Essentially, there are three fundamental architectures for delivering fiber directly to a subscriber’s house: point-to-point, switched, and Passive Optical Network (PON).  All three Fiber to the Home (FTTH) architectures require an aggregation device in the CO (the Optical Line Terminal or OLT), and all three require an optical to electrical converter (Optical Network Terminal or ONT) in or on the house. These three architectures differ mainly in what type of device (if any) is installed between the CO and the house.

One common characteristic of Fiber to the Home network architectures is the use of bidirectional transceivers (BIDI) allowing the use of a single fiber to serve each home. One wavelength is used for downstream (CO to home), and another wavelength is used for upstream transmissions. Bidirectional transmission on a single fiber increases the cost of optical transceivers somewhat, but it reduces the quantity of fiber (and labor to splice the fiber) needed to serve a home by half.

Point-to-Point FTTH

Point-to-Point (P2P, also known as Active Fiber) is the simplest of all three FTTH fundamental architectures. With a P2P network architecture, a fiber (typically only a single fiber) is installed from each subscriber’s house directly into the Central Office serving that subscriber.  This architecture has the advantage of simplicity, but it does require terminating lots of fiber cables in the Central Office (CO). The CO contains a high port count aggregation device (one port per subscriber) known as an Optical Line Terminal or OLT. An Optical Network Terminal (ONT) is installed either on the side of the subscriber’s house (typical in the US) or inside the subscriber’s house (not typical in the US). P2P has an advantage that no port is shared in any way, thus troubleshooting problems on the network is greatly simplified.  With P2P, problems can be easily isolated.  Additionally, this architecture has the highest bandwidth potential.  Links are easily upgraded to higher rates (requires new optics and electronics on both ends however), and each additional fiber linearly adds more aggregate bandwidth to the network.

Switched FTTH

A switched FTTH architecture (commonly Active Ethernet) has many of the advantages of P2P, but it dramatically reduces the number of fibers terminated in the CO. Of the three architectures, it has the potential to have the fewest fiber terminations in the CO, but it requires the largest investment in the OutSide Plant (OSP). To aggregate fibers delivered directly to subscribers, a switched architecture requires switches be installed in secured cabinets between the CO and the subscriber homes.

Passive Optical Network FTTH

A Passive Optical Network or PON network architecture is similar to the switched architecture, but it requires no OSP electronics. Instead, an optical splitter is used in place of the OSP switch. The splitter divides the light coming from the OLT, and it combines the light coming from the ONTs. This greatly reduces the cost of OSP aggregation since the splitter is inexpensive, requires no power and very little, if any maintenance. Maximum concentration of subscribers is generally limited to 32 per fiber (with a 32 port splitter) delivered to the CO, though up to 64x and even 128x splits are possible. BPON, EPON, and GPON are common types of PON networks in use today. 10G EPON and 10G GPON are new technologies to be deployed in the next few years. WDM PON is similar, but instead of a splitter, it requires an Arrayed WaveGuide (AWG) to divide wavelengths for individual delivery to subscribers. Note that a PON may support RF overlay analog video, which is generally not feasible in the other two architectures.

© 2009, The Product Group LLC. All rights reserved.

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1. John Nixon says:

   September 22, 2009 at 6:19 am

   Hello John,
   Your article popped out in my daily searches on this subject.
   Please check my website: http://www.onefibre.com
   Just a couple of corrections: RF Overlay can happily function on P2P networks, and also with GEPON and GPON.
   With BKtel GmbH, we have developed a very interesting extension of RF Overlay, enabling one to transmit native L-Band QPSK Satellite signals on 1550 nm, as well as normal free-to-air channels. This also includes the ability to switch between multiple satellites and two polarities over a FTTH network.
   You will find a complete description on my web site.
   It has been demonstrated recently in both Australia and New Zealand.
   Thanks for helping to propagate FTTH to the public!
   John Nixon
   Optical Network Engineering
   http://www.onefibre.com
2. cyberdoyle says:

   September 28, 2009 at 2:36 pm

   I think there’s a fourth. A community owned fibre to the home network, using any of the above three methods, but owned by the people who just connect to a fat pipe. Ideal solution for a rural community to JfDI themselves instead of waiting for a telco to provide it. There should be a way of enabling this approach, as despite the best efforts of the users there is no sign of any input from telcos into the more rural locations.
3. Irit Gillath says:

   October 29, 2009 at 9:38 am

   Hi John,
   Thanks for the good article.
   It is interesting that you separate between P2P and switches FTTH. What we in our Active Ethernet deployment see if that customer choose a combination of architecture and combine star, tree and rings (either in the aggregation level or in the CPE level) or even take the Fiber only to the poll or basement and go with Ethernet 10/100 Copper to the house.
   I agree also with the previous comment that while RF over Active Ethernet is not the most efficient thing it does exist and available in the market.
   My other comment would be on the higher cost on the OSP while this may be true the cost comparison should be in the per subscriber level and considering various take rate. And in this case Active Ethernet may be equal or better to PON.
4. Whitcomm_ says:

   November 7, 2009 at 3:36 pm

   Cyberdoyle, that’s exactly what we at Whitcomm have done. We have a passive optical network and connect to a fat pipe to the main exchange. We’re the first community owned organisation in the UK to build such a thing.

   On a personal level, I also live in a rural area and suffer the consequences technology-wise and have wondered about trying to organise a debate (if nothing else) about building our own fibre network. Having said that, building your own PON isn’t without its challenges.