This edition provides an introduction to modern digital telephone networks. It meets the requirements of the BTEC NII and NIII transmission principles unit, covering transmission systems, transmission lines (both copper and optical fibre), noise, modulation and signals. Worked examples are included to help students tackle problems, as well as examples of data networks using both analogue and digital circuits. The book also includes CCITT recommendations for modems, updated treatment of digital modulation and detailed treatment of pulse-code modulation systems.