erence from one channel to the next. Of course, that bandwidth allocated to guardbands is non-revenue-bearing and thus must be minimized. 21 CVSD stands for continuous variable slope delta (modulation), a form of digital modulation where the coding is 1 bit at a time. It is very popular with the armed forces. CONCEPTS IN TRANSMISSION TRANSPORT in the cable, particularly for supplying power, can be a conductive path for ground loops. Another approach is to supply power locally to repeaters with a floating battery backup. A key advantage to fiber over metallic cable is fewer repeaters per unit length. In Chapter 6 we showed that repeaters in tandem are the principal cause of jitter, a major impairment to a digital system such as PCM. Reducing the number of repeaters reduces jitter accordingly. In fact, fiber-optic systems require a small fraction of the number of repeaters, compared with PCM for the same unit length, on either wire-pair or coax- ial cable. 9.4.9.1 Design Approach. The first step in the design of a fiber-optic communication system is to establish the basic system parameters. Among these we would wish to know the following: Signal to be transmitted: digital or analog, video/CATV; bit rate and format System length, fiber portion end-to-end Growth requirements (additional circuits, increase of bit rates) Availability/survivability requirements Tolerable signal impairment level, stated as signal-to-noise ratio on BER at the output of the terminal-end detector The link BER should be established based on the end-to-end BER. In the past we had used X 10-9 as a link BER. Telcordia now requires 2 X 10-10 end-to-end. One link value might be 1 X 10-12. However, there is still another saving factor or two. If we have operation at the zero- dispersion wavelength, about 1300 nm, dispersion may really not be a concern until about the 1-Gbps rate. Of course, at 1300 nm we have lost the use of the really low loss band at around 1550 nm. There is an answer to that, too. Use a fiber where the minimum dispersion window