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.PPP links can then be used bynetwork protocols such as IP and IPX to establish internetwork connectivity.PPP solutions can support bandwidthaggregation using MultiLink PPP to provide greater throughput for internetworking applications.DDR: Dial-On-Demand RoutingWhen building internetworking applications, designers must determine how ISDN connections will be initiated,maintained, and released.DDR is a sophisticated set of Cisco IOS features that intelligently establishes and releasescircuit switched connections as needed by internetworking traffic.DDR can spoof internetwork routing and directoryservices in numerous ways to provide the illusion of full-time connectivity over circuit switched connections.Refer to"Designing DDR Internetworks," for a discussion of DDR design.Security IssuesBecause your internetwork devices can now be connected to over the PSTN, it is imperative to design and confirm arobust security model for protecting your network.Cisco IOS uses the AAA model for implementing security.ISDNoffers the use of Caller-ID and DNIS information to provide additional 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 leasedlines or frame relay).As such, it is very important to evaluate your data traffic profiles and monitor your ISDN usagepatterns to ensure your WAN costs are controlled.Dialer Callback can also be implemented to centralize billing.Each of these building blocks of ISDN (connectivity, data encapsulation, DDR, security, and cost containment) isdiscussed in further detail in the remaining sections of this chapter.ISDN Connectivity IssuesBased on application need and traffic engineering, BRI or PRI services are selected for ISDN connectivity from eachsite.Traffic engineering may require multiple BRI services or multiple PRIs at some sites.Once connected to the ISDNfabric by BRI or PRI interfaces, design of ISDN end-to-end services must be implemented.This section covers thefollowing issues related to ISDN connectivity:Establishing BRI ConnectivityEstablishing ISDN Primary Rate Interface (PRI)ISDN End-to-End ConsiderationsDatagram Encapsulation Issueshttp://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2011.htm (5 of 25) [9/16/2000 5:11:15 PM] Designing ISDN InternetworksEstablishing BRI ConnectivityThe BRI local loop is terminated at the customer premise at an NT1.The interface of the local loop at the NT1 is calledthe U reference point.On the customer premise side of the NT1 is the S/T reference point.The S/T reference point cansupport a multipoint bus of ISDN devices (Terminal Adapters).Figure 11-6 shows a typical BRI installation.Figure 11-6: The BRI local loop connected to ISDN.BRI HardwareTwo common types of ISDN CPE are available for BRI Services: ISDN routers and PC Terminal Adapters.Some BRIdevices offer integrated NT1s and integrated Terminal Adapters for analog telephones.LAN Routers---ISDN Routers provide routing between ISDN BRI and the LAN by using Dial-on-DemandRouting (DDR).DDR automatically establishes and releases circuit switched calls providing transparent connectivity toremote sites based on internetworking traffic.DDR also controls establishing and releasing secondary Bchannels based on load thresholds.MultiLink PPP is used to provide bandwidth aggregation when usingmultiple B channels.For more information on DDR, see "Designing DDR Internetworks."Some ISDN applications may require the SOHO user to take direct control over ISDN calls.EmergingCisco IOS features can bring this control to the user desktop.New Cisco 700 models provide a call buttonon the front of the router for direct control.Cisco 700 series and Cisco IOS based 1000, 1600, 2500 routers provide single BRI interfaces.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 Bus orexternally through the communications ports (such as RS-232) and can be used similar to analog (such as V.34)internal and external modems.PC Terminal Adapters can provide a single PC user with direct control over ISDN session initiation andrelease similar to using an analog modem.Automated mechanisms must be provided to support theaddition and removal of the secondary B channel.Cisco200 Series PC Cards can provide ISDN services toa 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 twofunctions at the local exchange: local termination and exchange termination.The local termination function dealswith the transmission facility and termination of the local loop.The exchange termination function deals with theswitching portion of the local exchange.First, the exchange termination function de-multiplexes the bits on the Band D channels.Next, B channel information is routed to the first stage of the circuit switch, and D channelpackets are routed to D channel packet separation circuitry.For proper ISDN operation, it is imperative that the correct switch type is configured on the ISDN device.For Cisco IOS releases up to 11.2, the configured ISDN switch-type is a global command (note this alsomeans you cannot use BRI and PRI cards in the same Cisco IOS chassis).In Cisco IOS 11.3T or later,http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2011.htm (6 of 25) [9/16/2000 5:11:15 PM] Designing ISDN Internetworksmultiple switch-types in a single Cisco IOS chassis are now supported.Cisco IOS switch typesThe following Cisco IOS command helps illustrate the supported BRI switch types.In North America, themost common types are 5ESS, DMS100, and NI-1 [ Pobierz całość w formacie PDF ]

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