This CCITT standards committee document specifies some 1984 data communications standards that were somewhat widely used before the Internet became popular. It is "Fascicle VII.3" of "Volume VIII" of a very wordy, voluminous, hard to read set of standards. Written in international bureaucratese for telco manufacturers. No wonder the Internet took over -- you could read its standards and actually understand how it worked, unlike these. The big kahuna in this book is "X.25" which was the interface used to a bunch of connection-oriented packet-switching networks that preceded the Internet. This was particularly used in Europe, but also saw use elsewhere.