Before you can begin to design a fiber optic cable plant, you need to establish with the end-user or network owner where the network will be built and which communications signals it will carry. Most contractors are more familiar with premises networks, where computer networks, local area networks (LANs) and security systems use structured cabling systems built around well-defined industry standards. Once the cabling exits a building, even for short links—for example, in a campus or metropolitan network—requirements for fiber and cable types change. Long-distance links for telecommunications, CATV or utility networks have other, more stringent requirements, necessary to support longer high-speed links, which must be considered.
But while the contractor generally considers the cabling requirements first, the real design starts with the communications system requirements established by the end-user. Start by looking at the types of equipment required for the communications systems, the speed of the network and the distances to be covered before considering anything related to the cable plant. The communications equipment will determine if fiber is necessary or preferable and which type of fiber is required.
Premises cable systems are designed to carry computer networks based on Ethernet, which currently may operate at speeds from 10 megabits per second to 10 gigabits per second. Other systems may carry security systems with digital or analog video, perimeter alarms or entry systems, which are usually low speeds, at least as far as fiber is concerned. Telephone systems can be carried on traditional twisted-pair cables or, as is becoming more common, use LAN cabling with voice over Internet protocol (VoIP) networks. Premises networks usually are short, often less than the 100 meters (about 330 feet) used as the limit for standardized structured cabling systems that allow twisted-pair copper or fiber optic cabling.
Premises networks generally operate over multimode fiber. Multimode systems are less expensive than single-mode systems, not because the fiber is cheaper (it isn’t) or because cable is cheaper (the same), but because the large core of multimode fiber allows the use of cheaper LED or VCSEL sources in transmitters, making the electronics much cheaper. Astute designers and end-users often include both multimode and single-mode fibers in their backbone cables (called hybrid cables), since single-mode fibers are very inexpensive, and they provide a virtually unlimited ability to expand the systems.
Telephone networks are mainly outside plant (OSP) systems, connecting buildings over distances as short as a few hundred meters to hundreds or thousands of kilometers. Data rates for telecom are typically 2.5 to 10 gigabits per second, using very high power lasers that operate exclusively over single-mode fibers. The big push for telecom is now taking fiber directly to a commercial building or the home, since the signals have become too fast for traditional twisted-copper pairs.
CATV also uses single-mode fibers with systems that are either hybrid fiber-coax (HFC) or digital where the backbone is fiber and the connection to the home is on coax. Coax still works for CATV since it has very high bandwidth. Some CATV providers have discussed or even tried some fiber to the home, but have not seen the economics become attractive yet.
Besides telecom and CATV, there are many other OSP applications of fiber. Intelligent highways are dotted with security cameras and signs and/or signals connected on fiber. Security monitoring systems in large buildings, such as airports, government and commercial buildings, casinos, etc., generally are connected on fiber due to the long distances involved. Like other networks, premises applications usually are multimode, while OSP is single-mode to support longer links.
Metropolitan networks owned and operated by cities can carry a variety of traffic, including telephone, LAN, security, traffic monitoring and control and sometimes even traffic for commercial interests using leased bandwidth or fibers. However, since most are designed to support longer links than premises or campus applications, single-mode is the fiber of choice.
For all except premises applications, fiber is the communications medium of choice, since its greater distance and bandwidth capabilities make it either the only choice or considerably less expensive than copper or wireless. Only inside buildings is there a choice to be made, and that choice is affected by economics, network architecture and the tradition of using copper inside buildings. Next time, we’ll look at the fiber/copper/wireless choices in more detail.
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.