The industry’s 2M channel of PCM devices is often referred to as E1. The E1 plays a critical role in the communication industry. Many people are unfamiliar with E1. Next, I will introduce you the basic knowledge of E1 in detail.
Overview of E1
E1 is a digital transmission system primary group (ie PCM30) standard formulated by ITU-T and named by the Confederation of European Posts and Telecommunications (CEPT). Among them, 30 speech channels transmit user information such as voice, and the other two are used as system overhead to transmit synchronization codes, signaling codes, and other auxiliary signals. The physical and electrical characteristics of the E1 interface conform to the G.703 standard of CCITT. Our country also adopts the E1 standard as the base group of the PCM system and N-ISDN. At present, the E1 interface based on G.703 has been widely used in packet networks, frame relay networks, GSM mobile base stations, and military communications to transmit voice signals, data, images, and other services.
Basic knowledge of E1
1. Brief introduction of E1
① An E1 link is 2.048M, encoded with PCM.
② The frame length of an E1 is 256 bits, which are divided into 32-time slots, and the one-time slot is 8 bits.
③ There are 8k E1 frames per second through the interface, that is, 8K*256=2048kbps.
④ Each time slot occupies 8 bits in the E1 frame, 8*8k=64k, that is, one E1 contains 32 64K.
2. E1 frame structure
E1 is divided into three modes: framing, multi-frame, and non-framing. In the framed E1, the 0th time slot is used to transmit frame synchronization data, and the remaining 31 time slots can be used to transmit valid data; In E1, except for the 0th time slot, the 16th time slot is used to transmit signaling, and only the 1st to 15th, 17th to 31st total of 30 time slots can be used to transmit valid data; In E1, all 32 time slots can be used to transmit valid data.
3. Brief description of frame structure of the E1 channel
In the E1 channel, 8 bits form a time slot (TS), 32-time slots form a frame (F), and 16 frames form a multi frame (MF). In a frame, TS0 is mainly used to transmit Frame Alignment Signal (FAS), CRC-4 (Cyclic Redundancy Check), and peer alarm indication, TS16 is mainly used to transmit Channel Associated Signaling (CAS), Multiframe Alignment Signal and Complex Frame peer alarm indication, TS1 to TS15 and TS17 to TS31 have a total of 30-time slots to transmit information such as voice or data. We call TS1 to TS15 and TS17 to TS31 “payload” and TS0 and TS16 “overhead”. If out-of-band common channel signaling (CCS) is used, TS16 loses the purpose of transmitting signaling, and this time slot can also be used to transmit information signals. At this time, the payload of the frame structure is TS1 to TS31, and the overhead is only TS0.
Introduction to E1 by PCM encoding
It can be known from the time slot characteristics of E1 in PCM coding that E1 is divided into 32-time slots TS0-TS31. Each time slot is 64K, of which TS0 is the frame synchronization code, and the Si, Sa4, Sa5, sa6, Sa7, and A bits are occupied. If the system uses a CRC check, the Si bit position is changed to transmit the CRC check code. TS16 is a signaling time slot. When signaling (common-channel signaling or channel-associated signaling) is used, this time slot is used to transmit signaling, and users cannot use it to send data. So the PCM code of 2M has
① PCM30: There are 30 time slots available for PCM30 users, TS1-TS15, TS17-TS31. TS16 transmits signaling without CRC check.
② PCM31: There are 31 time slots available for PCM30 users, TS1-TS15, TS16-TS31. TS16 does not transmit signaling and has no CRC check.
③ PCM30C: PCM30 users have 30 available time slots, TS1-TS15, TS17-TS31. TS16 transmits signaling with CRC check.
④ PCM31C: PCM30 users have 31 available time slots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling and has a CRC check.
CE1 is to divide the 2M transmissions into 30 64K time slots, generally written as N*64, you can use several of these time slots, that is, only use n 64K time slots, which must be connected to ce1/pri. CE1—-Up to 31 channels can carry data timeslots 1—-31 timeslots 0 Transmission synchronization.
Well, the above is some knowledge about E1. If you need E1 products, please come to consult.