ZyXEL Communications P-660HW-Tx v3 User Manual page 341

Chapter 24 Statistics
When data packets are transmitted they contain control bits that enable your
modem or router to check and correct a certain amount of erroneous bits. This
type of error checking is known as Forward Error Correction (FEC). FEC errors are
correctable.
Noise margin downstream is the signal to noise ratio for the downstream part
of the connection (coming into the ZyXEL Device from the ISP). It is measured in
decibels. The higher the number the more signal and less noise there is.
Output power upstream is the amount of power (in decibels) that the ZyXEL
Device is using to transmit to the ISP.
Attenuation downstream is the reduction in amplitude (in decibels) of the DSL
signal coming into the ZyXEL Device from the ISP.
Discrete Multi-Tone (DMT) modulation divides up a line's bandwidth into sub-
carriers (sub-channels) of 4.3125 KHz each called tones. The rest of the display is
the line's bit allocation. This is displayed as the number (in hexadecimal format) of
bits transmitted for each tone. This can be used to determine the quality of the
connection, whether a given sub-carrier loop has sufficient margins to support
certain ADSL transmission rates, and possibly to determine whether particular
specific types of interference or line attenuation exist. Refer to the ITU-T G.992.1
recommendation for more information on DMT.
The better (or shorter) the line, the higher the number of bits transmitted for a
DMT tone. The maximum number of bits that can be transmitted per DMT tone is
15. There will be some tones without any bits as there has to be space between
the upstream and downstream channels.
A Cyclic Redundancy Check (CRC) is an error detecting code that discovers
errors that cannot be corrected. Data packets containing CRC errors need to be
re-sent.
Header Error Correction (HEC) errors are similar to CRC errors, except only the
header of the ATM cell is checked.
Typically one type of error causes others too, and as mentioned, some errors are
normal on a DSL line.
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P-660HW-Tx v3 User's Guide