ADSL
ADSL is a popular residential service because it can carry both voice and
data over one twisted pair of wires. This capability is accomplished by using
either a splitter or a filter. A splitter takes the incoming analog signals and
splits off the frequencies under 4 MHz to a voice line. It sends all other
traffic to the DSL line. Splitters are more typically used at the CO than the
subscriber premises because they require a technician to install them. A
filter, or microfilter, requires no installation. It simply connects to the phone
line on one end and the telephone on the other. It passively filters out any
signals in the DSL range so that only voice reaches the telephone.
Figure 2-2 shows how ADSL components work together in a typical residential
implementation. The telephone company’s CO forwards both plain old
telephone service (POTS) and DSL data traffic over the same line to the
subscriber. The line enters at the network interface device (NIDS) and
branches toward the telephone and the PC. A low-pass filter blocks everything
but voice frequencies from reaching the phone. A DSL modem (or
router with a DSL interface) forwards data to the PC. When the CO receives
traffic from the subscriber, a splitter sends voice frequencies to the PSTN
switch and DSL frequencies to the DSLAM. The DSLAM sends data traffic
to a router for forwarding to the Internet