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| reply to artisticcheese
Re: How multiple ATAs can be behind same NAT router? said by artisticcheese:said by garys_2k:Another thing to keep in mind is that all SIP traffic, and the RTP traffic that the SIP traffic sets up, originates FROM the ATA which is behind the router. Port forwarding should not be needed for any ATA. It's the router's job to keep track of which internal IP sent a request out to which external IP and route the reply back -- it is the essence of NAT. You can have multiple PCs browsing the same web page and the router will keep track of which machine just asked for the page to be refreshed, it's no different. ATA traffic is UDP but those incoming packets are not unsolicited. How this will work for incoming call? Here's how it works...
Your ATA sends the SIP server an "I'm here" message, the SIP server sends back a "Got it, thanks" reply. This happens regularly, and it should happen regularly enough to keep your router's tables current. My Vonage ATA does this 3 to 4 times per minute, plenty often enough.
When a call comes in the SIP server (which is still current to the ATA in the table) sends the ATA a message "Hey, go take a call at this IP address." The ATA sends back an OK to the SIP server and then initiates contact to the RTP server's address that the SIP server delivered to it.
That goes something like, ATA: "Hey, RTP server, heard you have a call for me -- here's my phone number." The RTP server sends back "Yeah, let's get it going. Start the bell ringing and here's the caller ID info to throw on the wire."
The call proceeds entirely on the RTP server but communication with the SIP server is maintained. Once the call is over the ATA and the SIP server put things away and the regular traffic starts over again.
As for handling different ATAs, the router's NAT table remembers which internal IP is associated with which SIP and RTP traffic. All of that traffic is initiated OUT from inside the LAN so the router, if it doesn't get mixed up (which some cheap ones can, especially if they're handling tons of addresses as can happen with file sharing) sends the packets to the proper IP on both sides. |
