Packet Guide to Voice over IP

Chapter 5. Codecs

Audio and video streams such as those created when we speak into a computer microphone or use a digital camera start off as analog signals. There was a time when the voice signal traversed the entire telephone network in this form. However, this architecture is limited and cumbersome. Today, voice or video conversations, whether on a traditional telephony system or one based on Voice over IP, start off as analog but then are converted to a digital format. The digital representation of the voice or video is transmitted to the receiver and then converted back to analog in order to be understood by the human at the other end. For the most part, this process of analog-to-digital and digital-to-analog conversion is handled by a codec, or coder-decoder.

There are many different techniques used to handle these audio and video streams. Most of the popular codecs used today are standardized in the ITU-T (International Telecommunications Union–Telecom) recommendations, though there are many others. Much of the work done with codecs has been in an effort to reduce the amount of bandwidth consumed by the voice stream through the use of compression. Voice over IP, or VoIP, deployments require the same conversion processes to occur, though they do not always have the bandwidth concerns of more traditional topologies. Understanding common codecs and their basic operation can help with initial deployment and troubleshooting problems. In this chapter, communication frequencies and ...