While technology has improved, installing fiber in the last mile is problematic, particularly in the last 1,000 ft.
By Mitch Shapiro and Don Gall
During the past few years there have been several proposals, trials, and an occasional limited deployment of fiber-to-the-home (FTTH) network architectures. Most of these have had successes that could be pointed to but, because of economic and/or technical reasons, have not blossomed into the mainstream of deployable networks.
One reason for the "FTTH Blues" is that fiber optics has yet to make the transition from the "high-tech" headend and central-office environment to the blue collar, harsh reality of the "3rd rock from the sun" back easement conditions.
It's not the last mile, but the last 1,000 ft that presents the real problem. Transport fiber optics routed along major roads is less likely to be disturbed and is much easier to access than fiber running through the backyards of suburbia. This is the land of garden tillers, Dobermans, and 12-year-old kids with all kinds of time on their hands. Couple this with rain, sleet, snow, and wind, using only equipment you can lift over the backyard fence in the middle of the night . . . well, you get the idea.
Fiber management also becomes a contributing factor. We have seen FTTH network proposals that range from a dedicated pair of fibers from the central office (headend) to every individual household to as many as 768 households sharing a single strand. In the first case, a serving area of 20,000 passing would need to terminate a minimum of 40,000 fibers without any spares. When DWDM and time-division multiplexing (TDM) techniques are considered, this number drops to reasonable levels.
All these FTTH networks have varying degrees of difficulty in the last 1,000 ft. A typical utility easement passes four homes every 150 ft. Traditional methods using expensive, labor-intensive splice housings is not the answer. Running individual drops from a central location can quickly becomes a rat's nest, and we have our doubts as to the long-term operational management of these schemes. What is needed is an approach that is elegant, cost-effective, and maintainable.
Fiber-optic connectors are possibly the greatest hurdle to practical deployment of FTTH. The most reliable fiber-optic connection is a fusion splice. It is strong, permanent, and adds very little to the overall network optical power requirements. Unfortunately, it becomes impractical to install and maintain in the last 1000-ft environment. Even the least expensive fusion splicers cost over $2,000 each. They are also relatively fragile and need a controlled environment to achieve repeatable results. Both twisted-pair and coaxial cables, on the other hand, can be connectorized in almost any conditions with very inexpensive tools.
The size of the core is the obvious culprit. Aligning two fibers together with an error of less than 1 nm with a mechanical connector is not a trivial task. On top of that, the ends must be clean and the total package stable over time and temperature. This kind of performance is needed, however, for an FTTH network to be maintainable. The ideal connector would need to be as easy and repeatable as connectors in competing networks. Customers are not going to accept, "I'll be back when the weather clears up to repair your service!"
The time required to restore service is also an issue with fiber-optic cables. In either a twisted-pair or coaxial plant, almost every segment in the distribution is connectorized for easy maintenance. Also, the connectors can be installed in virtually any kind of weather with simple tools. This is not so with fiber-optic cable.
Since connectors are one of the most common failure points and can contribute significant losses and signal impairments to the optical signal, they are used sparingly in outside-plant distribution. If the optical cable is damaged, a technician repairing the fault has to replace the section with a fusion splicer or temporary mechanical connectors that should only be used for short-term emergency restoration.
Since the fusion splicer is relatively expensive, fragile, and sensitive to weather extremes, most repairs are done twice, the first time with temporary mechanical splices and then, when the weather permits, with a permanent fusion splice.
This works when the fiber plant is used as a signal transport network that is only a small percentage of the total plant. When you try to scale this method to the thousands of miles of network in the last 1,000 ft, however, it becomes too costly, and keeping track of the temporary repairs becomes an operational nightmare. Your telephony/high-speed data/ video entertainment customers are not likely to be very understanding.
Most FTTH network architectures utilize some form of DWDM technology to keep fiber counts reasonable and capital costs in line. If these DWDM devices are deployed in the field, they need to be stable over the entire temperature range that exists in an unconditioned environment. There are several manufacturers that have recently developed product that meets this need for passive devices. It is a little harder, however, to find International Telecommunication Union grid lasers that are stable over the same range. Most thermal electric coolers cannot cover the required temperature extremes. They also add significantly to the network's power requirements.
As a side note, we believe that, while the cost of DWDM systems has fallen, the cost per "color" port still needs to be much lower. A typical per-port cost today is approximately $600. This adds significantly to overall network costs and is one of the reasons why FTTH is much more expensive to deploy than competing technologies.
Keeping in mind that, to our knowledge, a singlemode connector meeting the overall needs for an FTTH distribution plant does not exist, we do believe that a connectorized tap meeting the exacting conditions necessary for the "last 1,000-ft" environment would go a long way toward making FTTH practical. It would solve much of the fiber management and meantime-to-repair issues. The necessary technology is available, with the exception of the rugged, repeatable, low-loss, temperature-insensitive, easy-to-install, environmentally stable connector.
Work is going on in developing a rugged fiber-optic drop, as well. The bottom line is that the drop needs to be robust enough to withstand at least as much abuse as a well-constructed coaxial drop cable (such as the Series 6 drop cable with a 100% bonded foil and 60%+ braided shield). Ideally, it would match the reliability of the traditional twisted-pair drop cable used by most telephony networks.
Currently, the largest capital cost issue surrounding FTTH architectures is the network interface unit (NIU). The NIU has to convert optical to electrical for downstream signals and electrical to optical in the reverse or upstream. It also has to convert these signals to the proper format needed to deliver services to the devices inside the home.
Depending on the network operator's business case, this could include telephony, high-speed data, video on demand, cable television, security service, electric meter reading, and energy management. The electrical interface to each of these devices has different requirements. In addition, the entire unit must be environmentally hardened and very energy-efficient. The only practical way to manage all of the above is to integrate as much of these functions as possible into large-scale-integration silicon circuits.
Our purpose in highlighting the challenges facing FTTH is not to discourage vendors and network providers from pursuing this technology. On the contrary, we are firm believers that this form of network will eventually be the best solution to deliver the kinds of bandwidth-intensive services that are on the near horizon. Our hope is that by pointing out some of the problems, we will challenge someone to solve the remaining hurdles.Don Gall has been involved with the cable-TV industry for the last 28 years. He was an integral part of the team at Time Warner that developed the first practical applications of analog fiber and hfc networks. He is currently a consultant with Pangrac & Associates (Port Aransas, TX) and can be reached at [email protected]