Ever since Voice over Internet Protocol (VoIP) was introduced, all calls going to the Public Switched Telephone Network (PSTN) have been plagued with line or electrical echo. I often get asked where these electrical echoes come from and why it is inevitable when using VoIP. We’ve given many seminars on the subject, especially in the Asterisk space where people are suddenly confronted with this problem when building equipment. I’ve put together a basic introduction explaining why echo cancellation is needed when using VoIP over PSTN.
Electrical or line echo is inherent to the PSTN, it has always been present and cannot be avoided because of the nature of the lines connected to homes and offices. These local or subscriber loop circuits use two wires to carry the voice signals while the voice channels use four wires for bi-directional communications beyond the first switch that is the local exchange. The conversion between the four wire and two wire electrical circuits is done by transformer called a telephone hybrid whose goal is to separate the signals’ directions and adapt the impedance of both circuits. Like all analog circuits, the hybrids can’t perfectly match the impedances and that causes part of the signal …
As video becomes one of the most demanding services for network traffic and Internet video becomes an important part of modern life, the quality of experience needs to meet the user’s expectations, regardless of the device or network being used. More and more video is being consumed on smartphones and tablets. The smaller screens allow for lower bit-rates, but the video playback need to start quickly and remain smooth throughout.
Internet Video delivery is challenging because of factors such as high bitrates and sensitivity to delay or packet loss.
Video Streaming
In the past, video streaming was typically associated to RTSP, with RTP used for transmission. This protocol uses “VCR-like” commands such as PLAY and PAUSE. In this scheme, the server has to keep track of the client’s state. The server starts playing a stream when giving the PLAY command, and has to maintain the state of each session in order to know what packet to send next. The video stream is based on a single “track”, a file with a fixed encoding profile that cannot change. The quality would quickly suffer when there was a shortage of bandwidth such as congestion leading to packet loss.
Today, HTTP based adaptive …
