umbering plan (Nunn, It is intended that provision will also be made for sub-addressing. 1952) and the country code for the USSR is '7'. Some examples of This will use additional digits following a subscriber's national international numbers are given in Table 29.3. number, which will be transmitted to the destination for use on the The existence of a world numbering plan places restrictions on the subscriber's premises. These can select the appropriate terminal for national numbering plan of each country to enable subscribers in an ISDN connection or provide DDI on a PBX without using up other countries to make international calls into its national network. numbers in the area numbering scheme. The sub-address field can The restrictions are: range from four digits for simple applications up to 40 digits for use in open systems interconnection (OSI). For OSI, a global scheme 1. The number of digits in a subscriber's world number is limited for the identification of network service access points has been to an absolute maximum of 12. In practice, with a few excep- developed by the International Standards Organisation (ISO). tions, world numbers are limited to 11 digits. The number of The CCITT has recommended (CCITT, 1989c) the full implemen- digits available for a national numbering plan is thus 11-N, tation of this plan by the end of 1996. It is hoped that, by that date, where N is the number of digits in the country code. stored-program-controlled digital exchanges will have penetrated 2. National numbers must consist of all numbers and no letters. all countries SO that they can handle 15-digit international numbers Countries which used letters on telephone dials chose different and the increased number of digits to be analysed. ways of allocating the 26 letters of the alphabet among the 10 decimal-number characters. Thus, exchange codes containing 29.7.4 Public data networks letters could have been dialled as different numbers by sub- scribers calling from outside the country. For public switched data networks