http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080149a1f.shtml
Echo Analysis for Voice over IP defines echo and describes where it occurs in a voice network. It examines the basic aspects of echo analysis and describes the effects of various network elements on echo. This document also explains how echo is measured and how echo cancelers work to estimate and eliminate echo. It looks at customer and service provider expectations about echo, and explains how to configure gateways to minimize echo. It points out that the normal characteristics of packet-based networks might unmask preexisting problems in the time-division multiplexing (TDM)-based voice infrastructure. Finally, it outlines a process for locating and eliminating loud echos and long echos, and concludes with a real-life case study involving PBX-based echo in an international voice network.
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_white_paper09186a00800d6b68.shtml
