Introduction to Telephony and VoIP

In its latest iteration, the Cisco CCDA exam expects that you not only be familiar with the traditional data networking concepts outlined throughout this book, but also voice networking as well. This includes a basic understanding of not only standard telephony concepts, but also an appreciation of some of the concepts and issues associated with the transfer of voice traffic over data networks, using technologies like Voice over IP (VoIP). While you are not expected to be a telephony expert for the exam by any stretch, you are required to have a basic understanding of how a traditional voice networks such as the public switched telephone network (PSTN) function, including an understanding of key terms, concepts, and equipment. Along the same lines, you will also need to be familiar with concepts relating to the transmission of voice traffic over data networks, including interoperability, planning, capacity, and equipment issues.

As you read through this section, it’s important to keep in mind that a traditional telephone connection between two users is circuit-switched. When a call is initiated, a dedicated end-to-end 64 kbps circuit is created from the originator to the recipient. Because it is circuit-switched, all of the circuit bandwidth associated with the call is reserved for the duration of the call, even if neither party is speaking. Similarly, the path over which the call travels is the same for the duration of the call. Later in this series you will learn how technologies like VoIP differ by transporting traffic across packet-switched networks.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.