Data Communications Networking Devices - 4th Ed.pdf

64 _______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSITU recommendations are designated according to the letters of the alphabet,from Series A to Series Z,with technical standards included in Series G to Z. Inthe ®eld of data communications,the most well known ITU recommendationsinclude Series I which pertains to Integrated Services Digital Network ISDN)transmission,Series Q which describes ISDN switching and signaling systems,Series V which covers facilities and transmission systems used for datatransmission over the PSTN and leased telephone circuits,the DTE±DCEinterface and modem operations,and Series X which covers data communicationsnetworks to include Open Systems Interconnection OSI).The ITU V-seriesTo provide readers with a general indication of the scope of ITU recommendations,Table 1.21 lists those promulgated for the V-series at the time this book wasprepared. In examining the entries in Table 1.21,note that the ITU RecommendationV.3 is actually a slightly modi®ed ANSI X3.4 standard,the ASCII code.For international use,the V.3 recommendation speci®es national currency symbolsin place of the dollar sign $) as well as a few other minor differences. Youshould also note that certain ITU recommendations,such as V.21,V.22 and V.23,among others,while similar to AT&T Bell modems,may or may not provideoperational compatibility with modems manufactured to Bell speci®cations.Chapter 5 provides detailed information concerning modem operations andcompatibility issues.ISOThe International Organization for Standardization ISO) is a non-governmentalentity that has consultative status within the UN Economic and Social Council.The goal of the ISO is to `promote the development of standards in the world witha view to facilitating international exchange of goods and services'.The membership of the ISO consists of the national standards organizations ofmost countries,with approximately 100 countries currently participating in itswork.Perhaps the most notable achievement of the ISO in the ®eld of communicationsis its development of the seven-layer Open Systems Interconnection OSI)Reference Model. This model is discussed in detail at the end of this section.De facto standardsPrior to the breakup of AT&T,a process referred to as divestiture,US telephoneinterface de®nitions were the exclusive domain of AT&T and its researchsubsidiary,Bell Laboratories. Other vendors which developed equipment for useon the AT&T network had to construct their equipment to those interfacede®nitions. In addition,since AT&T originally had a monopoly on equipment thatcould be connected to the switched telephone network,upon liberalization of that