 (CCITT, 1989d), an international 3. National numbering plans must include international prefix number consists of two parts: the data network identification code codes to avoid ambiguity when national and international num- (DNIC) of four digits and the network terminal number (NTN) of bers have the same initial digits. 10 digits. Thus, the complete number has 14 digits. The DNIC consists of a data country code (DCC) of three digits followed by a 'network' digit. By this means, a single country can have up to 10 Table 29.3 Examples of typical international numbers different data networks. Furthermore, a country can have more than one DCC. The UK has been allocated codes '234' to '237'; thus it can ultimately have up to 40 different public data networks. Country Country No. of digits Total no. of The format used for the 10-digit NTN can be determined by the in national digits network operator, since only the DNIC needs to be analysed to route number the call into that network from another country. However, the CCITT recommends that the last two digits shall be a sub-address for use by the subscriber. In the BT packet-switched service (PSS), Canada the first three digits of the NTN serve as a routing code to identify Egypt the packet-switching exchange and the next five digits identify the network terminating point serving the subscriber. The final two Liberia digits correspond to a sub-address for use within the subscriber's France own system. Portugal 7 or 8 10 or 11 8 or 9 10 or 11 29.8 Routing Switzerland National networks are hierarchical, as shown in Figure 29.5. The Brazil minimum configuration of star-connected exchanges is usually aug- Ecuador mented by direct routes inter-connecting those exchanges where a high community of interest results in sufficient traffic. There is thus a backbone route joining each switching centre to the highest level via intermediate centres, together with transverse branches between 29.7.3 Numbering plans for the ISDN era some centres at the same level. There may also be some 