Saturday, August 20, 2011

Voice over IP - VoIP Compression and Signaling

Voice Compression: The compression algorithms used in gateways analyze a block of PCM samples delivered by the voice encoder (voice codec). These blocks have a variable length depending on the encoder, for example the basic size of a block of G.729 algorithm is 10 ms, while the basic size of a block of G.723.1 algorithm is 30ms.

The chain analog voice is digitalized in PCM frame, and so delivered the same compression algorithm at intervals of 10 ms.

VoIP signaling: Has 3 distinct areas: signaling the PBX to the router, signaling between the router and router to the PBX signaling. For example for a corporate intranet, this appears as the backbone to the PBX, which will give the signal to intranet users. Therefore the PBX forwards the digits to the router in the same way that the digits had been forwarded to a central telephone switch.

When the remote router receives the call requesting Q.931, this sends a signal to the PBX. After the PBX sends an acknowledgment, the router sends the dialed digits to the PBX, and process a call acknowledgment to the source router.

In a network architecture is not connection-oriented (like IP), responsibility for establishing communication and signaling is the end stations (end stations). To successfully provide voice services through a IP re, it is necessary to make improvements in signaling.

For example, an H.323 agent is added to the router to provide support for the transport of audio and signaling networks. The Q.931 protocol is used for the establishment and disconnection of the call between terminals agents or h.323. RTCP (RTP Control Protocol) is used to establish channels of audio. A reliable protocol connection oriented, TCP is used between end stations to carry the signaling channels.

RTP transport protocol in real time, which is supported on UDP, is used for the transport of audio stream in real time. RTP uses UDP as a transport mechanism because it has less delay than TCP, and because voice traffic today, whether they are data or signaling, tolerate lower levels of loss and lack the ease of transmission.

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