ZyXEL Communications NBG410W3G Series User Manual

Chapter 18 ALG Screen

NBG410W3G Series User’s Guide

• SIP clients can be connected to the LAN or DMZ. A SIP server must be on the WAN. 
• You can make and receive calls between the LAN and the WAN, between the DMZ and 

the WAN. You cannot make a call between the LAN and the LAN, between the LAN and 
the DMZ, between the DMZ and the DMZ, and so on.

• The SIP ALG allows UDP packets with a port 5060 destination to pass through.
• The ZyXEL Device allows SIP audio connections. 

The following example shows SIP signaling (1) and audio (2) sessions between SIP clients A 
and B and the SIP server. 

Figure 184   SIP ALG Example 

Most SIP clients have an “expire” mechanism indicating the lifetime of signaling sessions. 
The SIP user agent sends registration packets to the SIP server periodically and keeps the 
session alive in the ZyXEL Device. 
If the SIP client does not have this mechanism and makes no calls during the ZyXEL Device 
SIP timeout default (60 minutes), the ZyXEL Device SIP ALG drops any incoming calls after 
the timeout period. 

If no voice packets go through the SIP ALG before the timeout period (default 5 minutes) 
expires, the SIP ALG does not drop the call but blocks all voice traffic and deletes the audio 
session. You cannot hear anything and you will need to make a new call to continue your 
conversation.

Click ADVANCED > ALG to open the ALG screen. Use the ALG screen to turn individual 
ALGs off or on and set the SIP timeout.