ddition to multimedia, tagging protocols such as 802. 1P are used for conferencing services, as well. VLANs shield endpoints from hackers by allowing only certain types of packets through firewalls to reach them. This is accomplished by means of policies for firewall logical ports that are dedicated to voice and video traf- fic. Logical firewall ports are defined in software rather than actual physical ports. IP PRIVATE BRANCH EXCHANGE ARCHITECTURE Internet Protocol Private Branch Exchange (IP PBX) architecture varies somewhat among different manufacturers, but the servers for call processing and gateway func- tionality are common to all. The gateway converts Voice over Internet Protocol (VoIP) signals to those compatible with the Public Switched Telephone Network (PSTN). These are located in a separate gateway or in a router (see Figure 2-4). In addition, Layer 2 switches transmit calls to other devices connected to the switch and backbone switches. Data Centers and IP Private Branch Exchanges Remote office Headquarters VolP system in a box Router Router 000000000 Layer 2 switch Gateway Media server Centralized voicemail/ Unified Communications server VolP = Voice over Internet Protocol WAN = Wide Area Network Figure 2-4 An overview of the IP PBX architecture. Communications Servers and Voice as an Application on the LAN Communications servers perform call processing, sending instructions on how to set up calls. They send instructions for features such as three-party conferencing, speed dial, and transfer. Communications servers host the software for the system, trunk- ing, and station features. These servers specify the trunks over which international, local, and interstate calls should be routed. They also have lists of user profiles with extension numbers and permissions. Permissions include specifying who is allowed to use particular features such as video conferencing and placing calls to international locations. Most IP servers run on UNIX, Linux, or Microsoft's Windows operating systems. These operating s