DSL provides high-speed networking over ordinary phone lines using broadband modem technology. DSL technology allows Internet and telephone service to work over the same phone line enabling a customer to use both Internet and voice simultaneously.
DSL Internet services are used primarily in homes and small businesses. DSL Internet service works over a limited distance from the exchange and the service quality becomes poor as the distance of the consumer increases from the exchange area where the DSL hub equipment is installed. DSL Internet service can be categorized as either asymmetric or symmetric.
1. Asymmetric DSL
Asymmetric types of DSL connections provide more network bandwidth for downloading (from the Internet service provider down to the subscriber's computer) than for uploading in the other direction. By reducing the amount of bandwidth available upstream, service providers are able to offer relatively more bandwidth downstream.
Asymmetric DSL technology is popular in residential DSL services as home Internet users predominately use downstream bandwidth. Typical asymmetric DSL services support 5 Mbps for downloads and 1 Mbps for uploads.
Common forms of asymmetric DSL include:
1. ADSL (Asymmetric Digital Subscriber Line): ADSL allows more data to be sent over existing copper telephone lines (POTS), when compared to traditional analog modem lines. A special filter, called a microfilter, is installed on a subscriber's telephone line to allow both ADSL and regular voice (telephone) services to be used at the same time. ADSL requires a special ADSL modem and subscribers must be in close geographical locations to the provider's central office to receive ADSL service. Typically this distance is within a radius of 2 to 2.5 miles (or 2.5 to 4 kilo meters approximately).
A typical ADSL system is shown in the figure above. ADSL typically supports data rates of from 640kbps up to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate).
ADSL2: ADSL2 (ITU G.992.3 and G.992.4) adds more speed and functionality to older ADSL specification. This is achieved as result of better noise reduction, and modulation techniques.
ADSL2+ (ITU G.992.5) doubles the bandwidth used for downstream data transmission, and achieving rates of 20 Mbps on telephone lines up to a distance of about one mile or 5,000 feet. ADSL2+ equipment is interoperable with ADSL and ADSL2. ADSL2+ will include all the feature and performance benefits of ADSL.