RF RETURN OPTIONS AN ENABLENCE ARTICLE WRITTEN BY JIM FARMER, CTO September, 2010 www.enablence.com
INTRODUCTION When Fiber-to-the-Home (FTTH) networks are used with an RF overlay, as is very common, an upstream path for control signals from set top boxes is usually needed. This upstream path supports video-on-demand (VoD) and other modern, high-value services. The upstream path is generically know as RF return, because the cable TV set top boxes have internal RF transmitters to send the upstream signals. The broadcast tier and RF return allows the FTTH operator to take advantage of the immense number of features that have been developed over the years for broadcast video systems. There are at least two ways to implement RF return: the first technique digitizes the appropriate upstream spectrum then sends the digitized signal to the headend as part of the normal data communications. The second technique uses an analog modulated return transmitter to send the entire upstream band to the headend. The latter technique is sometimes referred to as RFoG, though RFoG really has a related but different meaning. This paper contrasts the two ways of providing RF return A companion paper is available from the Enablence website, which describes the RF return system. Another companion paper describes RFoG, a type of FTTH network designed for compatibility with current cable TV HFC technology. ENABLENCE S TECHNOLOGY Enablence has developed a digital integrated RF return technology (patents issued and pending) that is vastly superior to the analog RF return technology being used by other vendors. It digitizes the return signal at the ONT, and transmits the resulting digital packet to the headend (or C. O.) along with all other data. At the headend, the packet is reconverted to RF in a device called an LMSR (Last Mile Subscriber Return), then supplied to the appropriate set top control system. Contrast this to the analog RF systems sold by other vendors, which use a special RF return optical module operating on a unique wavelength not in the PON standards. They sense RF coming out of the home and when they detect RF, they turn on the upstream analog transmitter to send the transmission to the headend. FEATURES OF THE ENABLENCE TECHNOLOGY Below summarizes features of the Enablence integrated digital solution. It works with every RF return solution in use today, including SCTE 55-1 (Motorola), SCTE 55-2 (Cisco), and DOCSIS (as standardized by CableLabs and SCTE in DOCSIS Set Top Gateway, or DSG). Of course, if someone wants to use IP to control the set top, this is available too. Enablence makes it easy to employ IP control channels, with multiple data-over-coax solutions. 2 P age www.enablence.com
Support for every system SCTE 55-1 SCTE 55-2 DOCSIS (e.g., DSG) IP (Ethernet) return Pick any combination you need Multiple solutions in any one ONT Download firmware from EMS If any future return protocols are developed, Enablence will support them with downloadable firmware You may pick any combination of RF return technologies you want, because Enablence is capable of supporting multiple technologies simultaneously. The processing is all done in firmware, so we can adapt to any new technology that may come along by simply downloading new software to the ONT. No need to visit homes, no need to make any physical changes at the subscriber s location. So what new RF return technologies may come along in the future? It is highly unlikely that any will come along. The existing standards account for 100% of systems on the market today. Any new system introduced would almost certainly result in commercial suicide for the manufacturer, as these existing standards are so ingrained in the industry, and run well ahead of any conceivable demand. In particular, DSG is a new technology developed and standardized by CableLabs and SCTE, that encapsulates either SCTE 55-1 or 55-2 into DOCSIS packets. It is envisioned for use in higher-end set tops, though due cost it is unlikely to penetrate into lower-cost set tops, which can implement SCTE 55-1 or -2 more economically. DSG is an integral part of a new standard suite known as Tru2Way, which codifies a number of SCTE standards in defining features that can be installed in TVs as well as set tops, so that set tops will not be needed with TVs that have incorporated Tru2Way technology. The first TVs to incorporate Tru2Way are expected to be on the market by the end of 2008. An upgraded set top control system will be needed at the headend to support them. If any other control protocols are ever developed, they will almost certainly be based on IP, which is globally accepted as the mechanism of choice. Enablence supports all variations of IP service, and provides integral data over coax support to facilitate connecting set tops to the data plane in our GPON and GE-PON FTTH systems. In the unlikely case that future RF-based transport technologies are developed, we can download new firmware to our ONTs to support it. THE SUPERIORITY OF THE ENABLENCE APPROACH Figure 2 summarizes some of the reasons that the Enablence integrated digital optical approach is superior to the analog separate wavelength approach used by other vendors. First and foremost, it is lower in cost. Other techniques require the use of a separate upstream laser located at every ONT. The laser cannot operate near the zero-dispersion 3 P age www.enablence.com
wavelength of the fiber (about 1310 nm) because that wavelength is already in use for the digital upstream. So another wavelength must be found, usually between about 1590 and 1610 nm. Because this wavelength is so far from the zero dispersion wavelength of the fiber, a DFB laser is almost certainly required, even for short paths. The DFB laser itself is relatively expensive. Furthermore, in many cases, the upstream transmitter is an add-on physical package, not necessarily even manufactured by the FTTH vendor. This adds cost to the device, and also adds labor cost to connecting the system. Why it is better: - Lower cost - Easier system design - No increased loss budget - Combine thousands of homes in one return path for headend efficiency - You are free to place the set top management system anywhere in your network you like, without having to worry about getting RF return signals to it. - So long as you have IP connectivity, you are good to go - Completely future-proof - Download software for new systems if they are ever developed - No wasted upstream spectrum o Why pay dearly for what you'll never use? o Supporting the entire upstream band costs dollars, cabling complexity, power, system design and maintenance complexity - Digital precision, home to headend no level maintenance worries Because the Enablence system is integrated with the digital upstream channel, there is no extra laser, no extra packaging. System design is easier, first because you don t have to deal with another wavelength and its loss budget. The set top management system may be located wherever you wish, without regard for whether or not you can get the upstream RF/optical signal to that location anywhere you can get IP connectivity you can locate the control system. The RF return packets in the Enablence system come back along with all other IP packets, so connecting into the control system is easy no mater where it is located. With analog return systems, you are limited in the number of return transmitters you can combine to one upstream receiver in the headend. This limitation arises from the combining loss you will suffer. With luck, you may be able to combine the outputs of a few hundred ONTs before you have to provide another headend optical receiver and you will need another WDM for each PON. These requirements can create additional cost and fiber management headaches in the headend. With the Wave7 system, you don t have any extra WDMs, and you can effectively combine the outputs of thousands of ONTs, to at least the same density as a cable operator would use you are limited by the capacity of the set top upstream system, not by optical loss budgets. This saves a lot of cost and complexity in the headend. You are completely future-proofed with the Enablence system. All functionality is contained in firmware (in a field programmable gate array FPGA to be precise). In the unlikely event a new system appears on the market, Enablence will simply develop the supporting software and download it to your ONTs. Sending the entire 5-42 MHz RF return spectrum back to the headend is wasteful and expensive. Cable TV systems use most of that spectrum for DOCSIS data transmission. DOCSIS is very limited, compared with FTTH, in the amount of data that can be returned, 4 P age www.enablence.com
even using the most optimistic DOCSIS 3.0 profiles. So it is pointless to spend effort and money to return this spectrum when it does no good. Finally, the Enablence technology is all-digital, and this offers several advantages over the analog return technology being used by others. The RF signal level of upstream set top transmitters in the home is moderately critical: demand too much level and the set top will not be able to deliver, and/or you may overload the upstream transmitter. Demand too low a signal level and you may suffer degraded performance due to noise and/or spurious signals generated in the home. All upstream RF transmitters in set tops are controlled through a long-loop AGC, whereby the signal level is measured in the headend, and a correction is sent to the transmitter at the home, telling it to increase or decrease its level until the level measured in the headend is correct. The signal level measured in the headend is not only a function of the signal level entering the ONT at the home, but is also a function of the optical level arriving at the headend optical receiver. This in turn is a function not only of the output from the ONT optical transmitter, but also of the loss in the network at the upstream wavelength. Because of the way optics work, the RF level change is twice the optical level change. This means that you will need to control the optical loss budget at the upstream wavelength fairly carefully in order to keep the RF transmitters at the home within their operating range. You can do it, but doing so does represent more work. On the other hand, the Enablence approach, being all-digital, affords you very high precision in setting up your return system. You can keep the signal levels at the home high (to help overcome any in-home generated noise), while not fearing that the long-loop AGE will either drive the set tops out of their range of output levels, or into transmitter overload. SUMMARY In summary, Enablence s approach to RF return is based on our many years experience with cable TV systems and their needs. Our digital RF return technology has been in use since 2002, with more units in the field than all other vendors combined. We support any of the existing standards, including the new Tru2way standard from CableLabs, which may see use in TVs late this year (though it will likely be much longer before there is a pressing need to support it). Any new standards that may be developed in the future are likely to be IP-based, so we provide native support for them. If any future RF-based standards are developed we will support them with downloadable software. However, the likelihood of new standards is low, given that the existing standards are licensable, and to go outside of them would be commercial suicide for any vendor. Enablence supports all of the RF return standards, and does it in a most friendly, economical manner. We digitize the RF return at the home, and put the digitized data in a return packet to the headend. The bandwidth required is negligible, because we only require it (typically one packet) when a set top is actually sending an upstream transmission. 5 P age www.enablence.com
Supports any conceivable future need - CableLabs new standard Tru2Way - Being embraced by TV manufacturers - DSG-based - Other set top manufacturers have licensed SCTE 55-1 or -2 - Any new applications will certainly use IP (Ethernet), the worldwide standard for data communications -To not do one of the above is commercial suicide - Enablence supports them all! For more information visit www.enablence.com 2010 Enablence Technologies Inc. The information presented is subject to change without notice. Enablence Technologies Inc. assumes no responsibility for changes or inaccuracies contained herein. Copyright 2010 Enablence Technologies Inc. All rights reserved. 6 P age www.enablence.com