Goff (of Gtel Consultancy Ltd, Suffolk, UK) Hill 
The Cable and Telecommunications Professionals” Reference [PDF ebook] 
PSTN, IP and Cellular Networks, and Mathematical Techniques

This book is for any telecommunications-convergence professional who needs to understand the structure of the industry, the structure of telephony networks and services, and the equipment involved.With the growing variety of networks and technologies now on offer it is inevitable that some convergence will take place between different networks, services and products. New VOIP (voice over internet protocol) networks must interwork with traditional networks. For instance, mobile phones can offer data services; wireless broadband connections to laptops will allow VOIP phone calls away from base; users could have the option of ”convergent phones” that can be used on a landline when at home or business, but which can be used as a mobile when on the move, and so on.