Hardware Reference
In-Depth Information
downstream (i.e., headend to user) traffic uses the frequencies at the high end, as il-
lustrated in Fig. 2-41.
54254 88
0
108
550
750 MHz
TV
FM
TV
Downstream data
Downstream frequencies
Figure 2-41.
Frequency allocation in a typical cable TV system used for Internet
access.
Note that since the television signals are all downstream, it is possible to use
upstream amplifiers that work only in the 5-42 MHz region and downstream
amplifiers that work only at 54 MHz and up, as shown in the figure. Thus, we get
an asymmetry in the upstream and downstream bandwidths because more spectrum
is available above television than below it. On the other hand, most of the traffic is
likely to be downstream, so cable operators are not unhappy with this fact of life.
As we saw earlier, telephone companies usually offer an asymmetric DSL service,
even though they have no technical reason for doing so.
Internet access requires a cable modem, a device that has two interfaces on it:
one to the computer and one to the cable network. The computer-to-cable-modem
interface is straightforward. It is normally Ethernet, just as with ADSL. In the fu-
ture, the entire modem might be a small card plugged into the computer, just as
with the old telephone modems.
The other end is more complicated. A large part of the cable standard deals
with radio engineering, a subject far beyond the scope of this topic. The only part
worth mentioning here is that cable modems, like ADSL modems, are always on.
They make a connection when turned on and maintain that connection as long as
they are powered up because cable operators do not charge for connect time.
To better understand how they work, let us see what happens when a cable
modem is plugged in and powered up. The modem scans the downstream channels
looking for a special packet periodically put out by the headend to provide system
parameters to modems that have just come online. Upon finding this packet, the
new modem announces its presence on one of the upstream channels. The headend
responds by assigning the modem to its upstream and downstream channels.
These assignments can be changed later if the headend deems it necessary to bal-
ance the load.
The modem then determines its distance from the headend by sending it a spe-
cial packet and seeing how long it takes to get the response. This process is called
