Hardware Reference
In-Depth Information
voice switch. The signal above 26 kHz is routed to a new kind of device called a
DSLAM
(
Digital Subscriber Line Access Multiplexer
), which contains the same
kind of digital signal processor as the ADSL modem. Once the digital signal has
been recovered into a bit stream, packets are formed and sent off to the ISP.
Internet over Cable
Many cable TV companies are now offering Internet access over their cables.
Since the technology is quite different from ADSL, it is worth looking at briefly.
The cable operator in each city has a main office and a large number of boxes full
of electronics, called
headends
, spread all over its territory. The headends are con-
nected to the main office by high-bandwidth cables or fiber optics.
Each headend has one or more cables that run from it past hundreds of homes
and offices. Each cable customer taps onto the cable as it passes the customer's
premises. Thus hundreds of users share the same cable to the headend. Usually,
the cable has a bandwidth of about 750 MHz. This system is radically different
from ADSL because each telephone user has a private (i.e., not shared) wire to the
telco office. However, in practice, having your own 1.1-MHz channel to a telco
office is not that different than sharing a 200-MHz piece of cable spectrum to the
headend with 400 users, half of whom are not using it at any one instant. It does
mean, however, that a cable Internet user will get much better service at 4
A
.
M
.
than at 4
P
.
M
, whereas ADSL service is constant all day long. People intent on get-
ting optimal Internet over cable service might wish to consider moving to a rich
neighborhood (houses far apart so fewer customers per cable) or a poor neighbor-
hood (nobody can afford Internet service).
Since the cable is a shared medium, determining who may send when and at
which frequency is a big issue. To see how that works, we have to briefly describe
how cable TV operates. Cable television channels in North America normally
occupy the 54-550 MHz region (except for FM radio from 88 to 108 MHz). These
channels are 6 MHz wide, including guard bands to prevent signal leakage between
channels. In Europe the low end is usually 65 MHz and the channels are 6-8 MHz
wide for the higher resolution required by PAL and SECAM, but otherwise the al-
location scheme is similar. The low part of the band is not used for television
transmission.
When introducing Internet over cable, the cable companies had two problems
to solve:
1. How to add Internet access without interfering with TV programs.
2. How to have two-way traffic when amplifiers are inherently one way.
The solutions chosen are as follows. Modern cables have a bandwidth of at least
550 MHz, often as much as 750 MHz or more. The upstream (i.e., user to head-
end) channels go in the 5-42 MHz band (but slightly higher in Europe) and the
