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18–5
cal modem both provide V.54 Remote Digital Loopback capability. The Control
Level field of the LCD screen of the initiating modem will indicate RLBST, and that
of the accepting modem will indicate LDL.
This test can be initiated by either modem when the modems are on-line.
Line Condition Status Display
The screen displays of this section only apply to the LCD models.
Regarding data being transmitted over telephone line circuits, there are many kinds
of line impairments that can affect the data being received by the remote modem.
The 2864 modem can measure four major line impairments. From the impairment
readings, you can understand the current line condition. The 2864 also logs line
events over a long period, so you know what the line condition has been. We will
describe these readings and logs as follows:
Signal to Noise Ratio (S/N)
This reading is expressed in 0.1 dB resolution. The higher the speed, the higher the
S/N ratio required. The modem measures the S/N ratio by measuring the distance
between the demodulated signal point and the ideal signal point. For V.32/V.32bis,
the modem-measured S/N ratio is generally about 2 dB higher than what is actually
on the line because the modem rejects some of the out-of-band noise. For V.22/
22bis, the difference can be as high as 8 dB because the modem only uses part of the
3KHz bandwidth and rejects more than half of the voice band noise.
Note: The readings displayed are just for reference and not to be taken for
measurements.
Transmitter
Receiver
B
LDL
Remote Modem
Transmitter
Receiver
B
RLBST
Local Modem
~
~
~
~
A C A
DTE
<< SN= 26.7 RX=-25.1 >>
PJ= 0.4 FO=-0.6