ervice using the band 5-30 MHz (see Figure 17.16a). In our example, the node has four incoming 5- to 30-MHz bands, one for each coaxial cable termination. It then converts each of these bands to a higher-frequency slot 25 MHz wide in a frequency division configuration for backhaul on a return fiber. In one scheme, at the headend, each 25-MHz slot is demultiplexed and the data and voice traffic is segregated for switching and processing. Access by data, voice, and special video users of the upstream and downstream assets is another question. There are many ways this can be accomplished. One unique method suggested by a consultant is to steal a page from the AMPS cellular radio specifications (see Chapter 18 for a discussion of AMPS). Because we have twice the bandwidth of an AMPS cellular operator, and because there are no handovers required, there are no shadowing effects and multipath (typical of the cellular environment); thus a much simpler system can be developed. For data communications, CDPD7 can be applied directly. in mind that each system only serves 500 users as a maximum. Those 500 users are allocated 25 MHz of bandwidth (one-way). An interesting exercise is to divide 25 MHz by 500. This tells us that we can allot each user a 50-kHz full period. By taking advantage of the statistics of calling (usage), we could achieve a bandwidth multiplier of from 4x to 10x by using forms of concentration. However, upstream video, depending on the type of compression, might consume a large portion of this spare bandwidth. There are many other ways a subscriber can gain access. DAMA techniques, where AMPS cellular is one, are favored. Suppose we were to turn to a digital format using standard 8-bit PCM. Allowing 1 bit/Hz and dividing 25 MHz by 64 kHz, we find only some 390 channels available. Keep in mind that these simple calculations are not accurate if we dig a little further. For instance, how will we distinguish one channel from another unless we somehow keep them in the frequency domain, where each channel is as