6.2.4.4 Inter-network connections: interconnection
In order to facilitate the communication between subscribers in different national networks, the national networks must be connected to one another. Such national "interconnections" are implemented in a form that is technically analogous to that of the international connections, where the operators of different countries are connected to one another via the large switching exchanges ("international exchanges") of the respective networks, on the basis of the relevant technical standards3 of ITU or ETSI, among others.
The networks of the different national network operators are connected to one another at defined locations that are called "Points of Interconnection" (PoI). The standard solution ("end of span" variant) for an interconnection between networks ("joining link") provides for an electric coupling by means of 2 Mbit/s transmission systems (PCM30) at the PoI. When interconnecting with the network of Telekom Austria, the latter will implement - as a rule – the interconnection link up to the specific PoI in the network of the interconnection partner. For network operators that have the necessary glass fibre infrastructure, an interconnection option on an optical level is also provided ("in span" variant).
With regard to the interconnection of two exchanges in different networks, one must also distinguish between the linkage on the level of the user channel and the signalling level. Here, the fundamental explanations in the general chapter on the core network apply. Concerning interconnection with the network of Telekom Austria in the signalling network, Telekom Austria uses two STP pairs, on account of the traffic volume. The STP Pair West with STPs in Salzburg and Graz, as well as the STP Pair East with STPs at Vienna-Arsenal and Vienna-Schillerplatz.
The recent enormous increases in traffic of the SMS services constitute a burden on the inter-network signalling network capacities. SMS messages between subscribers in different mobile networks are currently transited via the signalling network of Telekom Austria. The signalling network, which was basically designed for the transmission of intra-network messages to control the connections in the user channel network between exchanges, actually serves the SMS services as a combined user channel/signalling network. The corresponding capacity demand is difficult to calculate - if necessary a direct interconnection of the mobile networks would provide a remedy in this context.
Below, the fundamental routing mechanisms are briefly summarised. They are of relevance, last but not least, for various aspects in the field of interconnection.
Establishing a connection begins, as a rule, when the calling subscriber ("subscriber A") dials the individual digits of a telephone number (number of "subscriber B"). This dialling may be in the form of an independent transmission of the digits to the subscriber line ("overlap sending") or by the transmission of the complete call number as one message ("en-bloc sending"). A technical requirement for the latter is that there be a distinct data channel between the terminal equipment and the exchange, as it is the case with ISDN, for example, or in digital mobile radio communications.
Whereas it is ultimately a question of the subscriber’s terminal
equipment configuration in the case of ISDN, en-bloc dialling is used,
as a matter of principle, for (GSM) mobile radio communications, in order
to use more efficiently the scarce resource radio channel - after dialling
all digits, the en-bloc sending is triggered, as is known, by pushing
the "dial" button on the mobile phone.
At the first switching exchange ("subscriber switching exchange") the incoming dialled digits are evaluated. If it is clear, on account of the received digits, that the connection will leave the range of subscribers of the switching exchange, the connection (user channel) to the closest switching exchange is established, selected on the basis of the received dialled digits.
This is done by using the available 2 Mbit/s connecting lines, by means of the "Common Channel Signalling System No. 7" protocol. If necessary, a further connection is built up via several intermediate switching exchanges, in sub-steps from switching exchange to switching exchange. In the signalling messages the switching exchange, to which the user channel connection is built up, is always recorded as the destination. It should be mentioned that, when building up a connection, not only the signalling network (output capacity and computer capacity within the switching exchanges) is used, but the corresponding resources are also reserved in the lines and switching exchanges of the user channel network. If the connection is ultimately not established, for example, because the subscriber B is busy, then the entire path along the user channel is taken down in the opposite direction.
During the first phase of liberalisation, national interconnection with Telekom Austria was only possible at the central switching exchanges. Now, this is increasingly also handled by the local switching exchanges ("lower network level"). This makes it possible for new network operators to forward telephony traffic into the network of Telekom Austria close to the destination (i.e. close to the dialled subscriber), or to accept incoming traffic from the (carrier) network of Telekom Austria close to its origin. The consequence are correspondingly lower interconnection fees. This, in turn, has a positive effect on end-users, as telephone rates go down.
At the end of 2000, Telekom Austria presented an expansion programme regarding the interconnection points on a lower network level and the accessibility of all Austrian local networks. In the final expansion stage, i.e. at the end of the first quarter 2002, about 40 switching exchanges will cover all local networks. At the end of the period under review, approximately 90% of these switching exchanges had been adapted for interconnection, and almost 80% of all Austrian subscribers could be reached on a lower network level by means of interconnection.
