recipient might be using at a given moment (e.g., landline phone, mobile phone, or desktop computer). The ISDN Primary Rate Interface. In the US the PRI consists of 23 64 Kbps bearer or "B" channels and one 64 Kbps delta or "D" channel. In Europe, the PRI- equivalent is known as Primary Rate Access or PRA. The PRA consists of 30 64 Kbps B channels and two 64 Kbps D channels. prism Crystalline bodies with lateral faces that meet at edges that are parallel to each other. A prism is used to refract light, often dispersed into component wavelengths. private line A telephone line rented for the exclusive use of a business and which connects two locations on a point- to-point basis. Communications can only be exchanged between these two locations, as opposed to switched services in which calls can be placed to any other addressable location with compatible service. private network A collection of leased, private circuits that link multiple locations of an enterprise or organization. May be used to transmit different combinations of voice, data and video signals between sites over the point-to-point lines that exist between sites. proprietary Systems that use techniques and processes that are closely-guarded trade secrets and which, because of these highly-individual approaches to achieving a result, cannot interoperate with other manufacturers' equipment without a great deal of difficulty. protocol A standard procedure agreed upon by regulating agencies, companies, or standards-setting bodies to regulate transmission and, therefore, to achieve inter- communications between systems or networks. Public Switched Telephone Network. The conventional voice telephony network as provided by Local Exchange Carriers. Video Communication: the Whole Picture Post Telephone and Telegraph company. A generic term for the telephony providers in Europe and other non-US countries. publicroom A commercial facility that offers video communications services for rent. Pronounced "P times 64." Another (early) name for the ITU-T's H.261 codec.