Both Rates of G.723.1 (Q91)

The information in this article applies to:

Unlike H.324, H.323 requires that EPs who support G.723.1 be able to encode and decode it in both low- and high-rate modes. Virtually all EPs are able to decode both rates (each audio frame indicates the mode). However, some EPs that support G.723.1 always transmit it in either the high-rate or low-rate mode. Section 6.2.5/H.323v2: "If G.723.1 audio is provided, the audio codec shall be capable of encoding and decoding according to both the 5.3 kbit/s mode and the 6.3 kbit/s mode."

These non-compliant EPs could cause interoperability problems when involved in a multipoint conference where an MP mixes audio. This is because, in order to reduce the amount of transcoding, an MCU could force all G.723.1 participants to use the same mode via the RequestMode message. EPs that are only able to transmit one mode would have to reject this request, which they are not allowed to do (Section 7.11.5/H.245v2: "multipointModeCommand commands that a terminal in receipt shall comply with all requestMode requests issued by the MCU.").