User:Rich.japh

Purpose of this page
1)Identify VAD and CNG pattern or signature from G711 RTP Packets from wireshark/ethereal  a)There seems to be different way to identify VAD

- timestamp vs sequence number

- other sign?

CNG [ Relevant RFC 3389 ]
3.1 Noise Level

The magnitude of the noise level is packed into the least significant bits of the noise-level byte with the most significant bit unused and always set to 0 as shown below in Figure 1. The least significant bit of the noise level magnitude is packed into the least significant bit of the byte.

The noise level is expressed in -dBov, with values from 0 to 127 representing 0 to -127 dBov. dBov is the level relative to the overload of the system. (Note: Representation relative to the  overload point of a system is particularly useful for digital   implementations, since one does not need to know the relative   calibration of the analog circuitry.)  For example, in the case of a   u-law system, the reference would be a square wave with values +/- 8031, and this square wave represents 0dBov. This translates into 6.18dBm0.

Zopf                       Standards Track                     [Page 2] � RFC 3389            RTP Payload for Comfort Noise        September 2002

0 1 2 3 4 5 6 7                      +-+-+-+-+-+-+-+-+                       |0|   level     | +-+-+-+-+-+-+-+-+

Silence Supression( VAD )
RTP allows discontinuous transmission (silence suppression) on any audio payload format. The receiver can detect silence suppression on the first packet received after the silence by observing that the RTP timestamp is not contiguous with the end of the interval covered by the previous packet even though the RTP sequence number has incremented only by one. The RTP marker bit is also normally set on such a packet.'''

RTP info
RTP/UDP/IP header of 40 bytes (IP=20 bytes; UDP=12 bytes; RTP=8 bytes)

Relevant RFCs
RFC 3550 (obsoletes 1889): A Transport Protocol for Real-Time Applications

RFC 1889 - RTP: A Transport Protocol for Real-Time Applications

RFC 3389 Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)