Telecommunications systems are private switching systems that support various communications services. In contrast to private branch exchanges, which offer only voice communication, telecommunications systems, or PBXs for short, are suitable for data transmission ofvoice, data, text, graphics and moving images, etc., and can therefore also be used as switching systems in local area networks(LANs).
A distinction is made in these switching systems between single- service systems and multi-service systems. PBXs can be both, which is also expressed in the English designation ISPBX (integrated Services PBX); namely, service integration. The concept of a PBX takes into account the various public networks, such as the analogtelephone network, IDN, ISDN, etc., and private leased lines, as well as a wide variety of terminals. In addition, server functions are required for in-house applications and computer controls for administration are taken into account.
The topology of Tk systems
Topologically, a telecommunications system can be compared to a star-shaped network. All subscriber connections run via the central node, which is formed by a switching network that uses TDM. The public lines are connected to the subscriber lines via this switching network. The system periphery is used for control, maintenance, configuration and administration.
ISDN PBXs consist of the central switching network, which transparently connects the input and output channels. The network-side inputs of the switching network form the exchange access units (AAO) for the analog telephone network and for ISDN; on the subscriber side, they are the access units( TAO) for the various terminals with their individual interfaces. The system control of an ISDN PBX is computer-controlled, whereby the various signaling and D- channel protocols are evaluated here. The system control also performs the tasks ofsubscriber administration, configuration and control of the features, maintenance and diagnosis, charge recording and processing, and management of the electronic address book.
The principle of Tk systems
ISDN Tk systems generally operate on the principle of circuit switching, switching 64-kbit/s channels between input and output. The connections exist exclusively as a transparent connection with a fixed transmission rate for a fixed time. The traffic performance of PBXs is significantly higher than that of private branch exchanges. Since data connections require much longer connection times, a traffic performance of 0.5 to 1 Erlang should be applied per subscriber.
In the case of digital systems with stored program control, digitization extends into the subscriber lines. Terminals for a wide variety of information forms can be digitally connected. The ports offered by a telecommunications system can be specially designed for LAN interfaces, e.g. in accordance with IEEE 802.3, or universally, e.g. for ISDN.