 interface. The United States and Japan use 23B+D, and the ITU uses 30B+D. Also called Primary Rate Access (PRA). primary station On a point-to-point communication line, the station that gains control of the line first. On a multipoint line, the station that controls communications. private key The decryption (reception) or encryption (signature) component of an asymmet- rical key set. private line The channel equipment furnished to a customer as a unit for exclusive use, gen- erally with no access to or from the PSTN. Also called a leased line. private network A network based on leased lines or other facilities that provides telecom- munication services within an organization or within a closed user group, as a complement to or a substitute for the public network. proprietary Hardware or software that is privately owned. protocol A set of rules that govern network communications. Low-level protocols define transmission rates, data encoding schemes, physical interfaces, network addressing schemes, and the method by which nodes contend for the chance to transmit data over a network. High-level protocols define functions such as printing and file sharing. protocol header Information in a packet that the protocol needs in order to do its work. protocol stack (or protocol suite) A collection of protocols that computers use to exchange information. Glossary provisioned VPN provisioned VPN A packet-switched VPN that runs across the service provider's backbone, generally using Frame Relay or ATM. proxy ARP (proxy Address Resolution Protocol) A technique in which one machine, usually a router, answers ARP requests intended for another by supplying its own physical address. By pretending to be another machine, the router accepts responsibility for routing pack- ets to it. The purpose of proxy ARP is to allow a site to use a single IP network address with mul- tiple physical networks. proxy server A server that provides firewall functionality, acting as an intermediary for user requests and establishing a connection to the