croseconds, 256 bits Ch. 1 Ch. 2 Ch. 16 Ch. 30 TS 31 TS 16 TS 17 TS 31 Frame 8 bit sample alignment (frame Speech channels Signaling Speech channels synchronization) 1 to 15 time slot 16 to 30 Frame is repeated 8,000 times in a second which is the same as PCM sampling rate. Each frame contains one sample of 30 different speech signals. One time slot (TSO) is used for frame synchronization and one (TS 16) for signaling. Data rate: 8,000 * 8 bits * 32 = 2,048 Kbps Figure 4.21 The 2,048-Kbps frame structure from Recommendation G.704. Transmission Frame Synchronization Time Slot The frame alignment word is needed to inform the demultiplexer where the words of the channels are located in the received 2-Mbps data stream. The frame synchronization time slot (TSO) includes frame alignment information and it has two different contents that are alternated in subsequent frames (Figure 4.22). The demultiplexer looks for this time slot in the received data stream and, when it is found, locks onto it and starts picking up bytes from the time slots for each receiving user. Each user receives 8 bits in 125-us peri- ods, which makes 64 Kbps. A fixed alignment word is not reliable enough for frame synchronization because it may happen that a user's data from one channel simulates the synchronization word and the demultiplexer might lock to this user time slot instead of TSO. This is why there is one alternating bit (D2) in time slot 0 (see Figure 4.22) and due to this the demultiplexer is able to detect the situation where one channel constantly transmits a word that is equal to the frame alignment word (FAW). To make frame alignment even more reliable, the cyclic redundancy check 4 (CRC-4) procedure was added in the mid-1980s. C-bits are allocated to carry a four-bit error check code that is calculated over all bits of a few frames. The receiver performs error check calculations over all bits of the frames and it is able to detect false frame alignment even if the frame align- ment word is simulated by one user that alters bit tw