'Private networks' (Telecommunications, 1988), 'Telecommunications Traffic, Tariffs and Costs. An Introduction for Managers', Chap. 5, pp. 58-77, DOI: 10.1049/PBTE019E_ch5
IET Digital Library, https://digital-library.theiet.org/;jsessionid=27k0a9hq1mw9t.x-iet-live-01content/books/10.1049/pbte019e_ch5