E1 Line Knowledge

E1 Basics
　　Frame Structure of E1 Channel
　　In an E1 channel, 8 bits form a timeslot (TS), 32 timeslots form a frame (F), and 16 frames form a multiframe (MF). In 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 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, 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 to transmit signaling 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: PCM31 has 31 available user timeslots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling, without CRC check.
　③ PCM30C: PCM30C has 30 available user timeslots, TS1-TS15, TS17-TS31. TS16 transmits signaling, with CRC check.
　④ PCM31C: PCM31C has 31 available user timeslots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling, with CRC check.
　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 64K channels, and it must be connected to CE1/PRI.
　CE1----can carry data on up to 31 channels Ts1~Ts31. Ts0 transmits 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 2M as several 64k channels or their combinations, such as 128K, 256K, etc. This is CE1;

　3. When used as a digital trunk for a voice switch, this is also the most original use of E1, which is to use one E1 as 32 64K channels. However, timeslot 0 and timeslot 15 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 PRA signaling.
　Use WAN interface cards 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.
　Alternatively, the optical fiber from the transmission equipment can be extended to the optical terminal at the user side to provide E1 service.

IV. Usage Precautions
　When interconnecting E1 interfaces, both parties' E1 must not have signal loss/frame misalignment/multiframe misalignment/slip alarm. However, both parties must be completely consistent in E1 interface parameters, because inconsistencies in certain characteristic parameters will not cause any alarm on indicator lights or alarm consoles, but will cause data channel disconnection/errors/slips/misalignment, etc. These characteristic parameters mainly include: impedance/frame structure/CRC4 check. Impedance has two types: 75 ohm and 120 ohm. Frame structure has three types: PCM31/PCM30/non-framed. In Newbridge node machines, PCM31 and PCM30 are described as CCS and CAS respectively. When interconnecting, the network management personnel should be informed to select CCS. Whether to perform CRC check can be flexibly selected. The key is that both parties must be consistent, so as to ensure the normal operation of the physical layer.