signed a 64-kHz slot? This could be done by using QPSK modulation, which will leave some spare bandwidth for filter roll-off and guardbands. One approach that is widely used currently is to employ the format DOCSIS (Data over Cable Service Interface Specification). This is covered in Section 17.7. Why not bring the fiber directly into the home or to the desktop at the office? The most convincing argument against this approach is economic. A CATV system interfaces with the home/office TV set by means of a set-top box. As we discussed earlier, the basic function of this box is to convert incoming TV channels to a common channel on the TV set, usually either channel 2, 3, or 4. Now we will ask much more of this "box." It is to terminate the fiber in the AM system as well as to carry out channel conversion. The cost CDPD stands for cellular digital packet data. A brief description of CDPD is given in Chapter 18. 17.7 TWO-WAY VOICE AND DATA OVER CATV SYSTEMS BASED ON THE DOCSIS 2.0 SPECIFICATION of the set-top box in 2005 dollars should not exceed $350. With AM fiber to the home, the cost target will probably be exceeded. The reason that the driving factor is the set-top box is the multiplier effect. In this case we would be working with multipliers of, say, 500 (subscribers) by >$350. For a total CATV network, we would be working with 100,000 or more customers. Given the two- way and digital options, both highly desirable, the set-top box may exceed $1200 (2004 dollars), even with mass production. The amount is excessive to justify the business case. 17.6.2 Impairments Peculiar to Upstream Service 17.6.2.1 More Thermal Noise Upstream than Downstream. Figure 17.17 shows a hypothetical layout of amplifiers in a CATV distribution system for two-way opera- tion. In the downstream direction, broadband amplifiers point outward, down trunks, and out distribution cables. In the upstream direction, the broadband amplifiers point inward toward the headend and all their thermal noise accumulates and concentrates at the head- en