COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION)
Figure 17.16b. A small guardband was placed in the frequency slot from 170 to 174 MHz
to isolate downstream voice and data signals from conventional CATV television signals.
We assume that the voice service will be POTS (plain old telephone service) and that
both the data and voice will be digital.
In another approach, downstream voice, data, and special video are assigned the band
550 MHz to 750 MHz, which is the highest frequency segment portion of this system
(Ref. 6). In this case we are dealing with a 750-MHz aggregate system.
The optical fiber trunk terminates in a node or hub (see Figure 17.12). This is where
the conversion from an optical signal to the standard CATV coaxial cable format occurs.
Let a node serve four groupings of cable subscribers, each with a coaxial cable with the
necessary wideband amplifiers, line extenders, and taps. Typically, such subscriber groups
would consist of 200500 terminations (TV sets) each. Assume that each termination has
upstream service using the band 530 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, CDPD
can be applied directly. Keep
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 4
× to 10× 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 assigned
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.