New PDF release: IP Telephony: Deploying VoIP Protocols and IMS

By Olivier Hersent

ISBN-10: 047066584X

ISBN-13: 9780470665848

ISBN-10: 0470973269

ISBN-13: 9780470973264

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Additional info for IP Telephony: Deploying VoIP Protocols and IMS Infrastructure, Second Edition

Sample text

Another common situation is that some manufacturers want to sell some back-end server, while distributing software to clients for free. If the client includes a codec, then again intellectual property becomes a major choice factor. 4 Lookahead and frame size Most low-bitrate codecs compress voice in chunks called frames and need to know a little about the samples immediately following the samples they are currently encoding (this is called lookahead). There has been a lot of discussion (especially at the IMTC when they tried to choose a low-bitrate codec) over the influence of frame size on the quality of the codec.

Its format is completely free and must be defined by the application or the profile of RTP in use. It enables applications to distinguish a particular format from another without having to analyse the content of the payload. 225 and SIP. edu/in-notes/iana/assignments/rtp-; parameters PT 96 to 127 are reserved for dynamic payload types. Dynamic payload types are defined in the RTP audio-visual (A/V) profile and are not assigned in the IANA list. The dynamic PT meaning is defined only for the duration of the session.

So, codecs with a small frame length are indeed better than codecs with a longer frame length regarding delay, when each frame is sent immediately on the network. This is where it becomes tricky, because each RTP packet has an IP header of 20 octets, a UDP header of 8 octets, and an RTP header of 12 octets! 6-kbit/s overhead—much more than the actual bitrate of most narrow-band codecs! Therefore, most implementations choose to send multiple frames per packet, and the real frame length is in fact the sum of all frames stacked in a single IP packet.

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IP Telephony: Deploying VoIP Protocols and IMS Infrastructure, Second Edition by Olivier Hersent


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