Frame Formats Based on the Usage of Timeslot 0

Figure 1 shows the formats of timeslot 0 in E1 frames.

Figure 1 Formats of timeslot 0 in E1 frames

As shown in Figure 1, the format of timeslot 0 in an odd-numbered frame is different from that in an even-numbered frame. The signal contained in timeslot 0 of an even-numbered frame is called frame alignment signal (FAS); the signal contained in timeslot 0 of an odd-numbered frame is called not frame alignment signal (NFAS), which contains the A-bit indicating remote alarms and spare bits Sa4 to Sa8. The FAS and NFAS each contain an X-bit. Based on the function of the X-bit, E1 frames are classified into generic double-frames and cyclic redundancy check 4 (CRC-4) multiframes.

• When the E1 frame is a generic double-frame, the X-bit functions as the Si-bit.

• When the E1 frame is a CRC-4 multiframe, the X-bit is used to transmit CRC-4 multiframe check signal, CRC-4 check error bits, and multiframe alignment signal (MFAS).

Frame Formats Based on the Usage of Timeslot 16

E1 frames can be classified into PCM30 frames and PCM31 frames, depending on whether timeslot 16 transmits voice or signaling.

• A PCM30 frame uses channel associated signaling (CAS). In this mode, timeslot 0, as a synchronous timeslot, cannot carry voice; timeslot 16, to carry CAS, cannot carry voice either. As a result, one E1 frame can carry only 30 voice signals, and it is therefore called a PCM30 frame.

• A PCM31 frame uses common channel signaling (CCS). A multiframe in CCS mode does not need to transmit CAS. In this mode, except for timeslot 0 that carries synchronous signals, one E1 frame can carry 31 voice signals and it is therefore called a PCM31 frame.

The two classification methods focus on two attributes of E1 frames, and they can be combined. Specifically, there are four E1 frame formats in actual application:

• PCM30CRC (PCM30 + CRC-4 multiframe)

• PCM30NOCRC (PCM30 + generic double-frame)

• PCM31CRC (PCM31 + CRC-4 multiframe)

• PCM31NOCRC (PCM31 + generic double-frame)