2BASE-TL and enhanced SHDSL increased the bandwidth over SHDSL in two key
dimensions. First, a second constellation (or symbol encoding) is allowed that increases
the throughput by 33 percent without affecting the spectral properties of SHDSL. This
additional higher constellation cannot be used on the longest loops, but it does provide
a ???spectrally free??? throughput increase on loops up to 10 Kft (3 km), depending on
the noise environment. Second, 2BASE-TL and enhanced SHDSL increase the frequency
(number of symbols per second) as compared to SHDSL, thus allowing even
more throughput. This frequency addition increases the noise created by the technology,
but it still falls within North American and international spectral guidelines such
as ANSI T1.417.
10PASS-TS
The EFM short-reach solution is based on very high-speed DSL (VDSL) [4]. One of the
major technical decisions of the EFM task force was to decide which VDSL technology
was best suited for the short-reach Ethernet physical layer. At the time, there were two
VDSL candidates. One candidate was based on Discrete Multitone Modulation (DMT),
and the other was based on Quadrature Amplitude Modulation (QAM).
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