| 单词 | ADSL (Asymmetric Digital Subscriber Line) |
| 释义 | ADSL (Asymmetric Digital Subscriber Line) A broadband access technology designed to support voice, high speed Internet access, entertainment television over embedded telco unshielded twisted pair (UTP) local loops up to 18,000 ft in length. One of a family of xDSL standards, ADSL was developed by Bellcore (now Telcordia Technologies) at the request of the Regional Bell Operating Companies (RBOCs) in the United States, and was later standardized in 1999 by the American National Standards Institute (ANSI) as T1.413 and by the ITU-T as G.922.1. The term asymmetric refers to the fact that, in consideration of FEXT and NEXT crosstalk issues, ADSL offers considerably more bandwidth in the downstream direction than in the upstream direction.As ADSL employs discrete multitone (DMT) modulation, it sometimes is referred to as, G.dmt.Also known as orthogonal frequency division multiplexing (OFDM), DMT splits the signal over 256 narrowband subcarrier channels, within each of which quadrature amplitude modulation (QAM) is employed. ADSL involves a pair of matching modems, with the ADSL Transmission Unit-Centralized (ATU-C) located at the central office (CO) or other headend location, and the ADSL Transmission Unit-Remote (ATU-R) located on the customer premises.The ATUs multiplex voice, data, and sometimes video signals over three separate frequency channels. A bi-directional voice grade analog channel at 0 4 kHz is provided for full duplex (FDX) voice and facsimile applications, that is, POTS service. Upstream data transmission is over what technically is a bidirectional channel provided in increments of 64 kbps, up to 640 kbps, in a frequency band from 26 140 kHz. Downstream transmission is in increments of 1.536 Mbps up to 6.144 Mbps, based on T1 specifications, in a frequency band that runs from approximately 140–552 kHz. ( Note: The downstream increments are stated in maximum transmission rates. The available rates may be much lower, depending on carrier network design considerations and local loop characteristics.) In the context of the OSI Reference Model, ADSL is primarily a Layer 1 (Physical Layer) specification, although it includes Layer 2 (Data Link Layer) elements. The telcos currently avoid video over pure ADSL, although some provide video over hybrid fiber/ADSL loops in deployment scenarios known variously as fiber-to-the-curb (FTTC), fiber-to-the-neighborhood (FTTN), and fiber-to-the-node (FTTN). Table A-1 provides a comparative view of ADSL maximum downstream data rates and distance limitations based on acceptable options for wire gauges. |
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