6.2.4.1 Core network structure

Since early 2000, only computer-controlled switching exchanges that also have a digital switching array are used in Austria. The individual switching exchanges in a network are connected by means of 2 Mbit/s systems (PCM30). Within a PCM30 system, 30 voice and/or data connections (user channels) can be handled simultaneously, each having 64 kbit/s. In this connection, a time frame of 125 micro-seconds is divided into 32 time slots (two of these are used for special purposes and are not available for transmitting useful data). Within every time slot, one 8-bit value is transmitted 8,000 times every second.

The 2 Mbit/s systems between the network nodes of big switching networks should not be imagined to be independent individual systems with regard to their implementation. These fixed connections (which cannot be influenced by subscriber signalling) between the switching exchanges are implemented in the so-called transmission technology network which is below the switching network in the overall model. Modern long-distance transmission networks are based on SDH systems ("synchronous digital hierarchy") which, inter alia, use glass fibre lines as their physical medium. These glass fibres often connect the individual network nodes in a ring topology. For reasons of redundancy, two rings operating in opposite directions are typically used. As a rule, SDH systems also form the basis of ATM networks. The data rates of SDH systems are about 155 Mbit/s ("STM-1"), 622 Mbit/s
("STM-4" = 4 x STM-1) and 2488 Mbit/s
("STM-16" = 4 x STM-4). Via the so-called add/drop multiplexers the low bit-rate "transport units"
(e.g. 64 kbit/s, 2 Mbit/s, 34 Mbit/s) are input to, or output from, the SDH system. In the transmission network there are software-controlled network nodes, which are called cross-connectors. They serve for the "semi-permanent" switching (not for specific connections) of paths in the transmission network by means of network management functions.

The switching array of a digital switching exchange serves to connect ("to put through") the incoming 64 kbit/s user channels to the outgoing 64 kbit/s user channels, as determined by the software control of the switching exchange. During switching, the physically used PCM30 system and/or the number of the used time slot may change ("space/time switching array").

The individual switching exchanges of Telekom Austria are not fully intermeshed (i.e. not every exchange is linked to all the others), but according to a hierarchical network structure. Occasionally, one still refers to three network levels: central switching exchanges (HVSt), network switching exchanges (NVSt) and local switching exchanges (OVSt). In the future, there will only be a distinction between central switching exchanges (without any connected subscribers) and subscriber switching exchanges. With a view to the future, the TKK interconnection arrangements have basically provided for only two levels from the very beginning (HVSt level - without directly linked-up subscribers - and the lower network (hierarchy) level as the level of the subscriber switching exchanges).

The core network of Telekom Austria comprises the special feature of the so-called "dependent" switching exchanges (UVSt) in the area of the subscriber switching exchanges. From a technical viewpoint, the UVSt is a component of a switching system that is operated at a different location than that of the switching exchange, with the switching of the system components being achieved by means of 2 Mbit/s transmission systems. By using UVSts the number of the expensive switching exchanges can be kept small, on the one hand, while the length of the subscriber lines can be limited, on the other. At the sites of the UVSts, the subscriber lines are linked at the so-called main distribution frame to the respective lines from the subscriber modules of the UVSts (without UVSts, the subscriber modules are used directly at the site of the switching centre).

There are subscriber modules for analogue subscriber lines (POTS) and digital ISDN lines. At present, there are more than a thousand UVSts in addition to about 200 switching centres ("main exchanges") in the Telekom Austria network.

A UVSt is controlled by the central switching computer by means of signalling along the 2 Mbit/s systems. The main task of a UVSt is to concentrate traffic - in addition to analogue/digital conversion in the case of POTS. Traffic concentration means that the number of user channels between one UVSt and the attached subscribers (in case of POTS one channel per line, in case of ISDN two or 30 channels per line) is much higher than the number of channels between the UVSt and the switching exchange. It is therefore impossible for all subscribers to make calls at the same time. In practice, this is no problem. On the basis of the "traffic intensity" (average frequency/duration of calls) of the connected subscribers, the dimensions are structured according to the rules of "traffic theory" in such a way that the number of necessary 64 kbit/s user channels (or the number of 2 Mbit/s systems) between UVSt and switching exchange are kept at a minimum, without there being any noticeable constraint on subscribers. Difficulties may arise if the assumed traffic intensity, of several subscribers, suddenly goes up dramatically. The modem dial-up connections to the Internet are one subject that is being discussed intensively in this connection, since the traffic volume of individual subscribers may go up considerably, on account of the typically long connected periods - especially if the costs for such connections are very low or completely independent of use ("flat rate").

Within a switching network, one must distinguish between the user channel network and the signalling network. Whereas the user channel network serves to transport the useful data of the end-users (speech, data), the signalling network is used to exchange information between the switching exchanges - especially for controlling purposes when establishing or breaking a connection.

Just like the user channel network, the signalling network also uses the 64 kbit/s channels in the 2 Mbit/s systems to and/or from the switching exchange. However, whereas the user channel network always physically connects "joined" switching exchanges directly, the linking of switching exchanges in the signalling network is usually indirect.

To optimise the number of signalling connections between the switching exchanges, as well as the special hardware and software facilities required in the individual switching exchanges, the signalling channels of every switching exchange are typically only connected directly (failure protection) to two central transfer computers ("Signalling Transfer Points" - STP). As every message contains the address of the switching point at destination ("point codes" of the ITU Common Channel Signalling System No. 7), an STP can implement a correspondingly (transparent) forward transmission of the messages. In addition to the lower costs, the central function of the STP also promotes additional network monitoring functions, such as, for example, screening of the entire signalling traffic. Depending on the signalling volume, also several STP pairs may be implemented.

As already mentioned in the chapter on "circuit switching and packet switching", the process used in the ITU Common Channel Signalling System No. 7 (ZGV 7) for forwarding signalling messages corresponds to a datagram service, with the reliability of the technical components used in the signalling network of a classical voice telephony service being very high. Moreover, on account of the redundant double link-up of the switching centres to the STP pairs, the system is practically failure-safe.