What is E1? How to Use E1

What is E1
　　The European 30-channel Pulse Code Modulation (PCM) is abbreviated as E1, with a rate of 2.048Mbit/s. China adopts the European E1 standard.
　　An E1 time-division multiplexed frame (with a length of T=125us) is divided into 32 equal timeslots, numbered CH0CH31. Timeslot CH0 is used for frame synchronization, timeslot CH16 is used for signaling transmission, and the remaining CH1CH15 and CH17~CH31, a total of 30 timeslots, are used for 30 voice channels. Each timeslot transmits 8 bits, totaling 256 bits. 8000 frames are transmitted per second, so the data rate of the PCM primary group E1 is 2.048Mbit/s.
　　1. One E1 is a 2.048M link encoded with 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.
E1 Frame Structure
　　E1 has three modes: framed, multi-framed, and unframed. In a framed E1, timeslot 0 is used for frame synchronization data transmission, and 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.
Brief Description of E1 Channel Frame Structure
　　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 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 signals, 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 this timeslot can also be used to transmit information signals. In this case, the payload of the frame structure is TS1 to TS31, with only TS0 as overhead.
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. Among them, 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 code. TS16 is the signaling timeslot. When signaling (common channel signaling or channel-associated signaling) is used, this timeslot is used for signaling transmission and cannot be used by users for data transmission. Therefore, the 2M PCM code types are:
　  ① PCM30: PCM30 has 30 available user timeslots, TS1-TS15,
　　   TS17-TS31. TS16 transmits signaling, without CRC check.
　　② PCM31: PCM30 has 31 available user timeslots, TS1-TS15,
　　　 TS16-TS31. TS16 does not transmit signaling, without CRC check.
　  ③ PCM30C: PCM30 has 30 available user timeslots, TS1-TS15,
　　　 TS17-TS31. TS16 transmits signaling, with CRC check.
　  ④ PCM31C: PCM30 has 31 available user timeslots, TS1-TS15,
　　　 TS16-TS31. TS16 does not transmit signaling, with CRC check.
　　 CE1 divides the 2M transmission into 30 timeslots of 64K, generally written as n×64.
　　 You can utilize several of these timeslots, i.e., only use n channels of 64K, and it must be connected to ce1/pri.
　　 CE1----can support up to 31 channels for data transmission; timeslots 1----31; timeslot 0 transmits synchronization.
Interface Impedance
　　The impedance of the G.703 unbalanced interface is 75 ohm, and the impedance of the balanced interface is 120 ohm. There are three methods to use E1:
　　1. Use the entire 2M as one 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 voice switches, this is also the most original use of E1, which is to use one E1 as 32 channels of 64K. However, timeslot 0 and timeslot 16 are used for signaling, so one E1 can transmit 30 voice channels. PRI is one of the most common access methods, with the standard name being 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 usually extended to the user side via HDSL lines.
　　E1 can also be provided by an optical terminal at the user side, where the fiber from the transmission equipment extends to the user side.
Usage Precautions
　　When interconnecting E1 interfaces, both parties must not have alarms such as signal loss, frame loss of synchronization, multiframe loss of synchronization, or slip errors. However, both parties must be completely consistent in E1 interface parameters, because inconsistencies in certain characteristic parameters will not cause any alarms on indicator lights or alarm stations, but may cause data channel failures such as disconnection, bit errors, slips, or loss of synchronization. These characteristic parameters mainly include: impedance, frame structure, and CRC4 check. Impedance has two types: 75 ohm and 120 ohm. Frame structures include PCM31/PCM30/unframed. In Raytrans PCM optical terminals, PCM31 and PCM30 are described as CCS and CAS respectively. When interconnecting, the network administrator should be informed to select CCS. Whether to perform CRC check can be flexibly selected, but the key is that both parties must be consistent to ensure the normal operation of the physical layer.