Types of DSL
Asymmetrical DSL has higher downstream (from the provider�fs central
office [CO] to the subscriber) bandwidth than upstream (from the subscriber
to the CO.) Symmetrical DSL has the same bandwidth both downstream
and upstream. You will sometimes see these referred to as �gasynchronous�h
and �gsynchronous�h DSL.
The various types of DSL include the following:
�¡ ADSL.Asymmetric DSL supports both voice and data. Downstream
bandwidth goes up to 8 Mbps; upstream goes up to 1 Mbps. Two other
versions, ADSL2 and ADSL2+, provide 24 Mbps downstream and 1.5
Mbps upstream. The maximum distance from the CO is 18,000 feet, or
5.46 km.
�¡ RADSL.Rate-adaptive DSL changes the rate based on the local loop.
�¡ VDSL.Very-high-rate DSL can be either symmetric or asymmetric
and can carry voice along with data. Maximum symmetric bandwidth
is 26 Mbps; maximum asymmetric is 52 Mbps downstream and 13
Mbps upstream. The maximum distance from the CO is 4,500 feet, or
1.37 km.
�¡ IDSL.ISDN DSL carries only digital data (other forms of DSL send
analog signals). It uses both ISDN B channels and the D channel, for a
symmetric bandwidth of 144 kbps. The maximum distance for IDSL is
18,000 feet, or 5.46 km.
�¡ SDSL.Symmetric DSL carries only data, with a maximum for both
downstream and upstream of 768 kbps. The distance limitation is
22,000 feet, or 6.7 km. It is a proprietary technology that uses only one
twisted pair of wires.
�¡ HDSL.High-data-rate DSL uses two twisted pairs of wires to achieve
a maximum symmetrical bandwidth of 2.048 Mbps. Its maximum
distance from the CO is 12,000 feet, or 3.7 km. HDSL carries only
data, no voice.
�¡ G.SHDSL.Symmetric high-speed DSL has a symmetrical data rate
of 2.3 Mbps and the longest maximum distance: 28,000 feet, or 8.52
km. It also carries only data, no voice.