Digital Subscriber Line DSL

DSL is an "always-on" technology. This means that you don't need to dial up each time to connect to the Internet. DSL uses the existing copper telephone lines to provide high-speed digital data communication between end users and telephone companies. Unlike ISDN, in which digital data communication replaces analog voice communication, DSL shares the telephone wire with analog signals.
The telephone company limits the bandwidth of the analog voice on the lines. This limit allows the DSL to place digital data on the phone wire in the unused portion of the bandwidth. This sharing of the phone wire allows voice calls to be placed while DSL is connecting to the Internet.
You must consider two major points when selecting DSL. DSL has distance limitations. The phone lines used with DSL were designed to carry analog information. Therefore, the length that the digital signal can be sent is limited, and the signal cannot pass through any form of multiplexer used with analog phone lines. The other consideration is that the voice information and the data carried by DSL must be separated at the customer site. A device called a splitter separates the connection to the phones and the connection to the local network devices.
Asymmetric digital subscriber line (ADSL) is currently the most commonly used DSL technology. ADSL has different bandwidth capabilities in each direction. ADSL has a fast downstream speed—typically 1.5 Mbps. Downstream is the process of transferring data from the server to the end user. This is beneficial to users who download large amounts of data. The high-speed upload rate of ADSL is slower. ADSL does not perform well when hosting a web server or FTP server, both of which involve upload-intensive Internet activities.
The following are some of the most common DSL types:
■ ADSL: Asymmetric DSL currently is the most common implementation. It has speeds that vary from 384 Kbps to more than 6 Mbps downstream. The upstream speed typically is lower.
■ HDSL: High Data Rate DSL provides equal bandwidth in both directions. It is 1.544 Mbps in North America and 2.048 Mbps in Europe.
■ SDSL: Symmetric DSL provides the same speed, up to 3 Mbps, for uploads and downloads.
■ VDSL: Very High Data Rate DSL is capable of bandwidths between 13 and 52 Mbps downstream and 16 Mbps upstream.
■ IDSL: ISDN DSL is actually DSL over ISDN lines. It is a set of CCIT/ITU standards for digital transmission over ordinary telephone copper wire, as well as over other media, with a top speed of 144 Kbps. ISDN is available in areas that do not qualify for other DSL implementations. An ISDN adapter at both the user side and service provider side is required. ISDN generally is available in urban areas in the U.S. and Europe from the local phone company.
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