RFoG (DPON) Tutorial

RF over Glass (RFoG) replaces a Hybrid Fiber/Coax (HFC) access network with a passive optical network (PON). It is one form of what is known as DOCSIS PON or DPON (another technology also known as DPON is DOCSIS over EPON, which is being marketed by Salira). With RFoG, the HFC head end access fiber multiplexer is replaced by a WDM device that divides the upstream and downstream wavelengths and delivers them on separate fibers to other devices in the headend. With RFoG, fiber splitters, instead of HFC nodes, are deployed to serve neighborhoods. A fiber NID is installed at each house. So the RFoG architecture in the outside plant (OSP) is identical to that of a PON. The image below (click to enlarge) provides an architectural overview of a traditional HFC network.

The image below provides an overview of an RFoG implementation. Note that Set Top Boxes (STBs) within the home can remain unchanged in a shift from traditional HFC to RFoG.

With RFoG, downstream traffic is typically transmitted at 1550nm, and upstream traffic (with each STB transmitting in turn) is typically carried at 1310nm, 1590nm, or 1610nm. If 1310nm is used for the upstream, no upgrade path to GPON or most other PON technologies is possible since they use 1310nm for upstream transmission.

The RFoG form of DPON allows MSOs to use their existing operational infrastructure and much of their existing equipment with the new access technology. RFoG is compatible with most of the MSO headend gear (except for the HFC fiber mux), and it is compatible with the in-home gear (including all those STBs) currently in use with HFC networks. The RFoG ONU on the side of the house generates/receives RF coax signals identical to those used with HFC. The operational systems that tend the headend and the CPE with an HFC access network work just fine when RFoG is used instead.

Standards work is ongoing for RFoG in the Society of Cable Telecommunications Engineers (SCTE) subcommittee IP SP 91. The document they are developing is entitled RFoG System Overview.

So, what are the major advantages of RFoG?

1. RFoG provides better reach and requires much less equipment in the access network. From the HFC node, coax requires repeaters every 1000 feet or so. RFoG does not require an active piece of gear be installed in the access network, only passive splitters, and can reach as far as 20km.
2. RFoG is green. It does not require an active node or repeaters, so it avoids the power consumption of these devices.
3. With RFoG, the ultimate signal delivered to the subscriber is better because optical transmission is impervious to electromagnetic interference, especially important in the return path.
4. The RF bandwidth provided in an RFoG network is greater than what is provided by HFC. This allows a provider to deliver services above 1 GHz, even another complete channel lineup.
5. Since it requires no access network electronics, RFoG imposes much less of a hardware maintenance requirement than does HFC. Additionally, fiber is much better than coax at handling changes in temperature, humidity, and other environmental conditions further reducing the operational burden.
6. RFoG, if deployed with compatible wavelengths, can easily be upgraded to work with GPON or other PON technologies to provide tremendous data bandwidth.

RFoG is a smart way for carriers to deploy a simple, proven RF video solution along with high bandwidth PON services. The diagram below (click to enlarge for detail) shows what a combined PON and RFoG implementation looks like.

One of the largest advantages of RFoG is that it provides better service at a lower operational cost than comparable HFC networks.

For further reading, Broadband Properties has this article describing what Cable companies are doing with fiber. Cisco offers this whitepaper describing their RFoG+ solution. Tom Anderson of Alloptic wrote this article on Why RFoG?.  Motorola offers this whitepaper describing their RFoG solution.

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

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1. Raffaello Marcionetti says:

   November 22, 2009 at 9:14 am

   It seems that the RFoG burst transmission is not compatible with the doscis 3.0 return path bonding. Do you have some news about this problem ?

   Thank you
2. Srikanth says:

   December 20, 2009 at 11:03 am

   I went through the RFoG whitepaper by Motorola. The architecture has a video controller connected to the CMTS. Can somebody explain me what exactly is the purpose of that?