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Revision with unchanged content. Voice over Internet Protocol (VoIP) has become an increasingly popular way to provide phone services. At this moment there is a transition going on from delivering voice over the Plain Old Telephone System (POTS) to using VoIP technology. There are a number of problems that need to be taken into account in order to design a VoIP system. This book focuses on the problem of line echo. Line echo is created in the 2-wire to 4-wire hybrid circuit between the VoIP phone and the POTS phone and is much more apparent due to the large delay in the packet network; line echo cancellation is necessary to ensure satisfactory Quality of Service (QoS). It turns out that the performance limit of the amount of echo that can be cancelled is set by the non-linear portion of the signal. Line probing is a method of inserting a known signal at the far-end and recording the near-end signal. The two signals are then analyzed together for various impediments like, non-linearities, bad ERL, and noise. Line probing is used in this book to find the performance limit of the echo canceller as well as other useful metrics of performance. The book is addressed to telecommunications professionals that need to perform testing of echo cancellers in VoIP networks.