Hardware Reference
In-Depth Information
Within each channel the line quality is constantly monitored and the data rate
adjusted continuously as needed, so different channels may have different data
rates. The actual data are sent using a combination of amplitude and phase modu-
lation with up to 15 bits per baud. With, for example, 224 downstream channels
and 15 bits/baud at 4000 baud, the downstream bandwidth is 13.44 Mbps. In prac-
tice, the signal-to-noise ratio is never good enough to achieve this rate, but 4-8
Mbps is possible on short runs over high-quality loops.
A typical ADSL arrangement is shown in Fig. 2-40. In this scheme, the user
or a telephone company technician must install a
NID
(
Network Interface De-
vice
) on the customer's premises. This small plastic box marks the end of the tele-
phone company's property and the start of the customer's property. Close to the
NID (or sometimes combined with it) is a
splitter
, an analog filter that separates
the 0-4000 Hz band used by POTS from the data. The POTS signal is routed to
the existing telephone or fax machine, and the data signal is routed to an ADSL
modem. The ADSL modem is actually a digital signal processor that has been set
up to act as 250 modems operating in parallel at different frequencies. Since most
current ADSL modems are external, the computer must be connected to it at high
speed. Usually, this is done by putting an Ethernet card in the computer and oper-
ating a very short two-node Ethernet containing only the computer and ADSL
modem. (Ethernet is a popular and inexpensive local area network standard.)
Occasionally the USB port is used instead of Ethernet.
In the future, internal
ADSL modem cards will no doubt become available.
Voice
switch
Telephone
Codec
Splitter
Telephone
line
Splitter
NID
Computer
DSLAM
ADSL
modem
Ethernet
To ISP
Telephone company end office
Customer premises
Figure 2-40.
A typical ADSL equipment configuration.
At the other end of the wire, on the telco side, a corresponding splitter is in-
stalled. Here the voice portion of the signal is filtered out and sent to the normal
