186 CHAPTER 5. LIGHTWAVE SYSTEMS
Figure 5.2: (a) Hub topology and (b) bus topology for distribution networks.because of its large bandwidth compared with coaxial cables. The advent ofhigh-
definition television(HDTV) also requires lightwave transmission because of a large
bandwidth (about 100 Mb/s) of each video channel unless a compression technique
(such as MPEG-2, or 2nd recommendation of the motion-picture entertainment group)
is used.
A problem with the bus topology is that the signal loss increases exponentially with
the number of taps and limits the number of subscribers served by a single optical bus.
Even when fiber losses are neglected, the power available at theNth tap is given by [1]
PN=PTC[( 1 −δ)( 1 −C)]N−^1 , (5.1.1)wherePTis the transmitted power,Cis the fraction of power coupled out at each tap,
andδaccounts for insertion losses, assumed to be the same at each tap. The derivation
of Eq. (5.1.1) is left as an exercise for the reader. If we useδ= 0 .05,C= 0 .05,
PT=1 mW, andPN= 0. 1 μW as illustrative values,Nshould not exceed 60. A solution
to this problem is offered by optical amplifiers which can boost the optical power of the
bus periodically and thus permit distribution to a large number of subscribers as long
as the effects of fiber dispersion remain negligible.
5.1.3 Local-Area Networks
Many applications of fiber-optic communication technology require networks in which
a large number of users within a local area (e.g., a university campus) are intercon-