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| This article introduces the main components of ISDN. This includes, but is not limited to, the following topics:
What Are Integrated Services?Since the 1960s, the telecommunication networks backbone has been converting to digital. The end-user access, however, such as the telephone and modem connections, has remained mostly analog. ISDN takes advantage of the digital telecommunications backbone and replaces some of the analog service devices with new higher-speed digital equipment. So the beauty of ISDN is that it makes use of the existing backbone technology while enhancing it with cost-effective higher-speed services that were previously unavailable or unjustifiably expensive. When the digital network is extended end-to-end by ISDN, it eliminates the need to translate (or sample) the analog waveform into a digital pattern. This allows any application, whether voice, video, or data, to transparently transmit over the backbone, because there is no longer a need to differentiate between the various types of network traffic. As a result, diverse sets of services can be integrated into one cost-effective solution. Advantages of ISDNISDN provides a viable alternative to various forms of communication while allowing reliable high-speed access to the Internet and other services. The table below demonstrates how ISDN compares to a few of these forms of communication.
ISDN ServicesAs mentioned, ISDN can provide a number of different services:
ISDN Bandwidth and ChannelsThe discussion of ISDN revolves around two variations: BRI and PRI. Before we begin, let's examine the North American digital signal standards and their "T" assignments, because BRI and PRI adhere to those standards. You will also learn the European equivalents of their North American counterparts. The table below shows the DS level, its corresponding maximum speed, the "T" designation, and the number of channels for each level.
NOTE As mentioned, some equipment is incapable of supporting the 64 kbps DS0 standard and can extend to only 56 kbps operation. ISDN-BRIBRI specifies the following components:
ISDN-PRINorth American PRI specifies the following components:
European and other countries' PRI specifies the following components:
BRI Functional GroupsBRI defines the following functional groups (ISDN devices):
NOTE An NT1/NT2 combination device is sometimes called a Network Termination Unit (NTU). Which Devices Represent the BRI Reference PointsReference points are interfaces between functional groups. They might or might not manifest in actual physical interfaces. Reference points include the following:
Let's spend a few moments discussing how functional groups and reference points work together. First, you connect the wall jack to the NT1 with a standard two-wire cable. Then you connect the NT1 to an ISDN terminal or a terminal adapter with a four-wire connector. An eight-wire connector is used for the S/T interface because it requires both NT and TE capabilities. An S/T interface is a combination of the S and T interfaces. It defines a reference point between a TE1 (or TA) and an NT. You can think of it as a point-to-multipoint bus that multiple ISDN devices can share. The U interface is a two-wire interface between the NT and the provider cloud normally terminated with an eight-pin RJ-48 connector. In this case, the NAS has built-in NT1 functionality. U interface termination is mostly used in North America. As far as the Cisco IOS is concerned, there is no real difference between the S/T or U termination when it comes to BRI operation. What you have to keep in mind is that BRI consists of a single D channel for signaling and two B channels for data.
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