The graph can display two different scales using
Zoom In
:
The first scale displays all the tones on the same screen.
Tap
Zoom In
to display a smaller range of tones on screen. Move the pointer past 127 to view tones 128-255 in one
screen. Press
Zoom Out
to return to the original view displaying all the tones on the graph.
The
Table
displays tone measurements in a tabular format. Use the up/down arrows to scroll through the list of tones.
6.3.1 Bits per Tone
This tab displays the bits per tone distribution used by the modem to transmit at the provisioned rate. Bits allocated per tone are
displayed as a graph or table. The graph is a useful visual tool to quickly identify interference or cross talk issues. It provides
valuable insight to ADSL spectrums or VDSL2 channel plan implemented.
Frequency Division Duplexing (FDD) in ADSL/VDSL is used to divide upstream and downstream bandwidth.
Discrete Multitone (DMT) Overview
In traditional VDSL2/ADSLx, Discrete Multitone (DMT) divides the signal into multiple carriers (bins) centered in multiples of
4.3125 kHz. The spectrum of each bin overlaps with its neighbors, i.e. it is not confined to a 4.3125 kHz wide channel because
the orthogonality of COFDM makes this possible without interference. Up to 15 bits per symbol can be encoded on each bin on a
good quality line. Each tone (bin) represents a 4.3125 kHz bandwidth and is used to transfer up to 15 bits in a single direction. In
VDSL2 Bandplan 30a, the tone bandwidth is increased to 8.625 kHz.
In contrast, G.fast has a tone spacing of 51.75 kHz and typical maximum of 12 bits per tone. The usable spectrum for G.fast is up
to 106 MHz.
ADSL, G.fast Comparison
Specifications
VDSL
G.fast
Full form
Very high bit rate digital subscriber line
Fast access to subscriber terminals
Frequency
Up to 30 MHz
2-106 MHz, 2-212 MHz (future)
Maximum data rate
100 Mbps in both upstream and downstream
(VDSL2)
1 Gbps over distance of 70 meters
800 Mbps over distance of 100 meters
500 Mbps over old shielded type of cable
(distance of 100 meters)
Modulation type
DMT (Discrete multi-tone modulation) e.g. OFDM
(Orthogonal Frequency Division Multiplexing)
OFDM
Number of subcarriers
in a FFT
4000
2000 (in 106 MHz profile)
Duplexing method
FDD (Frequency Division Duplex)
TDD (Time Division Duplex)
OFDM symbol time
Approximately 250 µs (in 17 MHz profile)
Approximately 20 µs
Vectoring method
Using ITU-T G.993.5
Supported along with adaptations required for
high frequency
Transmit power
It depends on profile, 14.5 dBm
4 dBm
FEC type
RS Coding (Reed Solomon Coding), trellis coding RS Coding (Reed Solomon Coding), trellis
coding
Bits per DMT
frequency carrier
15 (in VDSL2)
12
Impulse Noise
Protection
Available in VDSL2
Available
Standard
ITU-T G.993.2, ITU-T G.993.5
ITU G.9700, G.9701
MTTplus-523_e-manual, D07-00-117P RevA00
