ADSL is an acronym for the Asymmetric Digital Subscriber Line. ADSL is the most common consumer form of DSL, the Digital Subscriber Line that makes high-speed Internet access possible over legacy communication equipment. While traditional modems are capable of a maximum 56 kb/s over copper telephone lines, standard ADSL technology increases this to over 1.5 MB/s, a 25-fold difference. However, most of the increased bandwidth is available for download only. Upload throughput typically varies from 16 kb/s to 256 kb/s. Specialized ADSL applications have published bandwidths of up to 9 MB/s download and 640 kb/s upload in some areas. Additionally, ADSL lines allow for simultaneous voice calls and data transfer.
ADSL makes the high throughput possible by transmitting data at frequencies outside the human range of hearing. As voice communication demands a very narrow frequency range, most of the frequencies that can be transmitted via copper are available for use even while a voice call is in progress. ADSL technologies divide the available frequencies into 4.3125 kHz channels, with active error correction discarding channels which suffer from noise. In standard 1.5 MB/s configuration, the channels between 138 kHz to 1104 kHz are reserved for downloading data, and the channels between 26 kHz and 138 kHz are used for uploading. Some ADSL hardware manufacturers have allocated additional channels above 1104 kHz to increase both upload and download throughput, however these high frequencies often suffer a relatively low signal-to-noise ratios. Therefore, consumers utilizing such configurations must be physically located very close to the telephone company's digital switching station.
ADSL differs from conventional DSL in the allocation of upload and download channels. Because of the synchronous data transfer protocol, any individual channel can be used for either uploading or download only, not both. As typical consumer use consists of proportionately very little data uploading, the majority of channels in ADSL are allocated to downloading. In contrast, conventional DSL allocates the same number of channels to uploading as to downloading. A side effect of this is that applications that demand high upload throughput, such as P2P applications or webhosting, are greatly restricted. Bittorrent is especially difficult to use with ADSL as users are expected to upload almost twice the amount of data that they download. Thus, ADSL connections are not recommended for those who wish to use P2P applications, host websites locally, or use other high-bandwidth applications such as VOIP.
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