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Cable modem channel and signal strength help

Posted on 2010-09-05
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Last Modified: 2012-05-10
Hi Experts!

I got 2 Comcast cable modems (SMC D3G) sharing a single coax line that's coming into the building. Each modem has its own customer account with Comcast.

1st modem: 20Mbps/5Mbps (is installed by Comcast tech)
2nd modem: 50Mbps/10Mbps (was installed by Comcast tech, but I moved it to another part of the building - 400ft run with RG11)

Both modems are working, but are not stable. When I only had 1 modem used at a time, no problems, but now they crash once every few days. Restarting the modem brings the connection back up.

I'm suspecting that the two modems may be interfering with each other, or the 2nd modem has improper signal strengh. I've noticed that both modems have the same Channel ID: 5 Is this a problem?


Is this modem status output helpful?

Modem Number 1:

Downstream Channel
Downstream Frequency       717.000000 MHz      723.000000 MHz      729.000000 MHz      735.000000 MHz
Lock Status       Locked      Locked      Locked      Locked
Modulation       256 QAM      256 QAM      256 QAM      256 QAM
Symbol Rate       5.360537Msym/sec      5.360537Msym/sec      5.360537Msym/sec      5.360537Msym/sec
Downstream Power       2.663050 dBmV       3.001565 dBmV       2.982143 dBmV       2.973500 dBmV
SNR       36.844463 dB      37.092701 dB      37.092701 dB      37.355988 dB
Upstream Channel
Upstream Frequency       30599659 Hz                  
Lock Status       Locked      Not locked      Not locked      Not locked
Modulation       64QAM                  
Symbol Rate       5120 KSym/sec                  
Upstream Power       54.0000 dBmV                  
Channel ID       5                  


Thanks in advance!
MODEM Number 2 (400ft away):

Downstream Channel
Downstream Frequency 	717.000000 MHz	723.000000 MHz	729.000000 MHz	735.000000 MHz
Lock Status 	Locked	Locked	Locked	Locked
Modulation 	256 QAM	256 QAM	256 QAM	256 QAM
Symbol Rate 	5.360537Msym/sec	5.360537Msym/sec	5.360537Msym/sec	5.360537Msym/sec
Downstream Power 	5.529790 dBmV 	5.396259 dBmV 	5.335433 dBmV 	5.210467 dBmV
SNR 	37.092701 dB	37.355988 dB	37.355988 dB	37.355988 dB
Upstream Channel
Upstream Frequency 	30599440 Hz			
Lock Status 	Locked	Not locked	Not locked	Not locked
Modulation 	64QAM			
Symbol Rate 	5120 KSym/sec			
Upstream Power 	39.7500 dBmV			
Channel ID 	5

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Question by:Kaptain1
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HRL earned 500 total points
ID: 33608257
When dealing with Cable Modems you have to have a balanced connection otherwise signal strength will be degraded. Channel ID is not an issue since they are operting at different frequency.
It might be a good idea to change the cable to RG6 or to add an amplifier to modem 2..
If you look at the power readings you can see that modem 2 has less power both up and downstream. You will have to balance this out.
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