a Service Unit Interface Specification, Bell System Reference 41450, AT&T, New York, 1981. 20. Information Processing Systems Open Systems Interconnection-Basic Reference Model, ISO 7498, Geneva, 1984. 21. Advanced Data Communications Control Procedures, X.3.66, ANSI, New York, 1979. 22. Interface Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE) for Terminals Operating in the Packet Mode and Connected to Public Data Networks by a Dedicated Circuit, ITU-T Rec. X.25, ITU Geneva, 1996. 23. ISDN User Network Interface-Data Link Layer, ITU-T Rec. Q.921, ITU Geneva, 1997. 24. Information Processing Systems-Local Area Networks, Part 2, Logical Link Control, IEEE Std. 802.3, 1994 edition, IEEE, New York, 1994. DATA COMMUNICATIONS 25. High-Level Data Link Control Procedures-Frame Structure, ISO 3309, International Standards Organization, Geneva, 1979. 26. Military Communication System Technical Standard, MIL-STD-188C, US Department of Defense, Washington, DC, 1966. 27. Reference Data for Engineers: Radio, Computers and Communications, 8th edition, SAM Pub- lishing, Carmel, IN, 1993. 28. A Digital Modem and Analog Modem Pair for Use on the Public Switched Telephone Network (PSTN) at Data Signaling Rates of up to 56,000 bits/s downstream and up to 33,600 bits/s Upstream, ITU-T Rec. V.90, ITU Geneva, 1998. 29. A Digital Modem Operating at Data Signaling Rates up to 64,000 bits/s for Use on a 4-wire Circuit Switched Connection and on Leased Point-to-point 4-wire Digital Circuit, ITU-T Rec. V.91, ITU-Geneva, May 1999. 30. List of Definitions for Interchange Circuits Between Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE), ITU-T Rec. V.24, ITU Geneva, 2000. ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS 11.1 WHAT DO ENTERPRISE NETWORKS DO? An enterprise network consists of an interconnected group of telecommunication facilities that are confined to a singular entity. There can be broad, midrange, and narrow inter- pretations of the definition. The "singular entity" may be