DSL
(ADSL) Basics...
At the customer premises, a DSL router or
modem connects the DSL line to a local-area network (LAN) or an
individual computer. The modem also organizes the aggregate data stream
created by multiplexing downstream channels, duplex
channels, and maintenance
channels together into blocks, and attaches an error correction code to
each block. The receiver then corrects errors that occur during
transmission up to the limits implied by the code and the block length.
The unit may, at the user's option, also create superblocks by
interweaving data within sub-blocks; this allows the receiver to correct
any combination of errors within a specific span of bits. This allows
for effective transmission of both data and video signals alike.
To create multiple
channels, DSL modems divide the available bandwidth of a telephone line
in one of two ways -- Frequency Division Multiplexing (FDM) or Echo
Cancellation. FDM assigns one band for upstream data and another band
for downstream data. The downstream path is then divided by time
division multiplexing into one or more high-speed channels and one or
more low speed channels. The upstream path is also multiplexed into
corresponding low speed channels. Echo Cancellation assigns the upstream
band to over-lap the downstream, and separates the two by means of local
echo cancellation, a technique well know in V.32 and V.34 modems. With
either technique, DSL splits off a 4 kHz region for POTS at the DC end
of the band.
Does DSL have any security issues like cable modems?
No. DSL is not a shared
network service like cable modems, which means that one end-user can not
see what's on a neighbor's computer using DSL. The service is a private
dedicated connection from the end-user to the network.
More information about DSL-vs-Cable
