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When using ISDN you will not have any problems with interaction with a usual telephony. If you call on ISDN number, connection will happen at the level of usual station. If you call on ISDN number of other city, connection will happen between ISDN modules of the different cities through the ISDN highway. Well and if you call on usual number of other city, connection happens between stations of the different cities.

The first ISDN stations which appeared in the mid-seventies were developed taking into account possibility of work with analog communication lines and DTI therefore they did not bring any dissonance in the industry of telephone services. The vast majority of subscribers continued to use ordinary analog phones, and the ISDN station had to provide support at the same time of both digital ISDN terminals, and ordinary analog devices.

One of basic elements of any communication system are the communication lines and the principles which are been the basis for their functioning. As for ISDN, historical realities of development and introduction of this technology inevitably led to use within ISDN at the same time several essentially various types of connecting lines, or interfaces. First of all it is connected with that "change of milestones" in the history of a telephony happens not in steps, and gradually, as if smoothly "flowing" from one (analog) state in another (. Approach of an era of ISDN happens without serious consequences for users of traditional telephone services, to gradual replacement of the principles of an analog telephony. For this reason the full-fledged digital automatic telephone exchange has to support, besides specific ISDN interfaces, and all types of the connecting lines existing in an analog telephony.

One more argument in favor of ISDN - the most direct and natural way to the organization of real videoconferences. However, the number of participants should not exceed four, however working meetings of two-three people can be held in real time.

In the North American market situation even more impressive. The modern telecommunication operator has to be able to provide the specialized services focused on satisfaction of requirements of concrete groups and certain users, keeping possibility of fast creation and expansion of new services.

In ISDN networks two specific types of interfaces are used: the interface of a basic level of BRI (Basic Rate Interface) regulating connection of ISDN station with the subscriber, and the interface of primary PRI level (Primary Rate Interface) providing communication between ISDN stations.

In a certain degree development of systems of multimedia has impact on growth of interest in ISDN. So, many commercial radio stations in the West use ISDN for transfer of a binaural sound with high quality. Besides, now in the market there were security systems and systems of video of supervision which work is based on use of the principles of ISDN.

Logically PRI is constructed by the same principle, as the BRI interface: certain number of V-channels and one D-channel. In other words, PRI can present in the form formulas nB+D (23B+D in the USA and Japan where the T-1 standard, and 30B+D in Europe where the E-standard works works. Thus it is necessary to remember that D-channels in PRI and BRI differ in capacity: if in BRI speed of the D-channel equally 16 Kbps, in PRI - 64 Kbps.