Q: What is the difference between a LAN and a WAN?

The explosive growth of personal computing in the business marketplace and the increasing sophistication of multiple-function local area networks are forcing system developers into an examination of what would be the optimum cable/system design.
    The growing requirements for bandwidth in computer applications, and the need to adapt to other inter and intra-building telecommunications needs such as telephone, security, alarm and video have all dramatically increased the demand for optical fiber.
    Fiber optic LANs generally have a maximum link distance between transmitter/receiver pairs of 2km. They may be isolated to only one floor or one building, or be interconnected with other networks among several buildings.
    A system can be low-speed, low capacity such as telephone, or high-speed, high capacity such as video. Although copper and fiber can both be used or intermixed in a LAN system, the high information capacity and upgrade ability of fiber is increasingly making it the choice. Instead of rewiring to add future capacity, changing the electronic hardware at the system ends is all that's necessary to alter these systems. Many designers add extra fibers to a system for this purpose.
    Star LANs are arranged around a single hub that may act as a central controller for the network. Transmission sent from one node or terminal must first pass through the hub. This hub can simply be a passive star coupler or an active controller.
    In a ring type network, all terminals are linked in a point-to-point series. If one part fails, the system is down unless bypass components are used. To avoid conflicting data demands, such systems use a bit pattern, called a token. The token is circulated to each node allowing that node to capture the token and the right to capture data. IBM has a ring architecture shown below. Other systems and software are also on the market.

Networks based on a bus topology also use a token passing scheme, or an access scheme known as carrier-sense multiple access with collision detection (CSMA/CD), or collision avoidance (CSMA/CA). Like a ring, messages on the bus are broadcast to all terminals. Since all the terminals tap into a single main trunk channel like branches on a tree, messages do not have to be repeated.
    Most LANs use combinations of bus and star networks today because of speed, easy installation of retrofit, and the fact that each node can be passive so that if one fails the network keeps functioning.

Interconnecting Components
    LAN networks can be easily configured because the fiber optic cable can be easily strung in a plenum on a single floor, up a raceway between floors, or among several buildings.

Figures 13 & 14 show typical examples of LAN layout for multi-premise and single locations. There are fiber optic components pieces corresponding to every piece of electronic hardware used with any other LAN type. These devices appear in a system wherever user connects, or where several lines join together at a node. These devices can be active, such as the transmitters and receivers that have already been discussed, or passive such as taps, distributors, couplers, concentrators, switches, relays, multiplexers and cross-connection cabinets. They are available from a variety of vendors as discrete components, in rack-mounted modules, or as fully integrated systems.

Optical taps or "T"s, and optical mixers or "star" couplers are shown above. Both are examples of concentrators which actively or passively combine signals at nodes or user connection points in a LAN system.

Simple LAN systems use "T"s, stars, and other passive components between transmitter/receiver pairs. More complex systems require active components to combine, route and sometimes re-amplify the signal. Data transmissions trends as outlined below are moving towards more active nodes as the need for greater fiber optic system flexibility, data speed, and link length increases.
    New standards such as the FDDI (Fiber Distribution Data Interface) ANSI X3T9.5 describes a LAN system which operates at 100 Mb/s. Still other options include the potential use of wavelength division multiplexing (WDM) to transmit various signals via different optical wavelengths.
    Optical power losses occur whenever a fiber is terminated or coupled. Therefore allowances for tapped bus or other LAN configurations require that connectors must be factored into the system's loss budget analysis. Since many connectors are used in typical LAN networks, each must have a known loss factor.