nvolved in, establishing a temporary connection via the PSTN. dialup line A circuit that is established by a switched-circuit connection. The term generally refers to the PSTN. DIAMETER An AAA protocol for applications such as network access and IP mobility. DIAMETER provides a base protocol that can be extended in order to provide AAA services to new access technologies. DID (direct inward dialing) A system in which incoming calls from the exchange network can be completed to specific station lines without attendant assistance. Also called direct dialing in (DDI). Diffie-Hellman A public key algorithm used mostly for exchanging keys. Its security rests on the difficulty of computing discrete algorithms in a finite field, generated by a large prime number. DiffServ (Differentiated Services) An approach to providing QoS in networks that use a small, well-defined set of building blocks from which a variety of services can be built. DiffServ evolved from IETF's IntServ. It is a prioritization model with preferential allocation of resources based on traffic classification. digital Communications procedures, techniques, and equipment whereby information is encoded as either binary 1 or 0; the representation of information in discrete binary form, dis- continuous in time, as opposed to the analog representation of information in variable, but con- tinuous, waveforms. digital certificate A method for registering user identities with a third party, a CA. A digital certificate binds a user to an electronic signature that can be trusted like a written signature and includes authentication, access rights, and verification information. digital loopback A technique for testing the digital processing circuitry of a communica- tions device. It can be installed locally or remotely via a telecommunications circuit. The device being tested echoes back a received test message, after first decoding and then reencoding it, the results of which are compared with the original message. digital network A network that incorporates both di