A.1 Introduction
23.1073GPPQuality of Service (QoS) concept and architectureRelease 17TS
This annex provides some basic information with respect to the error resilience of different encoded media streams when considering the support of unequal error protection for real-time packet multimedia services. It provides some indicative figures for the residual bit error rates that could be tolerated by audio-visual H.323 payloads in a 3G environment.
H.323 employs the H.225.0 packetisation scheme, which in turn uses UDP/IP and RTP to transport each media stream. The structure of an H.323 packet is shown in figure A.1.
Figure A.1: Structure of H.323 packet
Figure A.2: Structure of compressed H.323 packet.
Class 1 bits can tolerate medium BER; Class 2 bits can tolerate high BER
It is assumed that some elements of the H.323 header information, which comprises the IP, UDP and RTP headers, can be compressed. It is also assumed that this information will require reliable transmission, such that any errors in the header will result in the loss of the complete H.323 packet. However, for real-time multimedia streams that cannot accommodate a large delay (and therefore packet retransmission), codecs can be used that are tolerant to residual bit errors.
This annex highlights the error resilience of audio and visual codecs, and provide some example tolerance figures for media streams of the type that are likely to comprise H.323 payloads.
A.1.1 Factors affecting error resilience
Specific error resilience figures will depend on a number of factors, including:
– the media type;
– the quality of service (QoS) required;
– the specific codec used.
Media streams may also be sub-divided into different classes on the basis of bit error sensitivity as shown in figure A.2. In some cases the most sensitive bits may be protected by in-band checksum information. It should also be noted that, in addition to the effect of residual bit errors in the media stream, the QoS will be further degraded by packet loss due to errors in the H.323 header.