About the Megabit Modem CRA-C
2 Megabit Modem CRA-C User Manual
Use the Megabit Modem CRA-C with these other PairGain products:
•Megabit Modem CRA (at subscriber site)
•S1 Network Interface Device (network interface at subscriber site)
About Rate Adaptive Transmission
The following definitions are useful for understanding the operation of the
Megabit Modem:
•Bit Error Rate (BER) is the ratio of received bits that are in error
relative to the total number of bits received, measured over time.
For example, 10-7 BER means that on average one error occurs per
107bits received.
•Signal to Noise Ratio (SNR) is the ratio (typically expressed in dB) of
the received signal power to the received noise power. It is a measure
of the quality of the transmission. The theoretically achievable BER
can be extrapolated from the measured SNR.
•Margin (SNR margin) is the amount of degradation in SNR that the
system can tolerate under the current conditions and still achieve a
10-7 BER. A margin of 6 dB, for example, would mean that the SNR
can degrade by 6 dB and still provide a performance of 10-7 BER. The
PairGain Megabit Modem CRA-C has a margin configuration option
that defaults to 6 dB, but may be set anywhere between -3 dB to 9 dB.
•Reach is the longest loop length that the system can support with
a given margin and a BER of less than 10-7 at the given data rate.
Rate Adaptation
With PairGain's rate adaptive technology, the Megabit Modem CRA-C
can automatically adjust to the fastest speed possible, given the transmission
distance and line conditions. Alternatively, the provider can manually set
the modem to a specific rate. The available data rates are:
•downstream (transmit) rates: 3.168 Mbps, 2.528 Mbps, 2.208 Mbps,
1.888 Mbps, 1.568 Mbps, 1.248 Mbps, 928 Kbps, 608 Kbps
•upstream (receive) rates: 1.080 Mbps, 944 Kbps, 808 Kbps, 672 Kbps,
536 Kbps, 400 Kbps, 264 Kbps, 82.66 Kbps
