What is the relationship between codec and VoIP?
Codec is short for 'encoder and decoder'. When a person speaks, a continuous signal is generated. This signal can be directly sent over an analog line. To send speech over a digital interface, the continuous signal has to be converted into a digital form. To do this, some samples of the analog signal are taken (enough to adequately reconstruct the original analog signal at the far end). These samples are then encoded for transmission. There are multiple ways to encode a speech signal. For example, the ITU standard G.711 is one way to encode so that the speech samples are not compressed. To save bandwidth, the ITU standard G.729 can be used which applies a compression algorithm to the speech samples. Once encoded, the speech is put into packets which are transmitted across an IP network. This is the action of transmitting voice over IP. At the receiver, the packet is received and the information is decoded using the appropriate codec (G.711 or G.729 are examples of codecs).
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