ZyXEL Communications 70 Series User Manual

ZyWALL 5/35/70 Series User’s Guide

469 

Chapter 29 ALG Screen

Figure 234   H.323 Calls from the WAN

 

with Multiple Outgoing Calls

• The H.323 ALG operates on TCP packets with a port 1720 destination.
• The ZyWALL allows H.323 audio connections. 
• The ZyWALL can also apply bandwidth management to traffic that goes through the 

H.323 ALG.

The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol that 
handles the setting up, altering and tearing down of voice and multimedia sessions over the 
Internet. SIP is used in VoIP (Voice over IP), the sending of voice signals over the Internet 
Protocol.

SIP signaling is separate from the media for which it handles sessions. The media that is 
exchanged during the session can use a different path from that of the signaling. SIP handles 
telephone calls and can interface with traditional circuit-switched telephone networks.

STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address 
Translators) allows the VoIP device to find the presence and types of NAT routers and/or 
firewalls between it and the public Internet. STUN also allows the VoIP device to find the 
public IP address that NAT assigned, so the VoIP device can embed it in the SIP data stream. 
See RFC 3489 for details on STUN. You do not need to use STUN for devices behind the 
ZyWALL if you enable the SIP ALG.

• SIP clients can be connected to the LAN, WLAN or DMZ. A SIP server must be on the 

WAN. 

• You can make and receive calls between the LAN and the WAN, between the WLAN 

and the WAN and/or 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 LAN and the 
WLAN, between the DMZ and the DMZ, and so on.

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