Hardware Reference
In-Depth Information
broadband means there are multiple signaling channels whereas baseband means
there is only one. Thus in theory, 10-gigabit Ethernet, which is far faster than any
telephone-company-provided ''broadband'' service, is not broadband at all since it
has only one signaling channel.
Initially, there were many overlapping offerings, all under the general name of
xDSL
(
Digital Subscriber Line
), for various
x
. Below we will discuss what has
become the most popular of these services,
ADSL
(
Asymmetric DSL
). ADSL is
still being developed and not all the standards are fully in place, so some of the de-
tails given below may change in time, but the basic picture should remain valid.
For more information about ADSL, see Summers (1999) and Vetter et al. (2000).
The reason that modems are so slow is that telephones were invented for carry-
ing the human voice and the entire system has been carefully optimized for this
purpose. Data have always been stepchildren. The wire, called the
local loop
,
from each subscriber to the telephone company's office has traditionally been lim-
ited to about 3000 Hz by a filter in the telco office. It is this filter that limits the
data rate. The actual bandwidth of the local loop depends on its length, but for typ-
ical distances of a few kilometers, 1.1 MHz is feasible.
The most common approach to offering ADSL is illustrated in Fig. 2-39. In
effect, what it does is remove the filter and divide the available 1.1-MHz spectrum
on the local loop into 256 independent channels of 4312.5 Hz each. Channel 0 is
used for
POTS
(
Plain Old Telephone Service
). Channels 1-5 are not used, to
keep the voice signal and data signals from interfering with each other. Of the re-
maining 250 channels, one is used for upstream control and one for downstream
control. The rest are available for user data. ADSL is like having 250 modems.
256 4-kHz Channels
0
25
1100 kHz
Voice
Upstream
Downstream
Figure 2-39.
Operation of ADSL.
In principle, each of the remaining channels can be used for a full-duplex data
stream, but harmonics, crosstalk, and other effects keep practical systems well
below the theoretical limit. It is up to the provider to determine how many chan-
nels are used for upstream and how many for downstream. A 50-50 mix of
upstream and downstream is technically possible, but most providers allocate
something like 80%-90% of the bandwidth to the downstream channel since most
users download more data than they upload. This choice gives rise to the ''A'' in
ADSL. A common split is 32 channels for upstream and the rest for downstream.
