THE COMPLETE TECHNICAL PAPER PROCEEDINGS FROM: A COMPARISON OF PON ARCHITECTURES James O. “Jim” Farmer Wave7 Optics, Inc. Abstract ONT Several Passive Optical Network (PON) standards have been proposed as new architec- . Splitter or taps tures for delivering video, voice, and data to . (16-128X) homes. PONs are being built in large numbers in Asia, and in increasing numbers in the Amer- Headend icas and Europe. Several cable operators are starting to deploy PONs in selected greenfield applications, typically in situations where re- Figure 1. Basic PON quired by the developer. cal Network Terminal (ONT). In many cases This paper shows the most popular the ONT is located on the outside of the home at forms of PONs in use today. We compare the the utility entrance. Alternate locations include performance of the PONs, and talk about how inside the home and in a purpose-built niche in and when one may want to consider PON archi- the outside wall. tectures. Frequently the splitting is done in a cen- tral location as shown. In other cases the split- ting may be replaced by a tapped architecture WHAT IS A PON? more like that used in HFC architectures. The number of homes served by one PON is limited PONs, or passive optical networks, are by the loss budget. While PONs are built with just that: fiber optics all the way to the home, more or fewer subscribers, 32 subscribers is with only passive (non power-consuming) de- considered the “sweet spot” in PON sizing to- vices in the field. With no powered devices in day. We show up to 128-way splitting, but the the field, you save on power costs, and mainten- optics available today don’t support this high a ance is much lower than with hybrid fiber-coax split ratio. (HFC). Since the network is all glass (usually called “all dielectric”), you eliminate problems Done correctly, the advantages of PONs such as sheath current. Lightning issues are include much lower operational expenses, high- generally limited to anything that comes into the er quality, elimination of leakage and the resul- home over the power line and, through sub- tant measurement requirements, and incredible scriber equipment, jumps to your equipment. bandwidth. Data bandwidth of at least 1 Gb/s in each direction, shared over just 32 subscribers is Figure 1 illustrates the basic PON. A the norm today. This bandwidth is delivered single fiber optic strand extends from the head- over separate wavelengths from that used for end to an optical splitter located near a group of broadcast video, so the entire 54-1,000 MHz RF homes. Outputs of the splitter supply optical band is available for video. signals to a group of homes. Signals are termi- nated on each home in a device called an Opti- 2008 NCTA Technical Papers - page 163 TYPES OF PONS them with the likely RFoG architecture. We say “likely” architecture because work on the RFoG We shall describe several types of PONs standard has just started this year, and while in this paper, including BPON (Broadband Pas- there are some pre-standard systems entering the sive Optical Network, approaching end-of-life), market, the standard system has not been de- GPON (Gigabit Passive Optical Network), and fined. Thus, what is described herein is the au- GE-PON (Gigabit Ethernet Passive Optical thor’s conjecture of what the system may be. Network). We shall mention a variant used in some places, called an active optical network. PHYSICAL LEVEL ARCHITECTURE We’ll also describe an emerging adaptation of an HFC network to extend fiber deeper. It is Figure 2 illustrates the physical layer ar- called RFoG (Radio Frequency over Glass), and chitecture of PONs. Figure 2a illustrates the is an option to consider when a developer re- BPON/GPON/GE-PON architecture, and Figure quires fiber-to-the-home (FTTH). 2b illustrates a possible RFoG architecture. In each case, the headend comprises what head- GPON and GE-PON systems (and ends usually comprise in the way of video, BPON) share a common physical layer architec- voice, and data equipment, except that in the ture, with some differences in optical levels and standard PONs of Figure 2a, there is no CMTS speeds, so we will cover them together while – this will be explained later. Downstream RF discussing the physical layer. We’ll compare signals are supplied to a downstream optical Figure 2. Physical Layer Architectures 2008 NCTA Technical Papers - page 164 transmitter, usually an externally-modulated lines (POTS – plain old telephone service) will transmitter and always at 1550 nm because am- be supplied. Other options are shown below. plification of the optical signal is needed. While you can amplify other wavelengths, amplifica- RFoG tion at 1550 nm is the most mature and econom- ical process today. All of the standards use A possible RFoG system is shown in 1550 nm for downstream RF broadcast. Figure 2b. The headend is identical to that of an HFC system, because RFoG is really an HFC GPON/GE-PON node serving one subscriber. The downstream is again a 1550 nm transmitter, because you will Unlike HFC networks, the network inter- need to amplify the optical signal. The up- face for data is not a CMTS – none is used – but stream receiver is similar to that used in up- rather is an analogous device called an Optical stream paths today. The upstream may be ana- Line Terminal, or OLT. It serves the same func- log or it may be digital; this has not been de- tion as the CMTS in that it converts data (usual- cided in the standardization effort as of this ly delivered as gigabit Ethernet) into the format writing. needed for PON transmission. That conversion includes conversion to the particular PON pro- An optical node in the field is shown as tocol being used, and conversion to light. The optional. Of course, if used, the network is no downstream signals are carried at 1490 nm and longer completely passive. If used, the optical the upstream at 1310 nm. These are combined node will likely contain optical amplification in with the 1550 nm broadcast signal in a wave the downstream direction, and combining (in the division multiplexer, or WDM. The WDM op- optical and/or RF domains) in the upstream. erates analogously to a diplex filter in the HFC Some proposals convert the upstream to digital. world. Again, for RFoG we show a 32-way Typically, the OLT includes many PONs split, though in practice, some may elect to go in one chassis, density being very important. with different split ratios. Optical budgets will There are some cases in which you may need a lead to these answers, and as of this writing, less-dense solution for outlying pockets of sub- optical budgets for RFoG have not been de- scribers, and some manufacturers have accom- cided. modated this. While we show only one PON, typically many PONs feed into an area and all The RFoG upstream wavelength issue is splitters may be located at a common point interesting. One naturally gravitates to 1310 nm called a local convergence cabinet. We can as an upstream wavelength, based on wide- show that this architecture, particularly in green- spread availability of low-cost lasers and the fields, results in a very economical deployment zero-dispersion wavelength of standard cable. of equipment. Since this is the nominal zero-dispersion wave- length of the fiber, it may be possible to use Fa- After splitting, individual fibers supply bry-Perot lasers, at least for shorter distances. optical signals to the ONTs at individual homes. On the other hand, there are applications in An ONT may have one RF output that looks just which you may want to have some GPON or like the downstream signals from an HFC net- GE-PON and some RFoG ONTs on the same work, and it may have one or more data connec- network. For instance, you may want to serve tions, usually 10/100Base-T and sometimes some businesses with GPON and some nearby 1000Base-T. Also, several analog telephone residences with RFoG. Or, you may someday 2008 NCTA Technical Papers - page 165 want to upgrade from RFoG to GPON or GE- THE ONT PON. Since GPON and GE-PON use 1310 nm for upstream data transmission, you cannot put Figure 3 illustrates the Optical Network RFoG with a 1310 nm upstream on the same Terminal (ONT) at the home. In Figure 3a we PON. illustrate a fully-featured GE-PON or GPON ONT, and in Figure 3b we illustrate a possible These considerations would lead to a dif- RFoG ONT. In Figure 3a we show the optical ferent wavelength choice for RFoG upstream input to the ONT coming from a 32-way split- signaling. 1590 nm is a candidate, but the next ter, common practice today. In Figure 3b we generation of GE-PON (and perhaps GPON) has are showing a tapped architecture. While already staked out this wavelength for faster up- people deploying FTTH today tend to favor the stream. Any other wavelength that can be splitter architecture, some in the cable TV passed through the fiber with low attenuation community are leaning toward a tapped archi- could be used, so the RFoG working group may tecture. choose some other wavelength. While the lasers might be more expensive at first, presumably Experience has shown that centralizing with volume and competition the cost will drop. splitters from a common point within the net- Of course, it is more likely that DFB lasers will work and dedicating fiber to each home in a star have to be used since we are well away from the configuration provides the most cost effective zero dispersion wavelength of the fiber.