ZyXEL Communications IES4005M, Руководство пользователя

Страница: 168 / 236

Chapter 24 VDSL
IES4005M User’s Guide
168
Figure 43
DPBO Resolves Downstream Far-End Crosstalk
UPBO/DPBO Electrical Length
The distance between a cabinet and the central office is an important parameter of DPBO settings
as mentioned in
Section on page 167 . The electrical length is used instead of the real physical
distance according to G.997.1 format. Depending on the cable type, the line used and physical line
length, you can calculate the electrical length (in dB). For example, if the distance is 1 kilometer
and you use 24 AWG cable type, the electrical length 20.5 dB is recommended.
The following table displays the calculation from a real length to an electrical length.
Rate Adaption
Rate adaption is the ability of a device to adjust from the configured transmission rate to the
attainable transmission rate automatically depending on the line quality. The VDSL transmission
rate then stays at the new rate or adjusts if line quality improves or deteriorates.
The switch determines line quality using the Signal-to-Noise Ratio (SNR). SNR is the ratio of the
amplitude of the actual signal to the amplitude of noise signals at a given point in time. A low SNR
indicates poor line quality.
RFI (Radio Frequency Interference)
RFI is induced noise on the lines by surrounding radio frequency electromagnetic radiation from
sources such as AM and HAM radio stations. Since VDSL uses a much larger frequency range that
overlaps with other radio frequency systems, signals from VDSL lines and other radio systems
interfere with each other. To avoid performance degradation due to RFI, set the switch to not
transmit VDSL signals in the RFI band defined by the regulatory bodies (ETSI and ANSI). You can
also configure your own RFI bands on the system.
Table 114
Real length to electrical length
CABLE TYPE REAL LENGTH TO ELECTRICAL LENGTH A B C
22 AWG =16.2*(cable length in kilometer) 0 0 0
24 AWG =20.5*(cable length in kilometer) 0 1 0
26 AWG =25.8*(cable length in kilometer) 0 1.0039065 -0.0039065
VDSL
CPE (B)
Line 1 (150 m)
Line 2 (600 m)
No-DPBO
DPBO
ADSL
CO (A)
ADSL
CPE (A)
VDSL
CO (B)