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Cable Modem Signal Levels
By Wolf Paulus <wolf@wolfpaulus.com>

Posted Friday, March 24th, 2006

If you have a router to connect multiple computers to your high-speed Broadband Internet connection, your certainly have configured the router's DHCP and NAT settings, using a networked computer's Web Browser. On a LinkSys router for instance, this is done by browsing to http://192.168.1.1. You can also use your Web browser to find out how good your cable connection is. The magic address for most cable modems is http://192.168.100.1, some models may require http://root:root@192.168.100.1.

Now, you may immediately have recognize that this address as a non-routeable IP address, which means that we either have to connect a computer directly to the cable modem or hack the NAT-router's routing table. Never, ever, should you connect a Windows box directly to the Internet. Personally, I don't even want to do it with my Macs or Linux systems.


Routing non-routeable IPs

To allow a computer on your LAN to access the cable modem's internal Web server at 192.168.100.1, we need to enter a route, to permit your router to pass traffic through, to a non-routeable IP address. Find out what your router's IP address or its Default Gateway's IP address is - the router's status page should have this information and the IP usually starts with 67 or 68.

Find your router's advanced routing settings page and enter a new route like this:

Route Name : CableModem
Destination LAN IP : 192.168.100.1
Subnet Mask : 255.255.255.255
Default Gateway : (enter your cable modems' or its default gateway's IP)

You should now be able to get to the info-page in your cable-modem and find out how good your connection is. Now, with good I don't necessarily mean fast, but we get to look at Downstream Power, Upstream Power, and Signal to Noise Ratio (SNR) levels.


Downstream Received Power

For a DOCSIS cable modem to work within the specification, the Downstream (or Received) Power level needs to be in the -15dbmV to + 15 dBmV range. The desirable signal level would be -6 dBmV to -3 dBmV.

Downstream Signal-to-Noise Ratio

The downstream SNR tells how much noise is on the cable. Again, for the cable modem to work within the specification, the SNR needs to be at least 23.5dB; a desirable ratio is 30 dB or higher.

Upstream Transmit Power

The cable modem's Upstream (or Transmit) Power needs to be in the +8 to +58 dBmV range. However, the better the return path is, the lower the upstream transmit power needs to be. Meaning we are looking for a value well below 50 dBmV.

Poor signal levels will lead to frequent packet loss, your router not being able to obtain an IP address from the ISP's DHCP server, and spontaneous cable-modem rebooting. However, if the signal levels are not that great, don't blame your cable provider yet. 2 or multiple-way splitters may be responsible for your weak signal as well.



Putting a splitter before the cable modem, without even attaching any additional devices like a TV or VCR, had a strong negative impact on my signal:

Downstream: -1.9 dBmV :: -6.9 (with splitter)
SNR: 35.6 dB :: 34.4 (with splitter)
Upstream: 41.5 dBmV :: 46.8 (with splitter)


Disclosure: This blog is written and edited by me, it contains my words and my opinions only, and does not contain any content which might present a conflict of interest.
I am not compensated to provide opinion on products, services, websites and various other topics. This blog does not accept any form of cash advertising, sponsorship, or paid topic insertions. However, I will and do accept and keep free products, services, and other forms of compensation from companies and organizations. All advertising is in the form of advertisements generated by a third party ad network and identified as such.
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Published on: Friday, March 24th, 2006  •  Category: [hardware]
Article URL : http://wolfpaulus.com/journal/hardware/cablemodem.html