Varieties of DSL
Types of DSL include:
ADSL - Asymmetric DSL
HDSL - High bit-rate DSL
IDSL - ISDN over DSL
RADSL - Rate-adaptive DSL
SDSL - Symmetric DSL
VDSL - Very high bit-rate DSL
DSL Lite
G.SHDSL - Single-pair High-speed DSL
ADSL (Asynchronous DSL)
ADSL (Asymmetric DSL) is a type of DSL where the upstream and downstream bandwidth are assigned different amounts of bandwidth. Typical configurations today are 2Mb downstream and 128Kb upstream.
Downstream refers to data which you are downloading across the network to your local systems. Upstream refers to data you are sending from your local systems across the network.
ADSL is the most common flavor of DSL.
HDSL (High bit-rate DSL)
HDSL (High bit-rate Digital Subscriber Line) is a variant of DSL which is not frequently implemented.
IDSL (ISDN over DSL)
IDSL (ISDN over DSL) is a 144Kb standard for DSL. IDSL is available where other forms of DSL, such as ADSL are not available.
IDSL is slow and relatively expensive, but it can sometimes be the best possible option.
RADSL (Rate-adaptive DSL)
RADSL (Rate Adaptive DSL) is an asymmetric DSL variant which can adjust the speed of the DSL connection depending on the distance from the Central Office (CO) and the quality of the connection.
SDSL (Symmetric DSL)
SDSL (Single line DSL) is a DSL variant where the upstream and downstream are both set to the same bandwidth.
SDSL typically operates at 1.5Mbps upstream and downstream.
SDSL is not nearly as common as ADSL.
VDSL (Very high bit-rate DSL)
VDSL (Very high bit-rate DSL) is an asymmetric version of DSL which operates at very high speeds.
VDSL operates at downstream speeds up to 55Mbps, but only at a distance of up to 1000 feet from the Central Office (CO).
At longer distances, VDSL transmission rates drop dramatically.
DSL Lite
DSL Lite is an asymmetric variant of DSL which places the DSL splitter at the telco central office instead of in the customer premisis.
G.SHDSL (Single-pair High-speed DSL)
G.SHDSL (Single-pair high-speed Digital Subscriber Line) is a variant of SDSL which is defined by ITU standard G.991.
G.SHDSL supports symmetric speeds from 192Kbps-2304Kbps on a single line pair and 384Kbps-4608Kbps over two pair.
Europeans refer to G.SHDSL as "SDSL", causing confusion with existing SDSL equipment.
DSL Splitters
Some DSL implementations move data traffic over the same pair of wires which carry voice telephone traffic.
Other DSL implementations move date traffic over a dedicated wire pair.
Most American homes have two pairs of wires from the local telephone company. If the home has only one telephone line in use, DSL can be installed on the second pair.
If the home has both telephone lines in use, DSL must share one wire pair with voice traffic. This is done using a DSL splitter.
A DSL splitter is a small box which attaches to the wire pair and splits it into two separate wire pairs, one for voice traffic and the other for DSL traffic.
For more information on telephone wiring, read How do I install telephone wiring?
DSL Modems and DSLAMS
A DSL circuit exists between the DSL modem on the customer premisis and a DSLAM (Digital Subscriber Line Access Multiplier). The DSLAM is usually located in the telco Central Office (CO).
One DSLAM can terminate several hundred DSL connections.
