What is an E1 line? What are the core knowledge points?

E1 Basics
　　Frame Structure of the E1 Channel
　　In an E1 channel, 8 bits form one timeslot (TS), 32 timeslots form one frame (F), and 16 frames form one multiframe (MF). Within a frame, TS0 is primarily used to carry the frame alignment signal (FAS), CRC-4 (cyclic redundancy check), and remote alarm indication. TS16 mainly carries channel-associated signaling (CAS), the multiframe alignment signal, and the multiframe remote alarm indication. TS1 to TS15 and TS17 to TS31, a total of 30 timeslots, carry 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 used, TS16 loses its signaling function and can also be used to carry information signals. In this case, the frame payload is TS1 to TS31, and the only overhead is TS0.
　　Introduction to E1 via PCM Coding:
　　From the timeslot characteristics of E1 in PCM coding, E1 is divided into 32 timeslots, TS0-TS31. Each timeslot is 64K. TS0 is occupied by the frame synchronization word, Si, Sa4, Sa5, Sa6, Sa7, and the A bit. If the system uses CRC checking, the Si bit position is used to transmit the CRC check code instead. TS16 is the signaling timeslot. When signaling (common channel signaling or channel-associated signaling) is used, this timeslot is used to transmit signaling and cannot be used by users for data transmission. Therefore, the 2M PCM line codes are:
　① PCM30: PCM30 has 30 user-available timeslots, TS1-TS15, TS17-TS31. TS16 carries signaling, without CRC checking.
　② PCM31: PCM31 has 31 user-available timeslots, TS1-TS15, TS16-TS31. TS16 does not carry signaling, without CRC checking.
　③ PCM30C: PCM30C has 30 user-available timeslots, TS1-TS15, TS17-TS31. TS16 carries signaling, with CRC checking.
　④ PCM31C: PCM31C has 31 user-available timeslots, TS1-TS15, TS16-TS31. TS16 does not carry signaling, with CRC checking.
　CE1 divides the 2M transmission into 30 64K timeslots, generally written as N*64. You can use several of these timeslots, i.e., only use n times 64K, and it must be connected to a CE1/PRI interface.
　CE1 — up to 31 channels can carry data, TS1-TS31. TS0 carries synchronization.

II. Interface
　G.703 unbalanced 75 ohm and balanced 120 ohm, two types of interfaces.

III. Three Methods of Using E1:

　1. Use the entire 2M as a single link, such as DDN 2M;

　2. Use the 2M as several 64K channels or combinations thereof, such as 128K, 256K, etc. This is CE1;

　3. When used as a digital trunk for a voice switch, this is the most original use of E1 — treating one E1 as 32 64K channels. However, timeslot 0 and timeslot 15 are used for signaling, so one E1 can carry 30 voice channels. PRI is one of the most common access methods, with the standard name PRA signaling.
　Use a WAN interface card such as the 2611, connected to the E1 line via a V.35-G.703 converter. This cost should be lower than an E1 card. Currently, DDN 2M rate lines are typically extended to the user side via HDSL lines.
　Alternatively, fiber from the transmission equipment can be extended to an optical terminal on the user side to provide E1 service.

IV. Usage Precautions
　When interconnecting E1 interfaces, neither side may have alarms for signal loss, frame loss of synchronization, multiframe loss of synchronization, or slip. However, the E1 interface parameters on both sides must be completely consistent, because inconsistencies in certain characteristic parameters will not trigger any alarm on the indicator lights or alarm console, but will cause data channel failures such as disconnection, bit errors, slips, or loss of synchronization. These characteristic parameters mainly include: impedance, frame structure, and CRC4 checking. Impedance has two types: 75 ohm and 120 ohm. Frame structure has three types: PCM31, PCM30, and unframed. In Newbridge node switches, PCM31 and PCM30 are described as CCS and CAS, respectively. When interconnecting, the network administrator must be informed to select CCS. Whether to perform CRC checking can be flexibly chosen; the key is that both sides must be consistent, which ensures normal operation at the physical layer.