DPC3939B Working Properly?

Question

How do I know if my modem is working properly? We sometimes get random modem reboots, dropped ssh connections to remote servers, can't get to google mail, can't login to wifi (2.4 or 5), computers connected by ethernet cable can't get to any website and then it sometimes works fine.

Also the firewall is set to Typical Security (Medium): Blocks P2P applications and pings to the Gateway, but allows all other traffic. But I can still ping the modem.

Also is there any documention describering what the values from the wan status are?

thanks

Downstream	Channel Bonding Value
Index	1	2	3	4	5	6	7	8
Lock Status	Locked	Locked	Locked	Locked	Locked	Locked	Locked	Locked
Frequency	651 MHz	657 MHz	663 MHz	669 MHz	675 MHz	681 MHz	687 MHz	693 MHz
SNR	38.605 dB	37.636 dB	37.636 dB	38.605 dB	37.636 dB	37.636 dB	37.636 dB	37.636 dB
Power	-2.100 dBmV	-3.100 dBmV	-3.700 dBmV	-3.900 dBmV	-3.900 dBmV	-3.800 dBmV	-3.000 dBmV	-1.100 dBmV
Modulation	256 QAM	256 QAM	256 QAM	256 QAM	256 QAM	256 QAM	256 QAM	256 QAM

Upstream	Channel Bonding Value
Index	1	2	3
Lock Status	Locked	Locked	Locked
Frequency	29 MHz	22 MHz	36 MHz
Symbol Rate	5120 KSym/sec	2560 KSym/sec	5120 KSym/sec
Power Level	44.250 dBmV	42.960 dBmV	45.000 dBmV
Modulation	64 QAM	16 QAM	64 QAM
Channel ID	11	12	10

CM Error Codewords
Unerrored Codewords	3079810687	3079810687	3079810687	3079810687	3079810687	3079810687	3079810687	3079810687
Correctable Codewords	0	0	0	0	0	0	0	0
Uncorrectable Codewords	0	0	0	0	0	0	0	0

rdinoff · Accepted Answer

To resolve the issues comcast repalced the DPC3939B with a SMCD3G-CCR and a Netgear N300.  We then replaced  the Netgear with our Asus AC2400 and no problems since.

akent0405 · Answer

I do not have a resolution for this but I do want to report we have been having similar problems with this modem at our business. I run it in a sort of bridge mode, though of course not true bridge mode (Why is this not allowed?) but it works fine MOST of the time with our Cisco ASA 5510.

I too am experiencing random reboots, probably on average 1-3 times per week. Also, at one point I was in the administrative web interface on the modem viewing settings, and clicked on a button to go back to the previous page or menu, and the modem rebooted. I did not click anything or apply anything that would have caused a reboot.

The modem is on a UPS battery backup so I know it's not a power loss of some kind. Most all of the other equipment I have with Comcast (an Adtran PRI gateway and arris modem and an Arris telephony modem) is solid and works fine unless there is an area wide outage. We have been visited once by a service tech and he installed a filter between the coax cable itself and the input for the coax on the modem, but no change.

Can anyone shed light on what's going on here and what a possible resolution is?

Thanks.

akent0405 · Answer

Was afraid I'd have to do that, but I did and I am glad I did!

It solved a host of problems for us and I'm kind of scratching my head as to why that DPC3939B is even out there. It seems very buggy.

Some things swapping to the SMC modem has appeared to resolve for us:

1. As I said in an above post, random reboots. These have probably gone away, but I will know more later.

2. Ping times and latency across VPN tunnels. With the Cisco/DPC I was noticing higher ping times across the tunnels and a lot of inconsistency there. I'd go from 20ms to 56ms to 40ms to 25ms. Now I'm getting several pings in a row at the same times. For example ill get about 6 at 19ms, then a couple at 21 or 22, then back to 19 or 20.

3. Lost packets across a particular VPN tunnel. For a period of time we had been losing a small amount of packets on a VPN tunnel that is an AT&T DSL on the other side of the tunnel. Now I'm getting a consistent 62-65ms ping across that tunnel with no dropped packets. Before it was bouncing around between 60 and 80ms with dropped packets.

4. WiFi dying out for no obvious reason. There was definitely a problem with the WiFi in the DPC3939B. The tech who arrived to install the SMC asked if we had tried disabling that. We did and it didn't solve the rebooting, but interesting that he asked that.

I can definitely see one reason why the SMC modem is stable. It is very simple.

Comcast: please look into why the DPC3939B has so many problems and fix them if you wish to keep issuing this modem to customers. Businesses need stability!

rdinoff

DPC3939B Working Properly?

rdinoff

akent0405

akent0405