Answer | Cable signal levels for all DOCSIS cable modems require to be within the acceptable level for the modem operate properly.  The three signal levels that need to be within the acceptable level are Downstream Power, SNR (Signal to Noise Ratio), and Upstream Power.  Downstream Power refers to the line signal sent from the cable provider to cable modem.  Downstream SNR (Signal to Noise Ratio) refers to the signal being sent in relation to the noise on the line.  Upstream Power refers to the line signal from the cable modem to transmit data back to the cable provider.  This document describes how to check the three signal levels to be within the acceptable level for the modem operate properly. Â
Checking Cable Signal Levels
NOTE: If the SB6121 is connected to a router, it is recommended to disconnect the router and connect the SB6121 directly to a computer with an Ethernet cable.
- Launch a web browser, such as Internet Explorer, Firefox, or Safari. Enter http://192.168.100.1 into the address box. The Cable Modem Status page will appear.
 - Click the Signal link on the top menu . The Signal page will appear.
 - In the Downstream section, verify Power levels are within the acceptable range of - 15 dBmV to + 15 dBmV for each downstream channel.Â
 - In the Downstream section, identify the Modulation and Power to find verify SNR levels are within the acceptable range for each downstream channel.Â
Acceptable SNR Levels (dB):Â
If QAM64, SNR should be 23.5 dB or greater.Â
If QAM256 and DPL( -6 dBmV to +15 dBmV) SNR should be 30 dB or greater.Â
If QAM256 and DPL(-15 dBmV to -6 dBmV) SNR should be 33 dB or greater.  Â
 - In the Upstream section, identify the Symbolic Rate to find verify Upstream Power levels are within the acceptable range.Â
Acceptable Upstream Power Levels (dBmV)
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| Upstream Transmit Power Level | | Channel | US Channel Type | Symbol Rate | Acceptable Upstream Power Levels | | Single | TDMA | 1280 Ksym/sec | 45 dBmv to 61 dBmV | | ATDMA | 2560 Ksym/sec | 45 dBmv to 58 dBmV | | 5120 Ksym/sec | 45 dBmv to 57 dBmV | | Â Two | TDMA | 1280 Ksym/sec | 45 dBmv to 58 dBmV | | ATDMA | 2560 Ksym/sec | 45 dBmv to 55 dBmV | | 5120 Ksym/sec | 45 dBmv to 54 dBmV | | Three or more | TDMA | 1280 Ksym/sec | 45 dBmv to 55 dBmV | | ATDMA | Â 2560 Ksym/sec | 45 dBmv to 52 dBmV | | Â 5120 Ksym/sec | 45 dBmv to 51 dBmV |
- Refresh the page at least twice to identify variations in SNR, Downstream Power, and Upstream Power.  Â
NOTE: If the either of the signal levels are outside of the acceptable range, power cycle or factory reset the modem.Â
 - Contact the cable provider to adjust the signal level to the acceptable level range.
Acceptable Signal LevelsÂ
| Acceptable Downstream Power Level | | -15 dBmV to +15 dBmV | | Downstream Signal to Noise Ratio (SNR) | | Modulation | Downstream Power | Acceptable Downstream SNR | | 64 QAM | n/a | 23.5 dB or greater | 256 QAM
 | -6 dBmV to +15 BmV | 30 dB or greater | | -6 dBmV to -15 dBmV | 33 dB or greater | | Upstream Transmit Power Level | | Channel | US Channel Type | Symbol Rate | Acceptable Upstream Power Levels | | Single (1) | TDMA | 1280 Ksym/sec | 45 dBmv to 61 dBmV | | ATDMA | 2560 Ksym/sec | 45 dBmv to 58 dBmV | | 5120 Ksym/sec | 45 dBmv to 57 dBmV | |  Two (2) | TDMA | 1280 Ksym/sec | 45 dBmv to 58 dBmV | | ATDMA | 2560 Ksym/sec | 45 dBmv to 55 dBmV | | 5120 Ksym/sec | 45 dBmv to 54 dBmV | | Three or more (>=3) | TDMA | 1280 Ksym/sec | 45 dBmv to 55 dBmV | | ATDMA |  2560 Ksym/sec | 45 dBmv to 52 dBmV | |  5120 Ksym/sec | 45 dBmv to 51 dBmV |
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