Making WAN Connections
the performance to about 1.5 Mbps (at best) in the downstream direction. Only an
estimated 50 percent of U.S. locations are within 3,600 meters of an RBOC CO switch.
The good news is that some newer implementations of xDSL might be able
to overcome the distance limitation. Also, there are extender devices (essentially
repeaters) that the RBOCs can install to let them offer DSL connections to more remote
As mentioned, ADSL can support up to 8 Mbps of receive data (also called downstream
data) and up to 1 Mbps of send data (also called upstream data). In addition to the data
channel, ADSL carves out an 8 KHz channel for POTS, which can coexist with the
ADSL data channels.
Specific implementations of ADSL vary in their data rates. Some of the slower
implementations function at only 1.5 Mbps downstream and 256 Kbps upstream. In
some cases, this speed might even decrease to 384 Kbps downstream and 64 Kbps
T-1/T-3 (DS1/DS3) Connections
More than 40 years ago, Bell Laboratories developed a hierarchy of systems that can
carry digital voice signals. At the lowest level in this hierarchy is a DS0 connection
(DS stands for Digital Signal), which carries 64 Kbps of bandwidth. A DS1 connection
aggregates 24 DS0 channels and can carry up to 1.544 Mbps when all channels are in
use. The next-common level is called a DS3, which carries 672 DS0 channels, for an
aggregate total of 44.736 Mbps.
The DS1 connection is commonly called a T-1 connection, which actually refers to the
system of repeaters that can carry the DS1 traffic over a four-wire twisted-pair connection.
Why Asymmetric DSL?
Many data access needs are asymmetrical. In other words, at any given time, a
system often needs to receive more data than it needs to send, or vice versa. Most
remote access connections, particularly Internet connections, are asymmetrical.
The emphasis is on being able to receive data rapidly, rather than on sending data
Because of this, ADSL is the most popular among the xDSL implementations,
simply because it offers more benefits within the same amount of total frequency
bandwidth. Many applications will work far better with the data rate being faster
downstream than upstream.
Some xDSL implementations are symmetric, such as SDSL and HDSL. These
connection types are more suited to uses where the exchange of data is roughly equal
in both directions, such as two remote LANs that are connected to one another.