Data Communications Networking Devices - 4th Ed.pdf

156 ______________________________ FUNDAMENTAL WIDE AREA NETWORKING CONCEPTSFigure 1.90Basic ISDN address structureD channel data to the NT. The number of set E bits the TE must receive dependsupon the type of information to be transmitted. Signaling information has a higherpriority than non-signaling information,resulting in eight set E bits that must bereceived for the TE to transmit signaling information,and 10 set E bits fortransmitting non-signaling information.Data carried by an ISDN D channel is encoded using the link access protocol D-channel LAPD) format. LAPD is a layer 2 protocol de®ned by the ITU-T Q.921recommendation whose frame format is identical to HDLC. The composition ofthe address ®eld of Q.921,however,signi®cantly differs from HDLC. ISDNaddressing is standardized by the ITU I.331 Recommendation. That recommendationspeci®es a primary address up to 17 digits in length and an optional subaddressup to 40 digits in length. Figure 1.90 illustrates the basic structure of anISDN address.The country code,national destination code,and subscriber number uniquelyidentify each ISDN subscriber. This variable length number can be up to 17 digitsin length,with the country code standardized in the ITU E.163 Recommendation.The optional sub-address ®eld shown in Figure 1.90 enables organizations toextend their addressing within a private network which is accessible via ISDN. Toprovide interoperability between ISDN and other networks,such as the PSTNand public packet networks,the ITU developed a series of additional recommendations.Its I.330 series of recommendations de®ne internetworking betweenISDN and the North American Numbering Plan NANP) for switched networktelephone calls as well as for D channel transmission to and from X.25 publicpacket networks which use the ITU X.121 Recommendation for network addresses.One of the more publicized features of ISDN is its calling line identi®cationCLID) capability. Essentially,CLID results in the transmission of the caller'stelephone number via the D channel where it will be displayed on a liquid crystaldisplay LCD) built into most ISDN telephones,which are commonly referred toas digital telephones. Since the calling line identi®cation is carried as a series ofbinary numbers,it becomes possible to integrate incoming numbers into acomputer system. This makes it possible for a business to use the incomingnumber as a database search element. For example,an insurance company couldroute the calling number to their mainframe computer. As the telephone operatoranswers the call,the computer could simultaneously search a database and retrieveand display policy information on the operator's terminal. This capability not onlyenhances customer service,but,in addition,increases the productivity oforganization employees.