5.1. SYSTEM ARCHITECTURES 187
Figure 5.3: (a) Ring topology and (b) star topology for local-area networks.nected in such a way that any user can access the network randomly to transmit data
to any other user [14]–[16]. Such networks are calledlocal-area networks(LANs).
Optical-access networks used in a local subscriber loop also fall in this category [17].
Since the transmission distances are relatively short (<10 km), fiber losses are not of
much concern for LAN applications. The major motivation behind the use of optical
fibers is the large bandwidth offered by fiber-optic communication systems.
The main difference between MANs and LANs is related to the random access of-
fered to multiple users of a LAN. The system architecture plays an important role for
LANs, since the establishment of predefined protocol rules is a necessity in such an
environment. Three commonly used topologies are known as bus, ring, and star con-
figurations. The bus topology is similar to that shown in Fig. 5.2(b). A well-known
example of bus topology is provided by theEthernet, a network protocol used to con-
nect multiple computers and used by theInternet. The Ethernet operates at speeds up
to 1 Gb/s by using a protocol based oncarrier-sense multiple access(CSMA) with
collision detection. Although the Ethernet LAN architecture has proven to be quite
successful when coaxial cables are used for the bus, a number of difficulties arise when
optical fibers are used. A major limitation is related to the losses occurring at each tap,
which limits the number of users [see Eq. (5.1.1)].
Figure 5.3 shows the ring and star topologies for LAN applications. In the ring