One of the most frequent problems telephony users have is echo. This echo occurs when a speaker hears his own voice shortly after he speaks. This echo is often the result of the signal leaking back into the transmission path on cheap conference phones or poorly designed cell phones. But it can also occur in the network where such things as impedance mismatches on a wire can cause electrical signals to be reflected back toward the speaker.
To combat echo we insert signal loss into various points along the path of the voice circuit. This loss happens naturally, to some extent, in analog networks. At each junction in the path, some signal is reflected, but the loss inherent in the network usually prevents that low-power signal from making it back to the speaker. When it doesn't, more loss must be added to the network. Practically, this can be implemented in a fixed or variable fashion, but the details depend on your hardware vendor and service provider. In a hybrid telephony network where you have a mixture of analog and digital circuits, planning for loss can be a challenge.
Although they are not free, the following references explain some of the standard details you may want to learn if you plan to implement a large VoIP network and need to plan for signal loss. It may be worth your while to check a local university's library for a free copy.
The signal loss is measured in an interesting unit called a decibel. There are many types of decibels. Definitions for the ones related to telephony and networking can be found in ITU-T Recommendation G.101.
An important concept and term known as Overall loudness rating (OLR) is explained in ITU-T Recommendation G.111.
Quite a lot of detail about echo cancellers can be found in ITU-T Recommendation G.131 and G.165
Information about fixed loss plans on analog networks can be found in ITU-T Recommendation G.171.
These ITU-T Recommendations can be found at http://www.itu.int/publibase/itu-t/ItutAllBySeries.asp?serie=G
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.