call request --An X.25 call packet sent from a DTE to a DCE that initiates a connection to a destination DTE.
closed user group selection facility --A specific encoding element that can be presented in a call request or incoming call. A CUG selection facility in a call request allows the source DTE to identify the CUG within which it is placing the call. A CUG selection facility in an incoming call allows the destination DTE to identify the CUG to which both DTEs belong.
CPE --customer premises equipment. Terminating equipment, such as terminals, telephones, and modems, supplied by the telephone company, installed at customer sites, and connected to the telephone company network. This equipment is available for customer modification and is considered insecure by the network.
CUG --closed user group. A collection of DTE devices for which the network controls access among members and between members and nonmembers. A DTE may subscribe to zero, one, or more CUGs. A DTE that does not subscribe to a CUG is referred to as being in the open part of the network.
DCE --data communications equipment. A network connection where a subscriber can be attached. A DCE is configured with the operational details for which a given subscriber (DTE) has contracted.
DTE --data terminal equipment. A network subscriber that can be reached at a specific network attachment point. A network identifies each DTE device by assigning an X.121 address.
incoming call --An X.25 call packet sent from a DCE to a DTE that presents a connection requested by the source DTE.
PAD --packet assembler/disassembler. Device used to connect simple devices (like character-mode terminals) that do not support the full functionality of a particular protocol to a network. PADs buffer data and assemble and disassemble packets sent to such end devices.
POP --point of presence. In the context of a public data network, a POP is the part of the network to which CPE is attached. A POP is configured and controlled by the public network and serves as the boundary equipment between the trusted network and insecure client attachments.
preferential closed user group --The CUG that is assumed when a CUG is not specified in call setup. A DTE that subscribes to more than one CUG and does not have incoming or outgoing access must designate a preferred CUG.