RFoG, or Radio Frequency over Glass, carries traditional RF and DOCSIS services over fiber instead of long coaxial runs. It can support FTTH and FTTB deployments, work as a standalone point-to-multipoint architecture or overlay an existing PON/GPON, and extend network reach while retaining compatible headend and customer-premises equipment.
Key Takeaways
- RFoG replaces long coaxial distribution runs with single-mode fiber while continuing to transport traditional RF services.
- RFoG can support fiber-to-the-home (FTTH) and fiber-to-the-business (FTTB) deployments using existing DOCSIS infrastructure.
- It can operate as a standalone point-to-multipoint system or as an optical overlay on an existing PON/GPON network.
- Passive and hybrid RFoG architectures allow operators to balance split ratios, reach and network performance.
- Optical beat interference (OBI) can occur when similar optical transmitters operate simultaneously, making OBI mitigation an important RFoG design consideration.
Many times, we speak with customers who ask us what RFoG is. We explain that in many cases the losses and potential interference with sending RF signals over long runs of coaxial cable result in inadequate hybrid fiber-coax (HFC) network performance. The reason is that the losses and interference susceptibility of the RF coaxial cables used to keep network infrastructure costs reasonable are much greater than those of glass fiber cables or even higher quality coaxial cables.
For a related explanation of common optical-link configurations, see our guide to duplex fiber and how it differs from single-fiber transmission.
What Is RFoG?
Radio frequency over glass (RF over glass, or RFoG) is a way to deliver RF signals over fiber instead of coaxial cable. A solution to the coax losses described above arrived when the Society of Cable Telecommunication Engineers (SCTE) published the specification for RF over glass fiber in 2010. It enabled single mono-mode fiber to be used instead of coaxial cable for long runs within a Passive Optical Network (PON) or Gigabit Passive Optical Network (GPON).
An RFoG network replaces the coaxial distribution segment of an HFC network with fiber, without necessarily replacing compatible DOCSIS, headend or customer equipment. This benefits the delivery of internet and other communication services to homes and businesses, because RFoG can use the existing fiber optic network with RF modulated signals to provide downstream bandwidth up to 1 GHz. This technology allows fiber-to-the-home (FTTH) or fiber-to-the-business (FTTB) delivery of communication services using the DOCSIS network, with RFoG transmitter and receiver nodes providing distance coverage that depends on the equipment and design. Some nodes are specified for coverage of up to 50 km.
How Does RFoG Work?
An RFoG architecture carries signals from the optical hub, through the optical distribution network (the fiber and splitters), to an R-ONU (RFoG optical network unit) at the subscriber location.
In the downstream direction, the R-ONU converts the optical signal back to RF so it can be delivered to the customer’s equipment. In the upstream direction, it converts RF signals from the customer into optical bursts sent back toward the hub. This lets the RFoG network keep using RF-based services over a fiber path.
RFoG is a network architecture, unlike active optical cables, which integrate optical conversion components directly into a cable assembly.
RFoG Wavelengths and Signal Direction
RFoG wavelengths are defined by direction. Downstream transmission uses 1550 nm. Upstream transmission uses 1610 nm, or 1310 nm with deployment caveats, such as possible overlap with other services that use the same wavelength on the fiber.
These values are optical wavelengths, not frequencies. Wavelength planning matters because the downstream and upstream signals, and any other services on the same fiber, need to be kept separate.
Standalone RFoG and PON Overlay Architectures
RFoG may be used as a standalone point-to-multipoint (P2M) system, or as an optical overlay for an existing PON/GPON that coexists with PON services through wavelength separation.
RFoG and GPON are different technologies. RFoG transports RF signals over fiber, while GPON is a separate passive optical network technology, and the two can share the same fiber plant in an overlay design. The RFoG standard is also backwards compatible with some previously existing RF modulation technology, which enables the reuse of existing headend or customer premise equipment (CPE).
Passive vs Hybrid RFoG Networks
An RFoG network may take one of two architecture types: passive or hybrid.
A passive RFoG design has no active devices in the plant. One commonly cited example is a 32-way split over the 20 km typical of a PON, but these are examples rather than universal limits.
A hybrid RFoG design may use active field optics to manage loss, preserve performance, extend the reach of the fiber or support additional splits. In both cases, the RFoG split ratio and RFoG reach depend on each other: depending on the split ratio, a system can reach further, or allow for more splits at a lower range. Actual limits depend on the optical budget and the equipment used.
Benefits and Limitations of RFoG
Benefits
- Extends fiber toward the subscriber while retaining compatible cable-network infrastructure, including DOCSIS, headend and CPE.
- Avoids the losses and interference susceptibility of long coaxial runs.
- Supports FTTH and FTTB delivery, with downstream bandwidth up to 1 GHz.
- Can be deployed standalone or as an overlay on an existing PON/GPON.
Limitations and design considerations
- Optical budget: Splits, fiber length and connectors all consume the available loss budget.
- Wavelength planning: Upstream and downstream wavelengths must be chosen and kept separate from other services.
- Split ratio: More splits typically mean less reach, so the balance has to be designed for each network.
- Upstream interference: Upstream transmissions can interfere with one another, as described below.
What Is Optical Beat Interference?
RFoG networks have historically suffered from optical beat interference (OBI). OBI can occur when multiple upstream R-ONU transmitters send at the same time at the same or sufficiently close optical wavelengths. This is a statistical probability event, and the more active users on a plant, the greater the probability of OBI. It affects upstream performance.
Though a concern, there are a number of OBI-free RFoG systems available that eliminate this risk, which supports wider rollout of RFoG technology. These OBI-mitigating designs coordinate how RFoG transmitters send upstream signals or keep their wavelengths sufficiently separated.
Frequently Asked Questions About RFoG
What Is RFoG (Radio Frequency over Glass)?
RFoG carries RF and DOCSIS services over fiber instead of coaxial cable. It replaces the coax distribution segment of an HFC network while allowing compatible headend and customer equipment to be reused.
How Does RFoG Work?
Signals travel from the optical hub through the optical distribution network to an R-ONU. The R-ONU converts downstream optical signals to RF and upstream RF signals to optical bursts.
What Is the Difference Between RFoG and GPON?
They are different technologies. RFoG transports RF signals over fiber, while GPON is a separate passive optical network technology. RFoG can be deployed standalone or as an overlay on a PON/GPON.
What Are the Benefits of RFoG Compared With Coaxial Cable?
RFoG avoids the losses and interference susceptibility of long coaxial runs. It extends fiber while keeping compatible cable-network infrastructure in place.
What Is Optical Beat Interference (OBI) in an RFoG Network?
OBI can occur when multiple upstream transmitters send at the same time at sufficiently close optical wavelengths. It can degrade upstream performance, and OBI-free designs are available to mitigate it.