[Serusers] Is STUN protocol usable behind firewall?

Hi all, In my test environment there are a firewall between SER server and SIP terminal(But I am not sure). I think so because in this envirmonent SJPhone can not register to SER server (I use ethereal on SER server and can't find register packets from SJPHONE). I had tested SJPHONE behind a NAT created by cisco router and success. So I think that thing is a firewall. Today I test X-Lite in the same environment with same setting but it works. I get the UDP packet by ethereal. I find it send packet to xtunnel.xten.net port 3478/3479. These ports are used by STUN protocol. I would why it works. Attached file can be open by ethereal. In the file no reponse register is sent by SJPhone and those success is sent by X-Lite. The file is bigger than 40K so I compress it by WinRAR. I trying X-Lite again and it says the firewall is port restricted single mapped port system. What's that means? Br, Wangji