 mobile content delivery. (Wide WDM) CWDM operating at 1310 nm, compared to traditional CWDM, which operates at 850 nm. www (World Wide Web) An Internet application that uses hypertext links between remote network servers for accessing and displaying multimedia information. An ITU-T standard that defines the basic functionality of an asynchronous PAD, usually used in conjunction with ITU-T standards X.28 and X.29. X.21 An ITU-T recommendation that defines a general-purpose physical interface between a DTE and DCE for full-duplex synchronous operation on circuit-switched or packet-switched data networks. Glossary X.25 A standard that defines the interface between a DTE and DCE for equipment operating in the packet mode on public data networks. It also defines a link control protocol. X.75 A standard for connecting X.25 networks, developed by the ITU-T X.121 The ITU-T standard for the addressing plan used with X.25 PDNs. X.400 An ITU-T standard for electronic mail exchange. X.500 The family of protocols that define the operation of the ITU-T/ISO directory service. X.509 The ITU-T/OSI recommendation for a directory authentication framework. xDSL (Digital Subscriber Line) The DSL family of broadband technologies that use sophisticated modulation schemes to pack data onto copper wires. They include ADSL, HDSL, and VDSL. XMPP (Extensible Messaging and Presence Protocol) An XML-based extensible open-source protocol that drives the Jabber IM client. ZigBee The IEEE 802.15.4 standard, a wireless communication protocol designed for WPANs. ZigBee operates, internationally, in the unlicensed frequency bands, and it supports a low data rate, a very long battery life (months to even years), and very low complexity. Index Numbers digital cellular radio, 586-588 10GigE, 479, 480-481 GSM, 567, 588-589 16:9 aspect ratio, 398-399 PDC, 592 1G (first-generation) cellular networks, 581-586 UWC, 589 analog, 581-583 3.5G (3.5-generation) cellular networks, 602-605 CDPD packet data protocol, 583-584 3A software, 375 defined, 580 3DES (Tr