Page 134 innovaphone gateway
decrypting. Here, the key for decryption is not computable from the key for
encryption (or is computable only with considerable effort). The key for
encryption can therefore be published. Such methods are referred to as
asymmetric or public key methods. It is named after its inventors Ronald L.
Rivest, Adi Shamir and Leonard Adleman.
RTP
The RTP protocol (Real-Time Transport Protocol) is a protocol for the
continuous transmission of audiovisual data (streams) over IP-based
networks. It is used to transport multimedia data streams (audio, video,
text, etc.) over networks, that is, to encode, packet and send the data. RTP
is a packet-based protocol and is normally operated via UDP. RTP is used for
the negotiation and observance of QoS parameters (Quality Of Service). It
is applied in many areas, for example, it is used in the IP telephony
technologies H.323 and SIP (Session Initiation Protocol) to transfer the
audio/video streams of the call.
S
SC
A telephone call is made up, for the most part, of pauses. It would be
unnecessary to operate at the full data rate in these time slots. Codecs, such
as the G.723.1 or the G.729, therefore contain an SC feature (Silence
Compression). Essentially, this feature consists of three components: VAD,
DTX and CNF.
The task of VAD (Voice Activity Detector) is to determine when a subscriber
is speaking and when he/she is silent. For this, the algorithm must respond
quickly to prevent the first syllable being lost after such a silence. To reliably
differentiate between conversation and silence, the codec requires a buffer
which causes an additional delay.
DTX (Discontinuous Transmission) allows a codec, in theory, to interrupt the
connection if VAD has detected silence. Because an interruption of this kind
would mean absolute silence on the call party side, the connection is not
really completely interrupted. Rather a small set of data is transferred, which
allows the generation of background noise on the receiver side.
CFG (Comfort Noise Generator) starts precisely at this point. It is capable of
generating background noise independently. For this, it uses the background
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