Networking: A Beginner's Guide
High-speed DSL (HDSL) allows from 768 Kbps to 2.048 Mbps
connections between two sites. HDSL is symmetric, meaning that the available
upstream bandwidth and downstream bandwidth are the same.
Rate-adaptive DSL (RADSL) allows for 600 Kbps to 12 Mbps of
data to be received and 128 Kbps to 1 Mbps of data to be sent. RADSL is
Symmetric DSL (SDSL) allows bidirectional rates varying from 160 Kbps
to 2.048 Mbps.
Very-high-speed DSL (VDSL) allows up to approximately 52 Mbps of
bandwidth. VDSL can be either symmetric or asymmetric.
ISDN-based DSL (IDSL) speed is about the same as ISDN. IDSL is used
for data almost exclusively, because it's an always-on connection to a single
destination (as discussed earlier, ISDN can be used to place calls to other ISDN
A lot of interest surrounds xDSL, particularly ADSL. The cost per megabyte of data
transmitted is far less than POTS and is even considerably less expensive than ISDN.
Presently, xDSL is available in most cities in the United States.
In this section, you learn about how xDSL works and about when you might be
able to implement its high-bandwidth capabilities. This discussion focuses on ADSL
because it is the most prevalent and the least expensive. For WAN links, however, you
should consider SDSL if your WAN data needs are similar in both the downstream and
How xDSL Works
The twisted-pair copper wire that carries POTS is capable of carrying signals with up
to a 1 MHz spread of frequencies. However, POTS uses only 8 KHz of that potential
frequency bandwidth. The RBOC's CO switch contains a card that interfaces with the
analog signal that the twisted-pair wire sends to the phone company's digital network.
This interface card allows only 4 KHz of signaling frequencies in each direction,
even though the wire itself is capable of carrying a far broader frequency range. This
limitation exists for standard telephone service because 4 KHz provides reasonable
clarity for voice communications, and much of the telephone system is designed
around those types of circuits.
xDSL works by opening up that 1 MHz maximum capability through the use of
new xDSL interface cards, which the RBOCs install in their CO switch in place of the
cards used for voice lines. The distance from the computer equipment to the CO switch
limits the data rate, however. Most xDSL implementations function optimally at up to
3,600 meters (12,000 feet, or about 2 miles). In particular, the 8 Mbps downstream and
1 Mbps upstream data rates of ADSL are possible only within the 3600-meter distance
to the CO. Longer distances are possible, but not at that full possible data rate. For
instance, running an ADSL connection at 5,500 meters (18,000 feet)--the distance at
which 95 percent of telephone locations exist in relation to their CO switch--degrades