PCM Line Equipment Knowledge, Interface Protocols

Currently in communications, many enterprises lease E1 (2M) lines and use PCM equipment to transmit voice calls, audio, and other services. Most enterprises may only understand the bandwidth of E1 lines, but are still unfamiliar with the specific technical details of E1 lines.

1. One E1 is a 2.048M link, encoded using PCM.
2. The frame length of one E1 is 256 bits, divided into 32 timeslots, with each timeslot being 8 bits.
3. 8k E1 frames pass through the interface per second, i.e., 8K*256=2048kbps.
4. Each timeslot occupies 8 bits in the E1 frame, 8*8k=64k, meaning one E1 contains 32 channels of 64K.

PCM equipment E1 frame structure     E1 supports three modes: framed, multi-framed, and unframed. In a framed E1, timeslot 0 is used for frame synchronization data transmission, while the remaining 31 timeslots can be used for valid data transmission. In a multi-framed E1, in addition to timeslot 0, timeslot 16 is used for signaling transmission, leaving only timeslots 1 to 15 and 17 to 31, a total of 30 timeslots, for valid data transmission. In an unframed E1, all 32 timeslots can be used for valid data transmission.

I. E1 Basics     Brief Description of E1 Channel Frame Structure     In the E1 channel, 8 bits form a timeslot (TS), 32 timeslots form a frame (F), and 16 frames form a multiframe (MF). In one frame, TS0 is mainly used to transmit the frame alignment signal (FAS), CRC-4 (cyclic redundancy check), and remote alarm indication; TS16 mainly transmits channel-associated signaling (CAS), multiframe alignment signal, and multiframe remote alarm indication; TS1 to TS15 and TS17 to TS31, a total of 30 timeslots, transmit voice or data information. We refer to TS1 to TS15 and TS17 to TS31 as the "payload," and TS0 and TS16 as the "overhead." If out-of-band common channel signaling (CCS) is adopted, TS16 loses its signaling transmission purpose and can also be used to transmit information signals. In this case, the payload of the frame structure is TS1 to TS31, and the overhead is only TS0.     Introduction to E1 via PCM coding:     From the timeslot characteristics of E1 in PCM coding, it can be seen that E1 is divided into 32 timeslots, TS0-TS31. Each timeslot is 64K, among which TS0 is occupied by the frame synchronization code, Si, Sa4, Sa5, Sa6, Sa7, and A bits. If the system uses CRC check, the Si bit position is changed to transmit the CRC check.