ed Updating. Forced updating is achieved by forcing the use of the INTRA mode of the coding algorithm. Recommendation H.261 does not define the update pattern. For the control of accumulation of inverse transform mismatch error, a macroblock should be updated forcibly at least once per every 132 times it is transmitted. 16.10.2.3 Video Multiplex Coder. The video multiplex is arranged in a hierarchical structure with four layers. From top to bottom these layers are: Picture Group of blocks (GOB) Macroblock Block For further description of these layers, consult CCITT Rec. H.261 (Ref. 20). 16.11 BRIEF OVERVIEW OF FRAME TRANSPORT FOR VIDEO CONFERENCING This section briefly reviews one method of transporting on the PSTN digital network the video conferencing signals developed in Section 16.10. It is based on ITU-T Rec. H.221 (Ref. 22). 16.11.1 Basic Principle An overall transmission channel of 4-1920 kbps is dynamically subdivided into lower rates suitable for transport of audio, video, and data for telematic purposes. The trans- mission channel is derived by synchronizing and ordering transmission over from one to six B-channels, 11 from one to five H channels or an H11 or H12 channel. The initial connection is the first connection established, and it carries the initial channel in each direction. The additional connections carry the necessary additional channels. The total rate of transmitted information is called the transfer rate. The transfer rate can be less than the capacity of the overall transmission channel. A single 64-kbps channel is structured into octets transmitted at an 8-kHz rate. 12 Each bit position of the octets may be regarded as a subchannel of 8 kbps. A frame reflecting this concept is illustrated in Figure 16.12. The service channel (SC) resides in the eighth subchannel. It carries the frame alignment signal (FAS), a bit-rate allocation signal (BAS), and an encryption control signal (ECS). We can regard an H, H11, H12 channel as consisting of a number of 64-kbps time slots (TS). The lowest numb