allowing for much longer distances.<aside aria-label="In Content advertisement"class=ad_wrapper><span class=ad_notice> Advertisement </span><div class="ad ad_instream"></div></aside><p>Applying WDM in an PON architecture requires ripping out the splitters in the field and replacing them with a wavelength filter that separates the colors to individual customers. The filter selects one wavelength per user, making the fiber plant dedicated to WDM. In a point-to-point architecture, all you need is patching in the aggregation point.<p>The options outlined above are just what are currently available and affordable. Most likely others will be developed in the coming years.<blockquote class="pullquote rules center"><p>One thing we can learn from history is that “things are difficult to predict, especially the future”. Indeed, we often cannot imagine what we possibly could do with new technology.</blockquote><h3>Unbundling</h3><p>European regulators like having the option to unbundle physical access lines, with wholesale bandwidth access as the second best option. But the various architectures have different unbundling path dependencies as well.<p>For fiber-to-the-curb, physical unbundling is more theory than practice. Allowing multiple ISPs with their own hardware inside a small cabinet is a questionable idea to start with, but the backhaul is more of a problem. With something like 800 connections per cabinet, you need a very deeply distributed backhaul network, something not many can afford, especially competitors of incumbents. Regulating backhaul access for competitors is an option, but it's not an easy situation for new entrants.<p>Unbundling Deep PON by requiring patch cabinets close to homes (aggregating homerun fibers in splitters from different operators) is an option that has not been tested in practice. It's not clear that this overhead and cost is worth the effort, compared to alternatives like homerun.<h3>Upgrading the architecture</h3><p>In theory and in practice nothing is forever: you can upgrade a fiber-to-the-curb network to full fiber-to-the-home, or you can upgrade an HFC network to full fiber-to-the-home.<p>The legacy of a fiber-to-the-curb network, however, is a large number of cabinets with active equipment out in the field. This equipment is something that old network engineers abhor, because the point of fiber is to get an outside plant which you can leave alone forever.<p>A HFC network can be transformed to either a SPON or DPON or even a point-to-point network. In fact, some small cable operators who have shareholders that let them keep their cash flow have embarked on a seven-year program to slowly and quietly extend their network fiber up to the home, in order to build a full point-to-point network. This is the cheapest way to get the best possible architecture, with approximately a 20 percent lower investment per connection compared to a new build-out.<h3>Path dependencies</h3><p>One thing we can learn from history is that “things are difficult to predict, especially the future.” Indeed, we often cannot imagine what we possibly could do with new technology. Yet it turns out that in ten years, tools and applications can change in massive ways that nobody imagined beforehand. Our appetite for communication bandwidth seems to be unlimited.<p>Because the rollout of fiber-to-the-home will take more than ten years it makes a lot of sense to think hard about increasing options and reducing path dependencies.<p>The main issue with halfway options (FttC, FttB and HFC) that do give more bandwidth for data is the time needed to upgrade the network to full optical when the need arises. It may take a decade to do so because it is physical work, home by home. If some other region or country has gone fully optical in the first place, then they have a huge advantage.<p>A Deep PON architecture creates a serious lock-in. This architecture is the first to hit the wall of limitations of the pulse width/distance/light-strength combination and is hard to unbundle. A Staged PON has more leeway and has less unbundling limitations.<p>The most expensive but most flexible architecture is point-to-point (home run). All technologies can be mixed and matched on a per-customer basis, without modifying the outside plant. 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