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.Cisco IOS uses the AAAmodel for implementing security.ISDN offers the use of Caller-ID and DNIS information to provideadditional security design flexibility.Cost Containment IssuesA primary goal of selecting ISDN for your internetwork is to avoid the cost of full-time data services(such as leased lines or frame relay).As such, it is very important to evaluate your data traffic profilesand monitor your ISDN usage patterns to ensure your WAN costs are controlled.Dialer Callbackcan also be implemented to centralize billing.Each of these building blocks of ISDN (connectivity, data encapsulation, DDR, security, and costcontainment) is discussed in further detail in the remaining sections of this chapter.11-4 Internetwork Design Guide ISDN Connectivity IssuesISDN Connectivity IssuesBased on application need and traffic engineering, BRI or PRI services are selected for ISDNconnectivity from each site.Traffic engineering may require multiple BRI services or multiple PRIsat some sites.Once connected to the ISDN fabric by BRI or PRI interfaces, design of ISDNend-to-end services must be implemented.This section covers the following issues related to ISDNconnectivity:" Establishing BRI Connectivity" Establishing ISDN Primary Rate Interface (PRI)" ISDN End-to-End Considerations" Datagram Encapsulation IssuesEstablishing BRI ConnectivityThe BRI local loop is terminated at the customer premise at an NT1.The interface of the local loopat the NT1 is called the U reference point.On the customer premise side of the NT1 is the S/Treference point.The S/T reference point can support a multipoint bus of ISDN devices (TerminalAdapters).Figure 11-6 shows a typical BRI installation.Figure 11-6 The BRI local loop connected to ISDN.S/T UISDNNT1BRIBRI HardwareTwo common types of ISDN CPE are available for BRI Services: ISDN routers and PC TerminalAdapters.Some BRI devices offer integrated NT1s and integrated Terminal Adapters for analogtelephones." LAN Routers ISDN Routers provide routing between ISDN BRI and the LAN by usingDial-on-Demand Routing (DDR). DDR automatically establishes and releases circuit switched calls providing transparentconnectivity to remote sites based on internetworking traffic.DDR also controls establishingand releasing secondary B channels based on load thresholds.MultiLink PPP is used toprovide bandwidth aggregation when using multiple B channels.For more information onDDR, see Chapter 10, Designing DDR Internetworks. Some ISDN applications may require the SOHO user to take direct control over ISDN calls.Emerging Cisco IOS features can bring this control to the user desktop.New Cisco 700models provide a call button on the front of the router for direct control. Cisco 700 series and Cisco IOS based 1000, 1600, 2500 routers provide single BRIinterfaces.Multiple-BRI interfaces are available for the Cisco3600 and Cisco4x00 Series." PC Terminal Adapters (PC-TA) These devices connect to PC workstations either by the PC Busor externally through the communications ports (such as RS-232) and can be used similar toanalog (such as V.34) internal and external modems.Designing ISDN Internetworks 11-5 ISDN Connectivity Issues PC Terminal Adapters can provide a single PC user with direct control over ISDN sessioninitiation and release similar to using an analog modem.Automated mechanisms must beprovided to support the addition and removal of the secondary B channel.Cisco200 SeriesPC Cards can provide ISDN services to a PC.BRI ConfigurationBRI configuration involves configuration of ISDN switch-type, and ISDN SPIDs, as follows:" ISDN switch types ISDN central office switches (also known as local exchange equipment)provide two functions at the local exchange: local termination and exchange termination.Thelocal termination function deals with the transmission facility and termination of the local loop.The exchange termination function deals with the switching portion of the local exchange.First,the exchange termination function de-multiplexes the bits on the B and D channels.Next, Bchannel information is routed to the first stage of the circuit switch, and D channel packets arerouted to D channel packet separation circuitry. For proper ISDN operation, it is imperative that the correct switch type is configured on theISDN device.For Cisco IOS releases up to 11.2, the configured ISDN switch-type is a globalcommand (note this also means you cannot use BRI and PRI cards in the same Cisco IOSchassis).In Cisco IOS 11.3T or later, multiple switch-types in a single Cisco IOS chassis arenow supported. Cisco IOS switch typesThe following Cisco IOS command helps illustrate the supported BRI switch types.In NorthAmerica, the most common types are 5ESS, DMS100, and NI-1.kdt-3640(config)#isdn switch-type ?basic-1tr6 1TR6 switch type for Germanybasic-5ess AT&T 5ESS switch type for the U.S.basic-dms100 Northern DMS-100 switch typebasic-net3 NET3 switch type for UK and Europebasic-ni1 National ISDN-1 switch typebasic-nwnet3 NET3 switch type for Norwaybasic-nznet3 NET3 switch type for New Zealandbasic-ts013 TS013 switch type for Australiantt NTT switch type for Japanvn2 VN2 switch type for Francevn3 VN3 and VN4 switch types for France Cisco 700 switch types.On Cisco 700 Series routers, use the set switch command, which has the following optionswhen running the US software image:SEt SWitch 5ESS | DMS | NI-1 | PERM64 | PERM128" Service profile identifiers (SPIDs) A service profile identifier (SPID) is a number provided bythe ISDN carrier to identify the line configuration of the BRI service.SPIDs allow multiple ISDNdevices, such as voice and data, to share the local loop.SPIDs are required by DMS-100 andNational ISDN-1 switches.Depending on the software version it runs, an AT&T 5ESS switchmight require SPIDs as well [ Pobierz całość w formacie PDF ]