Objects to be measured - Digital telecommunication devices

Digital telecommunication devices of exchange (PABX) manufactureres are compatible with ISDN in service, although their physical realization is more simple in many cases, than the ISDN S0 interface. One of the possible explanation for this is that the exchange services do not require the accessibility of eight devices to one terminal, but, for example, only two, a telephone, and a modem. So, it does not require implementation of the point-multipoint configuration.

In case of digital (the so called private ISDN) telecommunication devices signal transmission is digital between the device and the exchange, and the A/D conversion is realized in the device. Connection to the exchange happens through the so called digital device line, which can provide the following services:

• A full duplex signal transmission capability. There are two transmission channels with the capacity of 64 kbit/s and one transmission channel with the capacity of 16 kbit/s (2B+D) operating between the device and the exchange, according to the basic rate ISDN standard.

  One of the transmission channels with the capacity of 64 kbit/s serves for speech transmission, while another one serves for transmission of the signal of a data transmission terminal /T/ connected to the device through the interface marked with V. Most of the terminals provide signals with a rate smaller than 64 kbit/s. Rate conversion is made by the so called terminal adapter (digital modem), which is built into the device in many cases.

  Note: This common data transmission process with modem dialing slowly starts to be out of fashion. Only a few users used the initial point-to-point data transmission service in practice, and nowadays the Internet-services (web, e-mail) displaced it so very much, that this kind of data transmission with a modem already have not been found in a modern exchange.

  Device interfaces can operate on a two-wire, or a four-wire bearer.

  Two-wire duplex digital transmission is realized by provider in several ways:

  • Echo cancellation (EC) method (e.g. Magyar Telekom) Separation of directions in the device and in the exchange happens with hybrid circuits.
  • Time compression (TCM) (ping-pong) method (e.g. Datastar) Transmission route is used here alternately in time, either in direction of exchange -> subscriber, or in direction of subscriber -> exchange.

• Signal transmission capability. The channel with a capacity of 16 kbit/s is used for transmission of signals between the device and the exchange. Signals are coded here into HDLC framed digital messages, and these messages are forwarded through this channel. In contrast with the analogue ones, digital telephones have more services. This signalling channel with a large capacity is one of the techncial basis of this issue. Most of the services are assigned to the function buttons of the devices, which contributes to their comfortable handling.

• Remote feeding of the subscriber device.

- Old-fashioned Nortel telephone - used to the DATASTAR exchange