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Cisco AVVID Network Infrastructure Enterprise Quality of Service Design
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Chapter 1 Overview
What is the Quality of Service Toolset?
Compressed Real-Time Protocol
RTP is the Internet Standard (RFC 1889) protocol for the transport of real-time voice and/or video. At
40 bytes total, the header portion of an RTP packet is considerably large and can account for nearly
two-thirds or the entire packet. For example, G.729 VoIP payloads are only 20 bytes when sampled at
20 ms.
Tip
For more information on RFC 1889, see RTP: A Transport Protocol for Real-Time Applications.
To avoid the unnecessary consumption of available bandwidth, cRTP can be used on a link-by-link basis.
cRTP compresses IP/UDP/RTP headers from 40 bytes to between 2 and 5 bytes. Such compression is
illustrated in
Figure 1-16
.
Figure 1-16 IP/UDP/RTP Header Compression
Note
Distributed RTP Header Compression (dCRTP) is the distributed counterpart of cRTP.
Compression techniques minimize bandwidth requirements and are highly useful on slow-links. Due to
the additional CPU loads these compression techniques require, they need to be used with caution,
especially in scenarios with many remote sites attaching to a central WAN Aggregation router. For more
information on when to use cRTP, see
Appendix A, "Reference Information."
Tip
For more information on cRTP, see Configuring Compressed Real-Time Protocol.
TX Ring
The transmit (TX) ring is a FIFO queue located just prior to the physical interface. Its purpose is to hold
packets prior to the physical interface to insure that a packet will be available when the interface is ready
to transmit traffic. It is the unprioritized, final buffer used to hold frames prior to transmission in order
to drive link utilization to 100%.
The placement of the TX ring in the Layer 2 queueing subsystem is shown in
Figure 1-10
.
UDP HDR
RTP Header
IP Header
20 Bytes
8 Bytes
12 Bytes
2-5 Bytes
81040