Monday 27 October 2008

Dynalink RTA1046VW DMZ Config Kills VoIP

While trying to speed up my P2P downloads yesterday I decided to configure a DMZ host in my Dynalink RTA1046VW router (on advice from a mid-2006 forum thread). My P2P ports were forwarded correctly, according to, but I figured it would be worth the experiment.

As I'm now on a naked ADSL 2+ plan, I no longer have a traditional land line and all inbound and outbound calls are shunted across the internet; to facilitate this, my analog phone plugs directly into the back of my modem. My new ADSL account was live on Friday and I spoke with my sister in Canada successfully for an hour straight on Saturday morning. On Sunday I configured the DMZ host and from that point forward I noticed I could no longer hear anything in the phone's earpiece. The internet connection was otherwise fine, apart from a few dropouts every so often, but there was absolutely nothing coming out of the phone's earpiece; calls were otherwise going through (sporadically) but the caller could hear me fine. To top it all off I got home tonight and successfully replayed my phone messages. The problem was reproducible with another phone. Weird. 

The only thing left to do was remove the DMZ host configuration and behold--the tin cans work! I don't get it--I really don't know much about VoIP but my guess is VoIP runs over a port and the port was being forwarded by the router to the DMZ host... dunno, especially considering the default setting is to simply discard packets not belonging to an application configured in the port forwarding table. 

[Update: Silly me, of course VoIP runs over a port--the port I set when I configured VoIP in the first place, duh. Anyway, I still reckon it was being forarded the DMZ host but I've noticed the kitchen microwave interferes with the phone and produces a similar problem where, after successfully initiating a call, I can hear the person I'm speaking to but they can't hear me.

By the way, my P2P speed reached about 180KBps down--a dizzying new height for the likes of someone who had no internet before this point.]

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