E1 Line Knowledge, Interface Protocols, and Usage Methods

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 not familiar 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.

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

I. E1 Basic Knowledge
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). 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 this timeslot 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 Encoding:
From the timeslot characteristics of E1 in PCM encoding, it can be seen that 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 positions are changed to transmit CRC check codes. 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 to transmit data. Therefore, the 2M PCM code patterns include:
① PCM30: PCM30 has 30 user-available timeslots, TS1-TS15, TS17-TS31. TS16 transmits signaling, without CRC check.
② PCM31: PCM31 has 31 user-available timeslots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling, without CRC check.
③ PCM30C: PCM30C has 30 user-available timeslots, TS1-TS15, TS17-TS31. TS16 transmits signaling, with CRC check.
④ PCM31C: PCM31C has 31 user-available timeslots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling, with CRC check.
CE1 divides the 2M transmission into 30 timeslots of 64K each, generally written as N*64. You can use several of these timeslots, i.e., only utilize n times 64K, and it must be connected to CE1/PRI. CE1 can carry data on up to 31 channels, timeslots 1-31, with timeslot 0 for synchronization.

II. E1 Interface Protocols
The E1 interface generally runs the G.703 protocol and is available in two interface types: unbalanced 75 ohm and balanced 120 ohm.

III. There are three methods of using E1:

1. Use the entire 2M as a single link, such as DDN 2M;
2. Use the 2M as a combination of several 64k channels, such as 128K, 256K, etc. This is CE1;
3. When used as a digital trunk for voice switches, this is the original purpose 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 WAN interface cards such as the 2611, and connect to the E1 line via a V.35-G.703 converter. This cost should be lower than that of an E1 card. Currently, DDN 2M rate lines are usually extended to the user side via HDSL lines. E1 can be provided by optical fiber from transmission equipment extended to the user-side optical terminal, which provides E1 services.

IV. Usage Precautions
When interconnecting E1 interfaces, both parties' E1 must not have signal loss, frame loss of synchronization, multiframe loss of synchronization, or 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, 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 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.