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Digital Subscriber Line

Digital subscriber line (DSL) creates a relatively short (miles long, not tens of miles) highspeed
link WAN between a telco customer and an ISP. To do so, it uses the same single-pair
telephone line used for a typical home phone line. DSL, as a technology, does not try to
replace leased lines, which run between any two sites, for potentially very long distances.
DSL instead just provides a short physical link from a home to the telco’s network, allowing
access to the Internet. First, to get an idea about the cabling, think about typical home telephone
service in the United States, before adding DSL service. Each home has one phone
line that runs from a nearby telco CO to the home. As shown on the left side of Figure
3-15, the telephone wiring splits out and terminates at several wall plates, often with RJ-11
ports that are a slightly skinnier cousin of the RJ-45 connector.
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 3-15 Typical Voice Cabling Concepts in the United States

Next, think about the telephone line and the equipment at the CO. Sometime in the past,
the telco installed all the telephone lines from its local CO to each neighborhood, apartment,
and so on. At the CO, each line connects to a port on a telco switch. This switch supports
the ability to set up voice calls, take them down, and forward the voice through the
worldwide voice network, called the public switched telephone network, or PSTN.

To add DSL service at the home in Figure 3-15, two changes need to be made. First, you
need to add DSL-capable devices at the home. Second, the telco has to add DSL equipment
at the CO. Together, the DSL equipment at each side of the local telephone line can send
data while still supporting the same voice traffic.

The left side of Figure 3-16 shows the changes. A new DSL modem now connects to a spare
phone outlet. The DSL modem follows the DSL physical and data link layer standards to send
data to/from the telco. The home now has a small LAN, implemented with a consumer-grade
router, which often includes an Ethernet switch and possibly a wireless LAN access point.
(Note that the telephones may now also need a short extra cable with a filter in it, installed at
the wall jack, to filter out the sounds of the higher electrical frequencies used for DSL.)
Free CISCO CCNA Routing and Switching ICND1 Study Guide
Figure 3-16 Wiring and Devices for a Home DSL Link

The home-based router on the left must be able to send data to/from the Internet. To make
that happen, the telco CO uses a product called a DSL access multiplexer (DSLAM). The
DSLAM splits out the data over to the router on the lower right, which completes the connection
to the Internet. The DSLAM also splits out the voice signals over to the voice switch
on the upper right.

DSL gives telcos a useful high-speed Internet service to offer their customers. Telcos have
had other offerings that happen to use the same telephone line for data, but these options
ran much slower than DSL. DSL supports asymmetric speeds, meaning that the transmission
speed from the ISP toward the home (downstream) is much faster than the transmissions
toward the ISP (upstream). Asymmetric speeds work better for consumer Internet access
from the home, because clicking a web page sends only a few hundred bytes upstream into
the Internet, but can trigger many megabytes of data to be delivered downstream to the
home.

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