 when someone within their community of users is available for real time or near real time messaging. PRI (Primary Rate Interface) A form of ISDN with 23 paths for voice, video, and data and 1 channel for signals. Each of the 24 channels transmits at 64Kbps. www.TechnicalPDF.com protocol Defines how devices and networks communicate with one another. For example, a suite of protocols known as TCP/IP spells out rules for sending voice, images, and data across the Internet and in corporate networks. proxy server Authenticates callers to ensure that they are who they say they are before they are sent to their destination. They serve as intermediaries between callers and applications or endpoints, telephones, and other devices connected to the LAN. For instance, a proxy server in a VoIP environment ensures that external devices requesting to communicate with an IP telephone are who they say they PSAP (Public Safety Answering Point) Groups of agents that answer and dispatch 911 and E-911 calls for their town, county, or cluster of towns. They are often located at police stations. PSTN (Public Switched Telephone Network) The global network of circuit-switched telephone services that telephone companies operate. The PSTN comprises traditional copper-based telephone lines, central office switches, fiber-optic cabling, undersea cable systems, cellular systems, microwave connections, communications satellites, and any other system connected via public telephone switching centers. It does not include the Internet or carriers' private data networks. push-to-talk A walkie-talkie type of service pioneered by Nextel (now part of Sprint) in which customers reach one another by pushing a button on the side of their phone. They also dial an abbreviated telephone number. Push-to-talk is used to reach individuals or predefined groups. Walkie- talkies are used on non-mobile spectrum between people www.TechnicalPDF.com that perhaps hike together or for parents to monitor children in other rooms of a house. QoS (Quality of Service) The 