On Wed, Oct 31, 2007 at 04:56:54PM +0100, Simon Hobson wrote: > SIP does stuff that IAX can't - specifically, SIP was designed to > have the data and control as separate channels. That way, your > registrar/pbx/whatever you want to call it does NOT have to also pass > all the traffic. Eg, you can have your control in one places, but the > voice (or video, or ...) traffic does not have to go through it. > > I would see this as essential if (for example) deploying VoIP across > a WAN with limited bandwidth. It would allow you to have calls > between people in the same office NOT traversing the WAN, or to have > calls between people in two offices not having to go via the main > site. All this without having an exchange in each site.
This feature is not related to SIP's port scattering. It's called a "reinvite", and what happens is: Phone A contacts the PBX, and sends a message asking to set up a call to a given extension. The PBX says "Here's the IP address of your target. Go away." Phone A contacts the given IP address, which is the address of phone B, and sends another message asking to set up a call. It then proceeds as if phone A was given that address by the user in the first place, and the PBX is no longer involved. The entire control layer is transferred - you don't have control going to the PBX and audio going to the target phone. There's no reason why IAX couldn't do the same thing (although offhand I don't know if it does). > Remove NAT and SIP works quite well - even through firewalls as long > as each end will allow outbound traffic and the corresponding > inbound traffic. The outbound traffic from phone a to phone b will > open up the firewall at a, while the outbound traffic from b to a > will open up the firewall at b. After the first two packets are > exchanged, the link is complete and voice will flow. It 'works' FSVO 'works'. The problem is phones that implement silence-suppression: until both ends have generated noise, there are no first two packets, so the whole thing just sits there. This gives you a round of "Hello? Hello? Can you hear me?" at the start of each call. You have to configure the firewall to explicitly pass the inbound audio channel just to work around this. But this is meandering offtopic, so I'll leave it there. ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Shorewall-users mailing list Shorewall-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/shorewall-users
