Comparing possibilities of ISDN and ADSL, it should be noted that these technologies intended for performance of various tasks mutually supplement each other. For example, the GTE company actively introduces today in the market of the ISDN line and at the same time carries out two programs of tests of the ADSL technology. The first program developed in the city Earvin is carried out with the assistance of business firms. The second is organized in the city of Redmond (piece Washington) where the staff of Microsoft improves collaboration of the software and the ADSL protocol. Efforts of developers are directed on that this technology could support some household applications, that is it is aimed at the same consumer market, as the ISDN technology.
While digital subscriber lines are only going to enter on the market, most of removed users are compelled to be content with less exotic methods of network access. And the most widespread option there is a work with analog modems so far. Though transfer speeds in such lines (usually 14,4 or 28,8 Kbps) also leave much to be desired (especially from the point of view of any inveterate traveler on the Web network), analog modems possess two main advantages.
They are usually calculated on transfer of a traffic of the certain stack of protocols belonging to any one network operating system with a speed up to 128 Kbps (on two channels B). Bridges/routers have special means of a filtration of a traffic and can carry out partially or completely functions of routing of the last. Such devices cost not much and are intended generally for small and average local networks (on 10 — 50 workstations).
In turn, the cost of the organization of "the last mile" is defined by the size of rent payments to the owner of the used cables (a radio channel or costs of their laying (the organization of a radio channel, and also at the price of the equipment installed in a point of access for the operator and at the subscriber. Now are known and the following means for the organization of "the last mile" are widely used in city conditions:
Now about 500 000 ISDN BRI lines are operated, and their number continues to grow. It is possible to draw a conclusion that the telecommunication companies, most likely, will continue to support further this technology in spite of the fact that in parallel with it they begin introduction of the first ADSL lines.
Extent of development of communication networks in many respects determines possibilities of subscribers by use of all range of the services ISDN, and existence of the certificate on means of communication for connection to ISDN guarantees possibility of connection of the terminal equipment to this network.
Existence of the wide range of means of communication for connection to ISDN sets complex challenges for operators of communication networks of ensuring their compatibility among themselves. And it suggests an idea that it is necessary to bring an order to the chaotic market of means of communication of ISDN.
As for services of Internet access the only thing the protocol of network level, necessary for interaction, is the IP protocol, a chain of the legal transformations which are carried out by SPD can be represented by the following chart:
The ADSL technology possesses a number of the small shortcomings interfering widespread introduction of technology on networks of subscriber access. It is complexity of installation of ADSL devices; they demand serious control for a concrete subscriber line (as a rule, with participation of the technical employee of the company - the operator of a network), have rather big cost.
For the main access of ISDN: NT of production of Siemens firm; NTBA of production of ELCON firm (Germany), NT+2a/b (with the built-in terminal adapter for connection analog the terminal of production of Ascom firm, NT1+ (with the built-in terminal adapter for connection analog the terminal of production of CS Telecom firm (France).
The method with frequency division consists that the pass-band of frequencies is allocated to each of streams. The high-speed stream can be divided on one or more low-speed streams. Transfer of these streams is carried out by method of "discrete multivoice-frequency modulation" (DMT).