osition of four bits per channel, grouped in the two halves of time slot 16. errored sub-multiframes by changing the binary state of one E-bit It therefore requires 15 frames to carry the information of the 30 from 1 to 0 for each errored sub-multiframe. Recommendation channels. Completed by a 16th frame (frame 0) they constitute a G.704 requires the delay between the detection of an errored sub- multiframe. Time slot 16 of this 16th frame contains a multiframe multiframe, and the setting of the E-bit that indicates the error state, alignment pattern MAS which allows unambiguous numbering of to be less than 1 second. Table 21.3 shows the allocation of bits 1 to the frames within the multiframe. Each channel thus arranges the 8 of the frames for a complete CRC-4 multiframe. signalling of 4 bits every 16 frames, i.e. every 2 milliseconds. 21.5.2 CRC-4 cyclic redundancy check 21.6 T1 and PCM-30 alarms and error conditions Where there is a need to provide additional protection against simulation of the frame alignment signal, and/or where there is a 21.6.1 Principal alarms need for an enhanced error monitoring capability, then bit 1 of the frame, i.e. the International bit, is used for the 4-bit cyclic redun- The principal alarms defined by both T1 and PCM-30 are the red dancy check described below. alarm, generated by the receiving equipment to indicate that it has Each CRC-4 multiframe, which is composed of 16 frames num- lost frame alignment, and the yellow alarm, which is returned to the bered from 0 to 15, is divided into two 8 frame sub-multiframes or transmitting terminal to indicate that the receiving terminal has lost SMF, designated SMF I and SMF II which indicates their respective frame alignment. Normally, the terminal will use the receiver's red order of occurrence within the CRC-4 multiframe structure. The alarm to request that a yellow alarm be transmitted. The name of SMF is the Cyclic Redundancy Check-4 block size, i.e. 2048 bits. these alarms simply comes from the colour of the lights on the In the 