DSL is an Internet access method that uses a standard phone line to provide high-speed Internet access. DSL is most commonly associated with high- speed Internet access; because it is a relatively inexpensive Internet access, it is often found in homes and small businesses. With DSL, a different frequency can be used for digital and analog signals, which means that you can talk on the phone while you upload data.
For DSL services, two types of systems exist: Asymmetric Digital Subscriber Line (ADSL) and High-Rate Digital Subscriber Line (HDSL). ADSL pro- vides a high data rate in only one direction. It enables fast download speeds but significantly slower upload speeds. ADSL is designed to work with exist- ing analog telephone service (POTS) service. With fast download speeds, ADSL is well suited for home-use Internet access where uploading large amounts of data isn’t a frequent task.
. Asymmetric DSL (ADSL): Probably the most common of the DSL varieties is ADSL, which uses different channels on the line. One chan- nel is used for plain old telephone service (POTS) and is responsible for analog traffic. The second channel provides upload access, and the third channel is used for downloads. With ADSL, downloads are faster than uploads, which is why it is called asymmetric DSL.
. Symmetric DSL (SDSL): Offers the same speeds for uploads and downloads, making it most suitable for business applications such as web hosting, intranets, and e-commerce. It is not widely implemented in the home/small business environment and cannot share a phone line.
. ISDN DSL (IDSL): A symmetric type of DSL commonly used in envi- ronments in which SDSL and ADSL are unavailable. IDSL does not support analog phones.
Rate-Adaptive DSL (RADSL): A variation on ADSL that can modify its transmission speeds based on signal quality. RADSL supports line sharing.
. Very High Bit Rate DSL (VHDSL or VDSL): An asymmetric version of DSL and, as such, can share a telephone line. VHDSL supports high- bandwidth applications such as VoIP and HDTV. VHDSL can achieve data rates up to approximately 10Mbps, making it the fastest available form of DSL. To achieve high speeds, VHDSL uses fiber-optic cabling.
. High Bit Rate DSL (HDSL): A symmetric technology that offers iden- tical transmission rates in both directions. HDSL does not allow line sharing with analog phones.
Sry copy paste from one of the Ebooks
