o. A frame, 125 microseconds TS2-30 Frame alignment word Bit number TSO in every other frame that does not contain FAW. Bit number TSO word A = Far end alarm, alarm condition "1". D = Spare bits that can be used for specific point-to-point low data rate applications (for example, network management information). Bit 2 alternates from frame to frame to prevent accidental simulations of the frame alignment signal. C = CRC-4 procedure for protection against simulation of frame alignment and enhanced error monitoring. If not in use, C-bit is set to "1". Figure 4.22 The 2,048-Kbps frame alignment word in TSO. Introduction to Telecommunications Network Engineering Each receiver of the 2,048-Kbps data stream detects errors in order to monitor the quality of the received signal. Error monitoring is mainly based on the detection of errors in the frame alignment word. The receiver compares the received word in every other TSO with the error-free frame alignment word. In addition to the frame alignment word, the CRC-4 code is used to detect low error rates. Errors in the frame alignment word do not give reliable results when the error rate is very low. It may take a long time before an error is detected in TSO although many errors may have occurred in other time slots of the frame. The C-bit in Figure 4.22 is set to 1 if CRC is not used [5]. The TSO in every other frame also contains a far-end alarm informa- tion bit A as shown in Figure 4.22. This is used (set to 1) to tell the transmit- ting multiplexer that there is a severe problem in the transmission connection and reception is not successful at the other end of the system. This can be caused by, for example, a high error rate, loss of frame alignment, or loss of signal. With the help of the far-end alarm, consequent actions can take place. These actions include rerouting user channels to another operational system. D-bits can be used for transmission of network management informa- tion. At international borders they are usually set to 1. Multiframe Structure of the Si