3GPP, which supports UMTS technology, originally defined 3G-324M as part of Release '99 in December 1999. In August of 2002, it approved 3G-324M usage for CDMA2000. In 2003, TD-SCDMA become a 3GPP standard. It adopted 3G-324M and began to operate in China as the formal 3G standard.

3G-324M is an addressless protocol. It doesn't include the call setup with the baseband. The call setup for the protocol is defined in the following specifications:

• 3GPP TS 24.008: Mobile radio interface Layer 3 specification
• 3GPP TS 27.001: General on Terminal Adaptation Functions (TAF) for Mobile Stations (MSs)
• 3GPP TS 29.007: General requirements on interworking between the public-land mobile network (PLMN) and the integrated-services digital network (ISDN) or public-switched telephone network (PSTN)
• 3GPP TS 23.108: Mobile radio interface Layer 3 specification core network protocols; Stage 2 (structured procedures)

3GPP defines UMTS/W-CDMA solution architectures. The organization's codec working group (TSG-SA/WG4) was responsible for the specification of the visual phone. It defined 3G-324M. As a baseline of the terminal specification, ITU-T H.324M was adopted.

ITU H.324 was defined for visual PSTN phone terminals. Initially, it was developed for PSTN with the V.34 modem protocol. Its mobile extension is defined as H.324M (originality called H.324 with mandatory support of Annex C). H.324M was realized with the improvement of error resiliency to the multiplexing protocol, which is defined in Annex A/B/C to H.223.

The major sub-protocols and procedures of 3G-324M are:

• Error-resilience services
• H.223 multiplexing/de-multiplexing protocol
• ITU-T H.245 call control
• Optional codecs to be used: MPEG-4 Simple Profile, H.263 for video, and adaptive multi-rate (AMR) for audio

For mobile conversational multimedia communication, error resilience is essential for error detection and concealment on the fly. H.223 provides Annexes A, B, C, and D for such services. Annexes A and B define the handling of light to moderate BER levels. These annexes were made mandatory by 3GPP. They're commonly used by vendors today.

In addition, MPEG-4 video provides tools for error resilience. It thereby minimizes the video-quality degradation that is caused by errors. These solutions don't reduce errors like forward error correction (FEC) or automatic repeat request (ARQ). But they can reduce the damage on decoded video quality.

For instance, MPEG-4 Visual (ISO/IEC 14496-2) is a generic video codec. One of its target areas is mobile communications. Error resiliency and high efficiency make this codec particularly well suited for 3G-324M.

In contrast, MPEG-4 Visual is organized into profiles. Within a profile, various levels are defined. The profiles define subsets of toolsets. The levels are related to computational complexity. Among these profiles, Simple Visual Profile provides low complexity and error resiliency (through data partitioning, reversible variable-length coding (RVLC), a resynchronization marker, and header extension code). MPEG-4 allows various input formats including general formats like quarter common intermediate format (QCIF) and common intermediate format (CIF). It also is baseline compatible with H.263. Details on the MPEG-4 error-resilience services follow:

The resynchronization marker can reduce the error propagation caused by the nature of variable-length code (VLC) into a single frame. In MPEG-4, the resynchronization marker is inserted at the top of a new group of block (GOB) with the header information (macro-block, or MB, number and quantization parameters) and optional header extension code (HEC). Decoding can then be done independently. It's a good idea to place the resynchronization marker before important objects like people. This approach will improve error resilience with a minimum increase of overhead.

Byte alignment: Bit stuffing for the byte alignment provides additional error-detection capability through its violation check.

Data partitioning: A new synchronization code, which is named motion marker, separates the motion-vector (MV) and discrete-cosine-transform (DCT) fields. In this way, it prevents inter-field error propagation. Effective error concealment can therefore be performed. When errors are detected solely in the DCT field, the MB will be reconstructed using correct MV. Compared to the simple MB replacement of the previous frame, this approach results in better natural motion.

Reversible variable-length code (RVLC): The RVLC is designed to enable forward and backward decoding without significantly impacting coding efficiency. Ideally, this feature localizes error propagation into a single macro block.

Adaptive intra refresh (AIR): In contrast to the conventional cyclic version, AIR employs motion-weighted intra refresh. It results in better perceptual quality along with the quick recovery of corrupted objects.

Error detection and concealment: Errors can be detected by exceptions or violations in the decoding process. Concealment will then be applied. This functionality is included for mobile applications. The code point of H.324 can support MPEG-4 audio, thereby making it usable for an H.324 mobile-phone terminal.