Each segment has 46 bytes of overhead Ppp Ip Udp and RTP headers

The example shows the benefits of RTP header compression on slow links.

A 64-kbps link is used to transport VoIP using PPP as the Layer 2 framing protocol.

For the case study application (voice, using the G.729 audio compression codec), the payload size is 20 bytes. Because PPP has 6 bytes of frame header, the total header overhead is 6 + 20 + 8 + 12 = 46 bytes, including the PPP, IP, UDP, and RTP headers.

Class-Based RTP Header Compression Example (Cont.)

Overhead = 46 / (46 + 20) = 70% Overhead = 10 / (10 + 20) = 33%

Delay = (46 + 20) I 64 kbps * 8 = 8 ms Delay = (10 + 20) I 64 kbps * 8 = 4 ms

BW = (46 + 20) * 50 * 8 = 26.4 kbps BW = (10 + 20) * 50 * 8 = 12 kbps

Overhead = 46 / (46 + 20) = 70% Overhead = 10 / (10 + 20) = 33%

Delay = (46 + 20) I 64 kbps * 8 = 8 ms Delay = (10 + 20) I 64 kbps * 8 = 4 ms

BW = (46 + 20) * 50 * 8 = 26.4 kbps BW = (10 + 20) * 50 * 8 = 12 kbps

2 voice sessions 164 kbps 5 voice sessions I 64 kbps

Codec

Voice

RTP

RTP

CRTP

CRTP

Bandwidth

Bandwidth

Overhead

Bandwidth

Overhead

G.711

64 kbps

82 kbps

22%

68 kbps

3%

G.729

8 kbps

26 kbps

61%

12 kbps

33%

The figure shows the packet size before and after RTP header compression. The IP, UDP, and RTP headers are reduced to 4 bytes, resulting in 10 bytes of overall headers. The overhead is reduced from 70 percent to 33 percent when small packets are used. Because of the packet size reduction, the serialization delay decreases from 8 ms to 4 ms, and the bandwidth that is used to transport a single voice call (using the G.729 codec) is reduced from 26.4 kbps (66 bytes per frame * 50 frames per second * 8 bits per byte) to 12 kbps (30 bytes per frame * 50 frames per second * 8 bits per byte). Therefore, a 64-kbps link can support up to two G.729 voice calls without cRTP, but up to five G.729 voice calls with cRTP.

The table in the figure shows how RTP header compression impacts performance when a different audio codec is used. For the traditional G.711 voice codec, RTP header compression still optimizes its transmission over slow links. However, the difference is more obvious when using advanced, low-bandwidth codecs.

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