Hello,
Today I'm going to share with you, detailed summary of knowledge points of E1 line.
E1 is a 2.048m link and is coded in pcm.
The frame length of an E1 is 256 bits, which is divided into 32 time slots, and one time slot is 8 bits.
8k E1 frames pass through the interface per second, that is, 8k*256=2048kbps.
Each time slot occupies 8 bits in the E1 frame, 8*8k=64k, that is, there are 32 64k in an E1.
E1 frame structure:
E1 has three modes: framed, multiframed and unframed. 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 the multiframed E1 In addition to the 0th time slot, the 16th time slot is used to transmit signaling, and only the 30th time slots from 1 to 15 and 17 to 31 can be used to transmit valid data; and in the unframed E1 , All 32 time slots can be used to transmit valid data.
1- E1 basic knowledge
Brief description of the 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 multiframe (mf). In a frame, ts0 is mainly used to transmit frame positioning signal (fas), crc-4 (cyclic redundancy check) and peer alarm indication, ts16 is mainly used to transmit channel associated signaling (cas), multi-frame positioning signal and multiplexing Frame opposite end alarm indication, ts1 to ts15 and ts17 to ts31 total 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 as "overhead". If the out-of-band common channel signaling (ccs) is used, ts16 loses the purpose of transmitting signaling. 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.
introduce E1 by pcm coding:
From the time slot characteristics of E1 in pcm coding, it can be seen that E1 is divided into 32 time slots ts0-ts31. Each time slot is 64k, where 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 the crc check code. ts16 is the signaling time slot. When signaling (common channel signaling or channel associated signaling) is used, this
The time slot is used to transmit signaling, and users cannot use it to transmit data. So the 2m pcm code 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, no crc check.
③ pcm30c: There are 30 time slots available for pcm30 users, ts1-ts15,
Ts17-ts31. ts16 transmits signaling and has crc verification.
④ pcm31c: There are 31 time slots available for pcm30 users, ts1-ts15,
Ts16-ts31. ts16 does not transmit signaling and has crc verification.
CE1, is to divide the 2m transmission into 30 64k time slots, generally written as n*64,
You can use several of these time slots, that is, only use n 64k, which must be connected to CE1/pri.
CE1----A maximum of 31 channels can carry data timeslots 1----31
Timeslots 0 pass synchronization
2- Interface
G. 703 Unbalanced 75 ohm, balanced 120 ohm 2 kinds of interfaces
3- There are three ways to use E1
Use the entire 2m as a link, such as ddn 2m;
Use 2m as several 64k and its combination, such as 128k, 256k, etc., this is CE1;
When used as a digital relay of a voice switch, this is also the most original usage of E1. One E1 is used as 32 64k, but time slot 0 and time slot 15 are used for signaling, so One E1 can transmit 30 voices. pri is one of the most commonly used access methods, and the standard is called pra signaling.
Use a WAN interface card such as 2611 to connect to the E1 line via a v.35-g.703 converter. Such a cost should be lower than the E1 card, the current 2m rate line of ddn is usually pulled to the user side through the hdsl line.
E1 can be pulled from the optical fiber from the transmission equipment to the optical transceiver on the user side to provide E1 services.
4- Precautions for use
When the E1 interface is connected, the E1 of both parties must not have signal loss/frame out-of-synchronization/multi-frame out-of-synchronization/slip code alarms, but both parties must be completely consistent in the E1 interface parameters, because the individual characteristic parameters are inconsistent, and there will be no indicators or there are any alarms on the alarm console, but it will cause the data channel to be blocked, error code, slip code, and out of step. These characteristic parameters mainly include; impedance/frame structure/crc4 check, resistance has two kinds of 75ohm and 120ohm, frame structure has three kinds of pcm31/pcm30/unframed; in the new bridge node machine, pcm31 and pcm30 are described as ccs and cas, when docking, tell the network administrator to choose ccs, whether to perform crc verification can be flexibly selected, the key is that both parties are consistent, so that the physical layer can be guaranteed to be normal.
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