Donald V. Glen

Abstract: American National Standard (ANS) X3.100 will be adopted as Federal Information Processing Standards Publication (FIPS PUB) 100 and Federal Standard (FED STD) 1041. These standards are based on CCITT Recommendation X.25 and ISO Standards 7776 and 8208. CCITT X.25 is an interface that is oriented toward data circuit–terminating equipment (DCE) for packet operation. While ISO 7776 and 8208 are compatible with X.25, they also provide added guidance that allows two pieces of data terminal equipment (DTE) to exchange packets over a dedicated path or circuit–switched network without a packet–switched network in between. Hence, ANS X3.100 and FIPS PUB 100/FED STD 1041 provide for operation that may, or may not, include an intervening packet network. While X.25 requires a dedicated circuit for access, CCITT Recommendation X.32 essentially modifies X.25 and allows switched access to a packet-switched network. Consequently, X.32 has been called a "dial-in, dial-out X.25" through the use of, for example, a public switched telephone network (PSTN). Although X.32 is not part of FIPS PUB 100/FED STD 1041, it is described in this reference manual. This reference manual provides a tutorial description of terminals operating in the packet mode and the applicable standards at the physical, link, and packet levels. This should permit the reader to begin understanding an area of data communications that has become popular and is being implemented throughout the United States and other countries.

Keywords: Federal standards; Open Systems Interconnection (OSI); DTE/DCE interfaces; high-level data link control (HDLC); ISO 7776; ISO 8208; link access procedure,balanced (LAPB); link access procedure, half-duplex (LAPX); link level; logical channels; packet level protocol (PLP); packet switching; packet switched public data network (PSPDN); physical level; public switched network (PSN); X.21; X.21 bis; X.25; X.32

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