VOIP Tech Chat → Anyone with a Comcast DPC3939B gateway?

VoIP through this gateway (cable modem + router + wireless) is kind of strange.

SIP works fine using UDP, TLS (tcp 5061) or any alternate ports. Traffic is blocked in a specific kind of way when using SIP over TCP/5060. Several signaling packets pass through and then nothing. I performed a capture on my side of the gateway and again at the VoIP server.

ALG is the usual culprit with this kind of behavior but I am not seeing some of the tell-tale signs, e.g. packet rewriting. Rather, it just lets some packets through, and then starts dropping them.

»[Comcast Equip] DPC3939 indicates there is an ALG in some firmware. I'm just looking for more information and if you have this particular gateway, would be grateful if you'd do some testing with your own phone + VoIP provider over TCP and see what happens.

Do yourself a favor and return that piece of crap and buy a Motorola/Arris 6141 cable modem, or equivalent, and router. Make sure you get a receipt when you return the DPC3939, make several copies, and store the original copy in a safe place

DPC3939 seems to do 16x4 channel bonding, while SB6141 does 8x4 only so in that sense the DPC is superior.
Older DPC gateways have an option to disable the router functionality but you have to dig for it as it's not in an obvious place. Perhaps the 3939 does too.

Assuming that you have the desired firewall protection elsewhere in your network, try setting "Disable entire firewall" under Custom Security.

Also, try changing the client's local SIP port from (or to) 5060.

There are several credible posts that "true bridge mode" on this gateway is not compatible with static IP. It that's your case, try "True Static IP Port Management" as shown at »forums.businesshelp.comc ··· -p/20409 .

If neither of the above work and Comcast won't permit a customer owned modem on this account, try to talk them into supplying a different brand of gateway.

If applicable, using SIP TLS or having the server bind to a port other than 5060 may be an alternate solution.

I'm going to try to work things out with this gateway and Comcast, due to the 16x4 channel bonding mentioned above. However, so far it looks like it will be frustrating. I posted on their forum and got one of those professional forum posters who is giving technically nonsensical answers and claiming there can't possibly be any blockage because Comcast doesn't do that (heh). Maybe the bridge/passthrough mode is going to be the way to go with this one, but I hate to lose the wireless functionality because it's actually a pretty decent wireless access point (2.4G and 5G bands, and this part of the gateway performs quite well). As for changing SIP ports, yes, that works, but I need to test and use SIP on the standard ports in this particular case and TCP/5060 is our default deployment.

Unless your budget is awfully tight, I would not try to salvage this functionality. 5 GHz doesn't penetrate walls and other equipment very well. I like to mount APs on the ceiling or high on a wall, in a clear area. Your ugly modem, needing power, coax and possibly several Ethernet cables will probably be hidden in a closet with other gear. If you later add an AP (with the same SSID) for increased coverage or capacity, you'll likely have problems getting seamless handoff. If you add another Internet connection for backup, the built-in AP won't be able to access it. Take a look at e.g. »www.newegg.com/Product/P ··· 05-00048 .

If Comcast will permit a customer owned modem (and especially if that reduces monthly cost), consider »www.amazon.com/ARRIS-Mot ··· MA5U1FW/ . If not, maybe you can talk them into disabling the SIP ALG, installing more compatible firmware, or replacing the gateway with a model that's not such a turkey.

Here is the follow-up on this. The DPC3939B is just a dud when it comes to VoIP. We were having VPN problems, too. We replaced the device with an Arris SB6183 (16x4) modem and TP-Link Archer C7 wireless router. These are working great and our VoIP and VPN problems are gone. Thanks for the advice given here.