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5. Signal Levels and other tests
This is the amount of signal received by the modem from the transmitter in the cable company head-end.
For all modems:
-7 dBmV to +7 dBmV "Recommended"
-8 dBmV to -10 dBmV / +8 dBmV to +10 dBmV - "Acceptable"
-11 dBmV to -15 dBmV / +11 dBmV to +15 dBmV - "Maximum"
Lower than -15 dBmV & Higher than +15 dBmV - "Out Of Spec."
SNR (signal to noise ratio) levels:
This is how clear the signal is at either the modem receiver (downstream SNR) or the receiver in the cable company head-end (upstream SNR).
DOCSIS specifications list minimum CNR (carrier to noise ratio) levels not SNR levels. The SNR levels listed here are based on commonly recommended MER levels for digital cable signals. Not all QAM demodulator chipsets accurately calculate SNR levels that approximate actual MER levels, so these levels may vary depending on which chipset and/or firmware is used in the equipment.
QPSK: 12 dB minimum. 15 dB or higher recommended. (often used in upstream channels)
16 QAM: 18 dB minimum. 21 dB or higher recommended. (often used in upstream channels)
64 QAM: 24 dB minimum. 27 dB or higher recommended. (often used in downstream channels)
256 QAM: 30 dB minimum. 33 dB or higher recommended. (often used in downstream channels)
*There is no upper SNR limit; however, 40 dB is the highest most people see. Going above 40 dB is possible though.
Downstream SNR levels are read at the modem on the downstream data channel and can be viewed using the modem diagnostic screens.
Upstream SNR levels are read at the CMTS on the upstream data channel, not the modem or the modem diagnostic screens. The end-user cannot get the upstream SNR directly. Only the provider can read the upstream SNR level, directly from the CMTS. Also, the upstream SNR level provided by most CMTSs is not specific to any single modem, but is an averaged, aggregate level from all modems on that upstream channel on the upstream port.
Upstream (Tx) Transmit Power (a.k.a. Return Signal) level:
This is the amount of signal transmitted by the modem to reach the receiver in the cable company head-end.
+8 dBmV to +58 dBmV maximum for QPSK. (DOCSIS 1.0, 1.1)
+8 dBmV to +55 dBmV maximum for 8 QAM and 16 QAM. (DOCSIS 1.0, 1.1)
+8 dBmV to +54 dBmV maximum for 32 QAM and 64 QAM. (A-TDMA DOCSIS 2.0)
+8 dBmV to +53 dBmV maximum for S-CDMA DOCSIS 2.0 (All Modulations)
+8 dBmV to +52 dBmV maximum for A-TDMA & TDMA (DOCSIS 3.0)
*Recommended upstream signal levels are +35 dBmV to +49 dBmV.
A cable modem running a higher upstream modulation rate may downgrade itself to a lower modulation rate (i.e. 64 QAM to 16 QAM or 16 QAM to QPSK) if the upstream transmit level is higher than the maximum signal level allowed for the higher modulation rate and the CMTS is configured to allow such a change. This downgrade can cause slow speed, packet loss, and connection loss issues depending on the condition of the upstream channel.
A house or drop amplifier will NOT fix an upstream signal problem because most house amplifiers don't amplify the upstream signals; they only pass the upstream signal through with some loss.
Important notes concerning signal levels:
1. Signal levels not within the specifications listed above can cause slow speeds, connection problems, and connection loss due to packet errors, packet loss, and/or constant packet retransmission.
2. It's recommended to have the modem's signal levels at least 3dB away from the maximum/minimum levels listed above due to normal temperature related signal variation. If the modem's signal levels are at the maximum or minimum limits, they may be out of spec. if the temperature changes significantly. Signal levels that vary more then 3 dB in a 24-hour period usually indicate a problem that should be looked into.
3. Excess splits, bad connectors, and/or poor quality cabling will certainly effect cable signal levels and will cause problems
Updates to this FAQ were provided by Johkal
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from RCN or any of its affiliates.
1. Visit the DocsDiag home page and download the docsdiag.zip file. Unzip the downloaded file to extract docsdiag.jar.
2. Change your IP address to 192.168.100.2 and subnet mask to 255.255.255.0. You can use one of the following methods if it applies to your system (note your current settings before you change them):
If your PC is connected directly to the modem and you use Windows 98/98SE, click Start, Settings, Control Panel. Double-click Network. On the Configuration tab, click TCP/IP for your Ethernet adapter, then click Properties. On the IP Address tab, select Specify an IP Address, enter an IP address of 192.168.100.2 and a subnet mask of 255.255.255.0. Click OK and OK. Reboot if prompted.
If you have a Linksys router, go to the router's setup page and select specify an IP address, enter an IP address of 192.168.100.2, a subnet mask of 255.255.255.0, and a gateway of 0.0.0.0.
If you have a Netgear router, go to menu 4, change IP Address Assignment to Static, IP Address to 192.168.100.2, IP Subnet Mask to 255.255.255.0, and Gateway IP Address to 0.0.0.0.
3. In the MS-DOS window, enter the following command:
For machines with Microsoft's Java Virtual Machine
jview /cp docsdiag.jar docsdiag
For machines with Sun's Java Runtime Environment
java -cp docsdiag.jar docsdiag
This should produce output with your signal levels and firmware version.
4. Put your IP address back to dynamically assigned by restoring the original settings you changed in step 3:
For a direct connection to the modem, go back to the TCP/IP Properties dialog box and select Obtain an IP address automatically. Reboot if prompted.
For a Linksys router, go back to the router's setup page and select Obtain an IP address automatically.
For a Netgear router, go back to menu 4 and change IP Address Assignment to Dynamic.
In addition, the following works for Toshiba modems:
To access the built in diagnostics, set your ip statically to 192.168.100.2, set your subnet mask to 255.255.255.0 and set your gateway to 192.168.100.1. Enter 192.168.100.1 into your browser you'll see the diagnostics page.
FAQ credit to Bobcat
Go to »speedtest.rcn.net , pick any server.
At the bottom of the screen you'll see your modem's vital statistics including the highly sought after MAC.
Click for larger view