MULTIPLEXING/DEMULTIPLEXING
A 3G-324M protocol is initialized after a circuit-switched channel is opened between two communicating parties. The H.223 multiplexing protocol is the first to be established between those parties. After initiating this protocol, the multiplexing process must be synchronized between the communicating parties. It's also important to establish the call control (H.245) as the first logical channel to be opened (channel 0).

The basic function of the multiplexing protocol is to interleave multiple media streams into a single stream. Such media streams could include video, speech, user data, and control signals (H.245). That single stream can then be sent over a transmission channel. 3G-324M uses the ITU-T H.223 mobile extensions of Level 2 as its multiplex protocol.

H.223 has a flexible mapping scheme that's suitable for a variety of media and a variable frame length. In its mobile extension, it flaunts stronger synchronization and control against channel errors without losing its flexibility. Three operation modes exist from Level 0 to Level 3. They are categorized according to their degree of error resiliency.

Multiplexing Level 0 is identical to the H.223 specification. It provides multiplexing and QoS functions that are appropriate for each media data. Two layers, which are known as the adaptation and multiplexer (MUX) layers, realize these features (FIG. 3). Three types of adaptation layers are defined according to their media type (video, speech, or data):

• Adaptation Layer (AL) 1: User data and control signals (H.245). This AL assumes that the upper layer provides error control.
• Adaptation Layer 2: Speech. Error detection and a sequence-numbering mechanism are provided.
• Adaptation Layer 3: Video. This layer offers error detection, sequence numbering, and ARQ.

The multiplexer layer assembles multiple media packets into a single bit stream according to the selected multiplex pattern. That pattern is chosen out of up to 16 multiplex patterns. The MUX pattern can be defined arbitrarily through the session negotiation procedure.

Header information is attached in order to control such a flexible multiplexing mechanism. It consists of a 4-b multiplex code (MC), 1-b packet marker (PM), and 3-b parity (HEC). As a delimiter of multiplexer-protocol data units (MUX-PDUs), 8-b HDLC synchronization flags ('01111110') are inserted. To prevent the flag emulation inside the payload, stuffing is defined ('0' bit insertion after every five succeeding '1s').

In Multiplexing Level 1, a 16-b PN sequence is used instead of an 8-b HDLC synchronization flag. It thereby improves the MUX-PDU synchronization over error-prone channels. Stuffing is prohibited to enable an octet-oriented flag search. This modification remarkably improves the flag-detection performance over error-prone channels. But in the case of conflict, there is a slight probability of flag emulation conditions. This multiplexing level is described in H.223 Annex A to overcome light error-prone channel for detection and concealment services.

In Multiplexing Level 2, MUX-PDU payload length information and FEC for the header is added over the Level 1 modification. As a result, it promises much better synchronization and error resilience. An optional header field, which includes MC/PM/HEC for the previous frame, can be applied to improve error resilience against burst errors through time-diversity effects. This multiplexing level is described in H.223 Annex B. Its goal is to overcome moderate error-prone channels for detection and concealment services. All Freedom of Mobile Access (FOMA) devices, which are approved by NTT-DoCoMo, support Multiplexing Level 2. This multiplexing level is the standard de-facto choice today.

H.245 TERMINAL CONTROL
3G-324M uses ITU-T Recommendation H.245 as the multimedia-communications-control method. H.245 is used in a variety of applications, such as B-ISDN (within H.320), local-area networks (within H.323), and mobile communications. It has a wide variety of communications-control functions. Assuming it's used in an error-free environment, H.245 enables reliable control using in-channel request-response messaging.

Currently, ITU H.245 version 10 is ratified by the SG16 of ITU. A few vendors, such as RADVISION, support this version in their 3G-324M protocol toolkit. The minimal version to be supported is version 3. Support for the higher versions enables a richer set of call-control services. The rising video-codec protocol, H.264, requires advanced H.245 support with generic capabilities exchange.

Because 3G-324M rides on a channel that was opened between two communicating parties, it doesn't need any addressing like H.323. The gateway (e.g., between 3G-324M, H.320, H.323, and SIP) is expected to provide the interoperability between different networks. This gateway can be realized rather easily.

3G-324M for H.245 operation requires Numbered Simple Retransmission Protocol (NSRP) and Control Channel Segmentation and Reassembly Layer (CCSRL) sun-layers support. NSRP is defined in H.324/Annex A. Essentially, mobile terminals shall support the NSRP and the SRP mode. If both terminals start the session in Level 0, the SRP mode shall be used. If Multiplexing Level 2 is used, both terminals shall start with NSRP mode.

The CCSRL sublayer is used for carrying the large H.245 packets that are required for operation. H.245 provides the following functions: master-slave determination, capability exchange, logical channel management, multiplex table management, mode-change request, and miscellaneous commands and indications. The explanation for each function follows:

The master-slave determination figures out which terminal is the master at the beginning of the session. Due to the fact that H.245 is a symmetric control protocol, it's necessary to determine the master terminal. That terminal has the right to decide the conditions in case of conflict.