Datasheet

818
Introducing Digital Subscriber Line (DSL) Connections
✦ RJ-45–to–DB-25 adapter: Connects a router to a PC through a 25-pin
serial port.
Introducing Digital Subscriber
Line (DSL) Connections
Digital subscriber line (DSL) data transfer is the high-speed transmission of
digital data over standard telephone lines. The digital data is transmitted over
analog carrier signals using copper wiring. DSL technology enables home and
small business customers to use the same phone line for both high-speed
Internet access and telephone services. This combination of new DSL technol-
ogy with older analog signaling (POTS) allows high-speed Internet access with-
out requiring the installation of newer and more expensive communications
methods. The most popular types of DSL connections are as follows:
✦ Asymmetric digital subscriber line (ADSL): The most common method
of DSL communication that transfers both analog and digital information
over a pair of copper wires, using either POTS or ISDN signals.
Since a typical home user requires less upstream bandwidth than down-
stream bandwidth, ADSL allocates the majority of the telephone line
frequencies to downstream traffic. This provides a much larger capacity
for Internet downloading, while restricting a users’ upload bandwidth
considerably. This means data downloads to the end user will occur
much faster than data being uploaded from the user to the Internet.
ADSL line speeds may approach 24 Mbps downstream and 3.5 Mbps
upstream, with data transmission rates fluctuating based on the Internet
service provider (ISP) and line quality of the link. Distance also plays a
factor in data rates. By using a splitter, an ADSL subscriber may simul-
taneously access both the public switched telephone network (PSTN)
and the Internet on the same twisted-pair copper cabling. The splitter
provides a means to filter between high and low frequencies.
Two DSL modems or ADSL transceiver units (ATUs) are used to establish
a link. One unit, located at the service provider’s central office, is called
the ATU-C; the remote transceiver unit located at the home or business
customer is called the ATU-R. The location where the telephone company’s
copper wires terminate is called the main distribution frame (MDF). The
MDF connects incoming public or private lines to the internal telephone
company’s network inside a wire rack. The digital subscriber line access
multiplexer (DSLAM) is connected to the line at the telephone service
provider via the MDF, and is used to connect multiple customer DSL con-
nections to the Internet backbone using multiplexing. The DSLAM acts as a
Layer 2 switch that collects multiple customer DSL streams and multiplexes
the data into a single signal. This multiplexed traffic is then sent over the
Internet backbone switch called the Network Service Provider (NSP) at
speeds up to 10 Gbps. Figure 1-3 shows an example of a typical ADSL setup.
48_489628-bk07ch01.indd 81848_489628-bk07ch01.indd 818 3/1/10 10:15 PM3/1/10 10:15 PM