864 page stapled booklet, undated, but the following passage gives a clue: “British Telecom is already in the process of converting the telephone network to digital transmission. We started with the main trunk highways and will finish with the local lines, customer to his exchange in the 1990s”. Nevertheless, as well as its historical interest, this little book might still be a useful reference for students of computer networking. For example, it includes clear diagrams illustrating connection protocols and the contents of PAD messages. Condition: A little grubby externally, and the booklet refuses to lie flat as a result of poor storage, but pages are clean and sound.