Xiaguang XP XG2045 Manual page 59

ADSL2+ Tester
non-stationary noise due to AM radio interference to get better coding gain from the
Viterbi decoder.
ADSL2 systems reduce framing overhead by providing a frame with a programmable
number of overhead bits. Therefore, unlike the first-generation ADSL standards
where the overhead bits per frame are fixed and consume 32 kbps of actual payload
data, in the ADSL2 standard the overhead bits per frame can be programmed from 4
to 32 kbps. In first-generation ADSL systems, on long lines where the data rate is low
(e. g. 128 kbps), a fixed 32 kbps (or 25% of the total data rate) is allocated to
overhead information. In ADSL2 systems, the overhead data rate can be reduced to 4
kbps, which provides an additional 28 kbps for payload data.
On long lines where data rates are lower, ADSL2 achieves higher coding gain from the
Reed-Solomon (RS) code. This is due to improvements in the ADSL2 framers that
improve flexibility and programmability in the construction of the RS codewords.
ADSL2+
n
ADSL2+ (ITU G.992.5) doubles the bandwidth used for downstream data
transmission, effectively doubling the maximum downstream data rates, and
achieving rates of 20 Mbps on phone lines as long at 5,000 feet. ADSL2+ solutions will
most commonly be multimodal, interoperating with ADSL and ADSL2, as well as with
ADSL2+ chipsets. ADSL2+ will enable service providers to evolve their networks to
support advanced services such as video in a flexible way, with a singular solution for
both short-loop and long-loop applications. It will include all the feature and
performance benefits of ADSL2 while maintaining the capability to interoperate with
legacy equipment. As such, carriers will be able to overlay new, advanced
technologies without having to "forklift- upgrade" existing equipment, allowing for a
gradual transition to advanced services.
While the first two members of the ADSL2 standards family G.992.3 (G.dmt.bis) and
G.992.4 (G.lite.bis) specify a downstream frequency band up to 1.1 MHz and 552 kHz
respectively, ADSL2+ specifies a downstream frequency up to 2.2 MHz (Figure 32).
The result is a significant increase in data rates on shorter phone lines (Figure 33).
Fig 32: ADSL2+ doubles the bandwidth used to carry data
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5 Performing Measurements
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