6.2.4.2 Different types of subscriber lines

In the field of voice telephony, two types of lines are distinguished: "common" analogue lines ("Plain Old Telephone Service" - POTS), on the one hand, which account for about 90% of all lines in Austria, and the digital ISDN lines, on the other. In the case of ISDN lines, the analogue/digital conversion of voice already occurs in the terminal equipment, whereas in POTS lines the signals are transmitted in analogue form along the subscriber line, with the analogue/digital conversion only taking place in the switching exchange assigned to the line.

ISDN lines are offered in two versions: as ISDN basic access line (ISDN-BA) on the one hand, and as ISDN primary access line (ISDN-PRA) on the other. Basic access consists of two distinct user channels (B channels) at 64 kbit/s each, to transmit voice, fax or data, as well as a signalling channel (D channel) of 16 kbit/s which, inter alia, controls connection set-up. Basic access lines are primarily used by small and medium-sized enterprises, as well as private customers with more sophisticated demands, for example a voice connection is possible in parallel to an active Internet access. The primary access consists of 30 user channels (B channels), which are independent of one another, of 64 kbit/s each, as well as a signalling channel (D channel) of 64 kbit/s. The primary access line is used to supply big companies, which have their own PABXs, with voice telephony.

As the digitisation of switching exchanges has been completed since the beginning of 2000, a large number of service features is available to all POTS and ISDN subscribers, such as, for example, "Call Waiting", "Call Forwarding" or "Call Completion Busy Subscriber", with the services features of ISDN being more comprehensive on account of the signalling channel that is always available in parallel to the user channels.

Most recently, CLIP (Calling Line Identification Presentation), which is a common feature in ISDN and/or mobile networks, has now also been made available to POTS subscribers. It indicates the number of the caller to the called subscriber (whenever the available terminal equipment does not support this feature, a small additional unit can be used).

As was previously mentioned, the network of Telekom Austria used to rely on switching exchanges with analogue switching arrays and fewer end-user functionalities, as opposed to the "digital" switching networks of today. In the transition period, the expression "digital line" or "digital connection" has unfortunately become common terminology for analogue lines to digital Telekom Austria switching exchanges, which is not correct technically speaking and sometimes leads to misunderstandings. As described above, this term, technically speaking, only applies to ISDN lines. In the case of POTS lines, where additional new systems are implemented in the frequency range above the analogue voice signal (e.g. ADSL), the (use of a) line consists of an analogue and a digital component. The latter, however, is connected to a separate data network, upstream from the subscriber switching exchange - from the perspective of the voice network this continues to be an "analogue line".