Cable operators are starting to explore a variety of technologies for moving fiber closer to homes and businesses using Passive Optical Networks (PONs). Analog PON technologies like RF over Glass (RFoG) promise to increase bandwidth while keeping operational costs down. Meanwhile, digital PON technologies (including Ethernet PON [EPON] and Gigabit PON [GPON]) are being used to provide high-bandwidth commercial services.
“If cable operators can put money on construction into fiber instead of coax, it will have a longer service live and more long-term value,” said Bill Dawson, business development manager at Arris.
PON networks also promise numerous operational advantages, added Roger Hughes, senior operations engineer at Armstrong Cable. They eliminate RF amplifiers, which reduces the need for sweeps and command line interface (CLI) testing. Furthermore, the fiber plant is less impacted by thunderstorms because the lightning does not affect the fiber. During thunderstorms, the technicians end up spending time replacing fuses, which is not an issue in the fiber plant.
Almost everyone seems to agree that fiber will continue to be pushed closer to the customer. Jeff Heynen, directing analyst with Infonetics Research, believes operators will have to weigh the tradeoffs between running fiber deeper into the neighborhood to reduce coax splits and running it all the way to the home.
“Operators will push more fiber out to then optical nodes to split them logically and physically so you can reduce the number of homes supported per node,” he explained. “There is also a way of using PON as an aggregating device.”
In the short run, Heynen expects smaller cable operators, particularly in rural areas, to adopt RFoG because they don’t want to disrupt their heavy investment in set-top boxes. Digital PON technologies like EPON and GPON would require residential service providers to move to Internet-protocol (IP)based set-top boxes. On the other hand, PON is an attractive technology for telcos planning to offer triple-play services.
Bringing Fiber Home
RFoG is a class of PON in which a single analog signal is transmitted over a fiber-optic network directly to multiple homes. It uses the same types of protocols found in traditional hybrid fiber coax (HFC) system, only the optoelectronic (OE) conversion is done on the side of the house rather than inside the network; a single optical signal is shared among multiple homes. A smaller number of homes share a return path to the headend. Consequently, in a RFoG network, one broadcast fiber is shared by a large number of homes, while a return fiber is shared by a smaller number of homes (between 32 and 128, depending on service requirements).
RFoG systems cost less to build in rural areas than do traditional HFC networks. They also promise to reduce maintenance and powering costs greatly for rural and urban network operators over time. Once the fiber for RFoG is in place, it can be shared or reused for other types of networks in the future.
As such, subdivisions that would have been cost-prohibitive to build out in the past now can be justified with fiber. For example, Buckeye Cable is building RFoG out to areas with only 15 to 30 homes per mile. “It has been expensive to build out there, so they have not had have good cable-TV service,” said Joe Jensen, Buckeye’s CTO.
Armstrong Cable has used RFoG for all of its new networks during the last couple of years. The Armstrong network uses equipment from Aurora Networks for optical transport, All Optic’s ONTs on the side of the house, and splice enclosures from Preformed Line Products. The Armstrong network currently passes some 12,000 homes, of which about half already are on the RFoG network.
According to Hughes, RFoG deployments can be profitable in rural areas with as few as 15 homes passed per mile. “In general, RFoG networks cost between 10-percent and 15-percent less to build per mile. Furthermore, there’s no CLI or sweeping of the amplifiers,” he said. “This means that we expand the network without adding technicians to maintain it.’
The Society of Cable Telecommunications Engineers (SCTE) still is debating the emerging RFoG standard (SCTE IPS SP 910); finalization of the standard will help bring prices down, Hughes said. He expects two separate standards to be developed, one designed to coexist with EPON services using more expensive optics and a simpler approach that would not allow an RFoG network to share the fiber as easily. A RFoG-only network could use cheaper 1310 nm lasers for the return; the use of a more expensive 1610 nm laser would allow the same fiber to be shared with an EPON network.
Fiber to the Business
Digital PON technology like EPON and GPON are used by telcos to provide triple-play services. Cable operators in general are using these technologies today only to provide business services. EPON has been more widely used in Asia, but many believe it will gain traction in the United States, particularly with cable operators because of ease of integration with DOCSIS.
The data rate described for both EPON and GPON systems is shared by all of the subscribers on a network. For example, on a 1 gigabit/s EPON, each of 32 subscribers on a loop only would have access to 31.25 megabit/s.
The telco set of PON technologies include ATM PON (APON), broadband PON (BPON), GPON and 10G-GPON. These have seen the most traction domestically, such as Verizon’s deployment of GPON. The technology also is used widely by cable operators to provide business-networking services, but there are challenges with integrating GPON management and provisioning systems with DOCSIS, which has limited its use for wider deployments. Cable operators using GPON have to manage these services separately, unlike traditional HFC-based services. The IEEE-based EPON and its successor, 10G-EPON, are starting to attract interest from cable operators, driven in part by the development of tools and standards to provide better integration with DOCSIS.
Although Buckeye is just getting into RFoG, it is currently on its third-generation deployment of GPON for commercial services. When these services were launched five years ago, GPON was the most practical technology for reaching business subscribers with high-bandwidth services in a cost-effective manner, said Jensen. Using equipment from Calix, Buckeye has deployed GPON across about 70 percent of its hubs to hundreds of business customers.
Jensen said the GPON equipment is reliable and allows Buckeye to deliver a tailored Ethernet pipe to customers, but one of the downsides is that it has to use a separate management-and-billing system for the GPON services. “We do have some nice provisioning capabilities on the DOCSIS side that we don’t have on the PON side,” Jensen explained, “but, at this point, we are a small-enough operation that full automation is not an issue for us.”
If EPON had been available earlier, Buckeye may have chosen a different path. “I have an inherent distaste for ATM. If EPON had been available, it would have been the preferred solution,” Jensen said. “We have not seen fit to change it. We are contemplating on when we could make that transition, but we have an installed base that is strong. We have a lot of legacy equipment that we would like to get the return on our investment from.”
There are several different ways of splitting out the fiber in a PON or RFoG network. A PON hub typically is configured to support 32 homes. Although it is possible to run a single fiber from this hub to each node, many operators use a more distributed approach in which multiple splitters are deployed closer to the home. At the extreme, equipment makers like CommScope are developing systems that allow operators to deploy networks with the same kind of split characteristics as coax networks.
Buckeye chose this technology in its RFoG retrofit project in a neighborhood with buried cable. This project calls for retrofitting the existing HFC plant installed underground throughout a subdivision with 15,000 feet of cable. The project is using Alloptics transceivers and networking equipment from CommScope BrightPath.
The cable provider also liked the CommScope BrightPath technology because it allowed the operator to mimic the layout of the existing coax network with taps and splitters. With this layout, a main distribution cable runs down the street, which is connected to a series of 2-, 4- or 8-port taps in line with the distribution architecture.
Carl Meyerhoefer, vice president/marketing at CommScope, said, “This architecture has similar tap values as splits in the HFC world. From the installation-and-design standpoint, the layout looks and smells like a HFC network, which makes it intuitive for cable operators to understands and deploy.”
Other operators used EPON from the beginning. For example, Chesterfield, Mo.-based Broadstripe, with some 32,000 subscribers, launched a commercial data service over EPON about a year ago to an industrial park with 34 tenants. Fiber-optic cables were built from an Aurora VHub to each building in the park. Broadstripe already had dark fiber at the main street outside of the park, and all it had to do was run the fiber through the park.
“EPON gave us the most bang for our buck in terms of providing what the industrial park and the city were looking for,” said Dave Harwood, Broadstripe’s general manager. “But the technology only makes sense in areas where there is a concentrated need for high bandwidth. EPON is something that only works if you have enough business customers in a group. Otherwise, it is not cost-effective today.”
While PON has been gathering a lot of momentum, some early PON operators are moving toward active networks. For example, CT Communications, a triple-play provider with about 10,000 subscribers near Urbana, Ohio, has deployed a fairly substantial BPON network. It recently made a transition to an active network using Allied Telesis equipment.
Tim Bolander, director of network operations at CT Communications, noted, “The amount of bandwidth we can offer is not comparable with passive. We are offering 100 megabit/s now and could offer 1 gigabit/s if we needed to.”
Making that kind of transition is more difficult with PON, and Bolander believes a PON network requires more labor. The active network also is more user-friendly to monitor. When a problem occurs, it’s easy to identify the source when every customer has his or her own dedicated link, Bolander added.
Active networks use a single point-to-point fiber-optic link between the service provider and the customer. They are called active networks because an OEO switch can be used in the field to manage the connections with multiple homes or businesses. Because the line is not shared, the electronics and lasers can be cheaper than in PON networks.
Traditionally, a telco would deploy a single high-speed link to an OEO node in the field, which would manage the connections electrically with multiple customers. However, many operators are starting to build networks that blur the line between passive and active technologies by moving these OEO nodes into the central office and running a single dedicated fiber from the central office. In essence, they are building a PON using lower-cost active ONTs and ONUs.
Proponents of this approach argue that the biggest capital expense is the construction cost rather than the extra fiber. Each link can run as far as 80 kilometers. Furthermore, these types of networks can use less-costly transceivers than PONs. Phil Jopa, CTO at Allied Telesis, said PON chips are two or three times more expensive than active chips because they need special lasers and receivers for sharing the optical channel, while active systems don’t.
The latest trend is for operators to use dedicated point-to-point connections. Juan Vela, director/strategic products and markets at Occam, explained, “We are seeing the splitter is being designed closer and closer to the central office or headend so that if an operator wants to take the splitter out, they can replace it with active equipment.”
As noted previously, most of the cost of new builds is in construction rather than in the cable itself. When installing fiber, it costs the same to trench 144 fibers as it does to trench 12. However, some cable operators are skeptical about the operational issues associated with having to manage so many fibers. As Armstrong’s Roger Hughes noted, “I don’t like going directly to each home because, when you have a fiber cut, you have a lot of fiber to fix. In my model, there are only four to six active fibers to each home. In the event of a break, the technician can be in and out in a few minutes, but a cable with dedicated fiber for each home would take days to replace.”
A little bit of upfront planning can greatly improve the maintenance process in the long haul. For example, Hughes said his company spent a considerable amount of time and research developing a fiber plan that simplifies maintenance and makes it easy to restore service in the event of a fiber cut.
Armstrong has standardized on 12-, 24- and 36-count fiber, with some 60 percent of the Armstrong plant being 12-count fiber to help simplify ordering and inventory. Hughes said this makes sense even though the company only is using a fraction of the fibers strung between poles. According to Hughes, Armstrong wanted to have plenty of extra fiber to expand when required.
The cables were segmented in the order in which they were to be spliced. The main feed fibers were replaced first, followed by the distribution fibers. “We’ve come up with a way where you don’t need a written record at the fiber because you can go through them in a sequence,” Hughes said. “The first fibers carry the most traffic, while the last one will be single-customer. We designed this process with operations in mind so you don’t have to resort to operations sheets in the event of a break.”
Individual optical fibers are stored in a colored plastic buffer tube, with as many as 12 fibers carried in one blue, orange or green buffer tube. The blue buffer tube is for distribution fiber, used only for 12-count fiber; it is used to feed the couplers. The orange tube provides feeds from facilities-based locations; it goes back to the headend or it provides a return path to the facilities.
Cable operators also need to think about developing a wavelength plan as they begin to place multiple optical signals on a single fiber, recommended Bill Dawson, business development manager at Arris; it’s important to think through how to stack services like RFoG and EPON together. “This plan creates a framework for how to deploy wavelengths in the long run to get the most out of the fiber plant,” he said.
While the use of PON technologies promises to help cable operators shave their power bills, at the same time, it raises some new challenges. Arris’ Dawson said, “Powering RFoG is a good news/bad news story. You don’t have to pay the cost of power, but you do have to offer power integrity even if the power goes out.” Cable operators need to take into account a few power considerations with the transition to PON for powering the PON hubs and powering the electronics on the home.
One of the nice elements of PONs is that the equipment in the field requires significantly less power. The networks typically are built with an optical amplifier that can be mounted on a pole and requires some power. This can sometimes be powered by the existing coax plant. For example, when Broadstripe rolled out G-EPON services to a business park, it was able to mount the equipment on the pole with a module that is powered over the 60-volt AC current carried on the HFC network. A traditional power supply with 120-volt power would have required an electrician, a meter and a permit.
Al Humphrey, a regional engineer for Broadstripe, said, “We did not have to wait for permitting and the installation of an active OTN. With the traditional network design, we would have had to build a larger cabinet and install active equipment that required AC in the park. Part of our cost advantage came from not needing those permits and not having to build an active OTN.”
With PON, operators eliminate the need to power the network over the coax, but this creates a need to deliver power to the device mounted on the side of the home. Unless the box is installed in a garage with a power outlet, operators typically have to install a power unit inside the home and then transmit power to the side of the house over coax, Ethernet or twisted pair wires. If the network is used for telephone service, a UPS has to be built into the power supply to keep the phones working in the event of a power outage.
As an example, for its first trial, Buckeye is powering the optical nodes with a UPS and power inserter from APC that can provide an eight-hour backup for the device in the home. Armstrong uses the Alpha FlexPoint to provide better backup and power on the coax.
George Lawton is a frequent CT contributor. Contact him at firstname.lastname@example.org.