About the Megabit Modem CRA-C
Reach, Data Rate, SNR Margin, and Noise Environment
The maximum transmission rate of the Megabit Modem CRA-C is determined by distance, SNR margin, and the condition of the line (wire gauge, condition noise environment). Figure 1 shows the relationship between reach and data rate for a given set of conditions. The plots can be used to determine the achievable reach at a given data rate, or they may be used to determined the achievable data rates at a given distance. In all of the cases except the no noise case, a margin of 6 dB was allocated above the SNR that provides a Bit Error Rate (BER) of 10-7.
These line conditions fall within the range of the plots shown in Figure 1:
•no noise (0 dB margin)
•low noise (−140 dBm-Hz Additive White Gaussian Noise (AWGN))
•24 ISDN lines in the same binder group
•24 HDSL lines in the same binder group
•24 ADSL lines in the same binder group
•10 T1 lines in an adjacent binder group
•one T1 line in the same binder group
The performance shown is "typical" for PairGain Megabit Modem CRA products. Due to variations in environment and test setup, PairGain does not guarantee performance to the figures shown.
One of the configuration options in the Megabit Modem CRA-C is the startup margin setting. This setting defaults to 6 dB but may be set from -3 dB to 9 dB. The modems will attempt to come up at the programmed margin setting. Setting the margin high allows for a cushion against changing line conditions and impulse noise. Setting the margin low provides less protection but allows the modems to come up at a higher data rate (if in the rate-adaptive modes) or at a longer loop length (if in the fixed data rate mode). The default setting of 6 dB is recommended and provides an overall BER better than 10-10. Reducing the setting to 0 dB will only provide a BER of less than 10-7and is not recommended.
4 | Megabit Modem CRA-C User Manual |
