| 单词 | VDSL2 (Very-high-data-rate Digital Subscriber Line version 2) |
| 释义 | VDSL2 (Very-high-data-rate Digital Subscriber Line version 2) Specified by the ITU-T in Rec- ommendation G.993.2 (February 2006), is the specification for two versions of VDSL2. The long reach version runs at 12 MHz, divided among 2,872 subcarriers. Bandwidth is asymmetric, with transmission rates up to 55 Mbps downstream and 30 Mbps upstream over local loops up to 1,000 feet in length. Considerably reduced rates are achievable at distances up to 4,000–5,000 feet. The short reach version runs variously at 17.6 MHz and up to 30 MHz, divided among as many as 4,096 and 3,478 subcarriers. Bandwidth is symmetric, with downstream and upstream transmission rates as high as 100 Mbps over loops up to 500 feet. Considerably reduced rates are achievable at distances up to 4,000–5,000 feet.The short reach version will run in asymmetric mode, as well.VDSL2 employs the same discrete multitone (DMT) modulation scheme as ADSL.Trellis-coded modulation (TCM) yields higher throughput on long loops where the signal-to-noise ratio (SNR) is low, although data rates drop considerably beyond 500–1,000 feet. VDSL2 defines eight profiles for services, including asynchronous transfer mode (ATM) and Ethernet, and quality of service (QoS) features are integrated into the specification. See also ADSL, asymmetric, ATM, bandwidth, DMT, downstream, Ethernet, ITU-T, local loop, modulation, QoS, SNR, subcarrier, symmetric, TCM, throughput, transmission rate, upstream, and VDSL. |
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