Technology
xDSL (ADSL, RDSL, SDSL, HDSL) technologies represent a family of technologies
allowing the organization of high-speed access channels for subscribers.
In the abbreviation “xDSL” the symbol “õ” is used to denominate the first symbol
in the name of a certain technology, “DSL” means a Digital Subscriber Line. The
õDSL technologies provide for data transmission at rates from 64 Kbps to 8Mbps.
Many xDSL technologies make it possible to combine high-speed data transmission
and voice transmission over one and the same copper pair. The existing types
of õDSL technologies mainly differ in the modulation form used and data rates.
The xDSL technologies are the most efficient solution aimed at maximally increasing
the volume of data transmitted over the existing telephone lines. Usage of
these technologies for high-speed access is especially noteworthy due to their
use as a transmission medium for the existing cable infrastructure of local
telephone networks. This allows service providers to save money and quicker
(and at reasonable prices) create a big number of services for their subscribers.
The xDSL family includes the following technologies: HDSL, SDSL, ADSL, RDSL
etc.; it is also possible to attribute to the õDSL technology the digital channel
organization under LRE (Long Reach Ethernet) technology.
1.1. HDSL technology (High-speed Digital Subscriber Line) provides symmetrical
high-speed data transmission. The HDSL technology can be associated with the
category of traditional technologies, for it is the most prevalent one. In
most cases HDSL provides data speeds for two-way transmission of 1.5 Mbps or
2 Mbps over distances depending on the type of cable used.
1.1. Diagram of HDSL-based leased channel organization (traditional technology
of digital leased channel organization)
1.2. SDSL technology (Symmetric Digital Subscriber Line) is a version of HDSL
using one cable pair only. SDSL provides the same date rate to and from the subscriber.
Two modifications of this equipment are known: MSDSL (Multispeed SDSL) and HDSL
2, both with a built-in mechanism of data rate adapting for the physical line
parameters.
1.3. RDSL technology (Reach Digital Subscriber Line). The RDSL technology
is a version of SDSL with several significant differences. RDSL allows the
organization of data speeds for two-way transmission of up to 1 Mbps over distances
of up to 11 km.
1.3. Diagram of ReachDSL-based leased channel organization
1.4. ADSL technology (Asymmetric Digital Subscriber Line) was developed in order
to be used on the PSTN’s telephone lines. At present the ADSL technology becomes
more and more popular in Russia among the xDSL technologies. Its basic difference
consists in data speed toward the user being another than that from the user
(that’s why this digital subscriber line is asymmetric). In this case data speed
toward the subscriber considerably exceeds that from the subscriber. This ADSL
operation mode considers the main Internet peculiarity consisting in the information
steam from the Network being considerably higher than that from the subscriber
to the Network. In particular data speed to the user makes 1.5 to 8 Mbps, while
that from the user to the Internet is usually 64 Kbps to 1.5 Mbps. Since ADSL
was intended for use by individual users or in small offices, along with the
organization of high-speed transmission it keeps available analog telephone communication
over this subscriber line. This excludes a necessity of laying an additional
telephone line up to the user.
1.4. Diagram of ADSL-based leased channel organization
