Many applications require LANs in which a large number of users within a local area (e.g., a town or a university campus) are interconnected in such a way that any user can access the network randomly to transmit data to any other user [ 18]—[21 ]. Access networks used in a subscriber loop (often called a local loop) also fall into this category . As transmission distances are relatively short (<10 km), fiber losses as well as the dispersive and nonlinear effects occurring inside fibers are not of much concern for LANs. The major motivation behind using optical fibers is a much larger bandwidth offered by them compared with a coaxial cable.
System architecture plays an important role for LANs. Three commonly used topologies are shown in Figure 10.3 and are known as the bus, ring, and star topologies. In the case of bus topology, all users communicate with each other by tapping into a cental optical fiber (the bus) that transports all data in one direction. An optical tap diverts a small fraction of optical power at each node on the bus. The bus topology is often employed for cable-television (CATV) networks.
In the case of ring topology, users are connected at nodes located on a ring. The network functions by passing a token (a predefined bit sequence) around the ring. Each node monitors this token and accepts the datum if it contains its own address. It can also transmit by appending data to an empty token. The use of ring topology for fiberoptic LANs was commercialized in the 1990s with the standardized interface known as the fiber-distributed data interface (FDDI). This protocol transmits data at 100 Mb/s over multimode fibers using 1.3-^tm transmitters containing a light-emitting diode.
In the case of star topology, all nodes are connected to a star coupler (see Section 4.5.3 of LT1) at a central location. The star coupler receives the signal power transmitted by each node and distributes it equally to all nodes such that all nodes receive the entire traffic. The signal power reaching each node depends on the number of users and decreases as their number increases. This type of loss is known as distribution loss and is much smaller for the star topology compared with the bus topology. It can be virtually eliminated by integrating one or more amplifiers within the star coupler.
Some LANs distribute information to a group of subscribers, without requiring a two-way connection. A common example is provided by CATV networks in which multiple video channels are broadcast to a group of subscribers. Each subscriber selects one video channel from the entire broadcasted signal and can switch among channels as desired. Such networks fall into the category of broadcast-and-select networks, discussed later in Section 10.6.1. Although coaxial cables were originally used by the cable industry, the use of optical fibers permits the distribution of more video channels
Figure 10.3: Schematic illustration of the (a) bus, (b) ring, and (c) star topologies employed for local-area networks.
because of the larger bandwidth associated with them. The advent of high-definition television (HDTV) also requires transmission over fibers because of the large bandwidth associated with this video format.
Most CATV networks employ the bus topology. A single optical fiber carries multiple video channels on the same optical wavelength through a technique known as subcarrier multiplexing (SCM). In the SCM technique, a separate microwave subcarrier is used for each video channel before the entire microwave signal is transferred to the optical domain using a suitable modulation format. Distribution occurs through optical taps that divert a small fraction of optical power to each node located on the central bus.
A problem with bus topology is that distribution losses increase exponentially with the number of taps and limit the number of subscribers that can be served by a single optical bus. Even when fiber losses are neglected, the power available at the Mh tap is given by
where Pj is the transmitted power, Cf is the fraction of power coupled out at each tap, and 8 accounts for the insertion loss, assumed to be the same at each tap. If we use 8 = 0.05, Cf = 0.05, PT = 1 mW, and PN =0.1 juW as illustrative values, N should not exceed 60. Optical amplifiers can be used to boost the optical power along the bus periodically to solve the distribution-loss problem.
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