RF over glass or RFoG, is a technology receiving considerable attention as an access network solution to deliver RF-based services using a fiber-optic network. Both MSOs and telcos are using RFoG to increase usable spectrum (both upstream and downstream) for additional services, to optimize network bandwidth by separating video from data and voice services, and to increase the competitiveness of their networks through increased reliability and lower operating costs.

It is also emerging as a strong competitor to the more traditional in-building "5-Wire" Master Antenna television (MATV) systems and community access television (CATV) networks. While not a typical triple play FTTH solution, I am writing about this technology because it is basic Fibre to the Home that offers a painless and cost effective upgrade path to a full triple play solution.

Tom Anderson wrote an article for Broadband Gear on the technology:

What is RFoG compared to HFC?

RFoG is a deep fiber network in which the coax portion of the HFC network is replaced by a single-fiber passive optical architecture as shown in Figure 1 below. Downstream and upstream transmission uses different wavelengths to share the same fiber, typically 1,550 nm downstream and either 1,310 nm or 1,590 nm for the return path. Using 1,590 nm in the upstream direction allows the fiber infrastructure to simultaneously support a standards-based PON system, which operate with 1,490 nm downstream and 1,310 nm upstream wavelengths.

One of the most attractive aspects of RFoG is that it is compatible with the existing RF/DOCSIS/HFC network. RFoG works with the same:

* CPE - Set-tops, cable modems and EMTAs
* OSS/BSS systems
* Headend/hub equipment - Laser transmitters, EDFAs, return path receivers and CMTS

RFoG delivers the same services as an RF/DOCSIS/HFC network, with the added benefit of improved noise performance and increased usable RF spectrum in both the upstream and downstream.

Both RFoG and HFC systems can concurrently operate out of the same headend/hub. That makes RFoG an ideal solution for node-splitting and capacity increases on the existing network.

If Everything Is the Same, Why Deploy RFoG?

While service delivery methods and equipment are the same, the RFoG network has several advantages that include:

More downstream spectrum. RFoG systems support 1 GHz and beyond. This added spectrum can be used as the network operator chooses for added video channels, VOD/PPV services, data bandwidth and so forth.

More upstream bandwidth. Because of RFoG's improved noise characteristics (described below), the full 5-42 MHz return path spectrum can be used for data. Additionally, better RFoG systems not only support DOCSIS 3.0 with bonding, but also the low noise floor enables 64-QAM upstream transmission, dramatically increasing return path bandwidth.

Less maintenance. RFoG networks do not have active electronics such as nodes and amplifiers between the headend and the subscriber location. That means leakage and sweep testing is eliminated in the outside plant (OSP). It also means that OSP powering is not needed for the RFoG network. AC power, battery backup, emergency generators and all the associated truck rolls are eliminated for the RFoG portion of the network. It should be noted that RFoG transceivers are powered from the subscribers' site. Optional battery backup, if used, is inexpensive and can be maintained by the subscriber similar to the way in which alarm system batteries are maintained.

Along with the advantages of eliminating the OSP active electronics, the all-fiber RFoG network has another benefit: Fiber is more reliable than copper-based networks. Humidity, temperature, lightning, galvanic corrosion and other conditions all take their toll on coax and twisted-pair networks over time. Fiber, while not indestructible, is inherently immune from metallic problems. Alloptic's customers, who have been deploying RFoG for several years, are reporting decreases in maintenance rates of 90% and more compared to their HFC networks.

RFoG networks support other technologies. An advantage of RFoG networks is that its architecture supports technologies that can deliver additional services. The single-fiber passive optical network used by RFoG is the same as PON systems, whether BPON, GPON, GEPON, 10GEPON, or even DPON. By using the same fiber infrastructure a PON system can overlay the RFoG network, enabling advanced business services with gigabit bandwidth and rich Ethernet capabilities. Figure 2 illustrates how this is accomplished without disrupting existing services or subscribers.

How Is the RFoG Transmission Controlled to Prevent Collisions?

No new controls are needed when RFoG networks are deployed.

Downstream, control of the RFoG equipment is not required to deliver services. Services are broadcast through the RFoG network just as they are in a traditional HFC network. Instead of converting from optical to electrical RF in the node, the O-E conversion takes place where the RFoG transceiver is located, typically at or near the subscriber's location. RFoG transceivers are transparent to modulation techniques such as QAM or QPSK.

In the return path, RFoG transceivers operate in burst mode. With RFoG, the CMTS controls the cable modem in exactly the same way it does in an HFC network, allowing only one to transmit at any given time. In fact, the CPE controls when the RFoG device is transmitting. The RFoG transceiver detects RF transmission from the CPE and immediately turns the reverse path laser "on". When RF from the CPE stops, the laser turns "off." "Collisions" - two lasers being "on" simultaneously - are avoided because the lasers are extremely responsive. Laser on/off times are critical, requiring 1.6?sec operation to support DOCSIS 3.0 with channel bonding.

How Does RFoG Achieve Better Noise Performance Than HFC?

There are three ways RFoG has lower noise operation than HFC. First, RFoG networks have inherently lower noise. As shown in Figure 3 for return path transmission, an HFC network amplifies noise along with the signal level at each amplifier. An RFoG fiber network only has a single amplifier, maintaining the same signal-to-noise ratio (SNR) from end-to-end.

Second, the return path or upstream burst mode described earlier is key to exceptional noise performance in RFoG networks. Only one RFoG device transmitting on the network at any given time - when the CPE device is active - lowers ingress noise significantly. Non-transmitting transceivers squelch ingress noise.

Typically, the noise reduction is enough to reclaim the lower 10 MHz of the reverse path spectrum that is lost to ingress noise. In a 5-42 MHz return spectrum, this increases upstream spectrum by 27%.

There is another noise improvement with the advent of RFoG technology. Return path receivers that make use of low noise optical circuits have been optimized for RFoG networks. Combined with RFoG transceivers, these receivers are delivering return path noise performance more than 4 dB better than traditional HFC networks.

The low noise benefit can be used in several ways. For example, it enables 64-QAM rather than 16-QAM in the upstream direction, increasing bandwidth from 27 Mbps to 38 Mbps per 6 MHz channel. Another use is to increase the distance reached by the optical network.

RFoG is a relatively new technology that delivers a number of immediate benefits to network operators. In addition to the financial advantages, network technical performance is improved, giving engineers new tools to increase the competitive advantages of their networks.

Tom Anderson is the director of product marketing for Alloptic, an optical access solution provider headquartered in Livermore, CA.


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