ables based on three criteria. If the permitted GOS is B when the normal traffic is offered, it shall not exceed 2.5B International Telephone Service, Blue Book, (9th Plenary As- sembly). for a 10% overload or 6.25B for a 20% overload. CCITT (1989b) Recommendation E164, Numbering for the ISDN 0.016 Era, Blue Book (9th Plenary Assembly). CCITT (1989c) Recommendation E165, Timetable for co-ordinated 0.014 implementation of the full capability of the numbering plan for the ISDN era, Blue Book (9th Plenary Assembly). 0.012 CCITT (1989d) Recommendation X121, International numbering plan for Public Data Networks, Blue Book (9th Plenary Assem- bly). 0.010 Clarke, A.B. and Osborne, H.S. (1952) Automatic Switching for Nationwide Telephone Service, Bell Syst. Tech. J. 31 p. 823. 0.008 Clos, C. (1954) Automatic Alternate Routing of Telephone Traffic, Bell Lab. Rec. 32, p. 51. 0.006 Farr, R.E. (1988) Telecommunications Traffic, Tariffs and Costs, Peter Peregrinus. Flood, J.E. (ed.) (1975) Telecommunication Networks, Peter Pere- 0.004 grinus. Francis, H.E. (1959) The General Plan for Subscriber Trunk Diall- 0.002 ing, Post Off. Electr. Eng. J. (January), 51, p. 258. Hills, M.T. and Evans, B.G. (1973) Transmission Systems, Allen and Unwin. Hunter, J.M., Lawrie, N. and Peterson, N. (1988) Tariffs, Traffic and Percentage overload Performance, Com. Ed. Publishing. IEEE (1988) Journ. on Selected Areas in Communications, SA6, Figure 29.16 Effect of overload on grade of service (4), (Papers on Telecommunications Network Operations and Management). 29/20 Telephone networks Ingham, A.R. and Elvidge, A.M. (1989) Trunk Reservation with Roberts, G.J. and Brutnell, R.F. (1985) An Introduction to the Automatic Alternative Routing, In Sixth UK Teletraffic Sympo- Analogue Derived Services Network, Brit. Telecom. Eng. 4, (3), sium. p. 129. Katzschner, L. (1973) Service Protection for Direct Final Traffic in Roberts, G.J. (1987) The Digital Derived Services Network, Brit. DDD Networks, In 3rd Int. Teletraffic Congress, Stockholm. Telecom. Eng.