Timeslot is a part of serial self multiplexing dedicated to timeslot information of a single channel, usually referring to a voice channel (64kbps) in PCM E1 and T1 signals.It is a time slice in time division multiplexing mode (TDM).
PCM E1 and T1 are two international multiplexing systems: 30/32 channel frame structure (E1) and 24 channel frame structure (T1).China adopts a 30/32 channel frame structure, that is, each frame accounts for 125 µ s and is divided into 32 time slots, but only 30 channels of voice information are transmitted.
2. One E1 frame is 256 bits long, divided into 32 time slots, and one time slot is 8 bit time (bit time is the time required to transmit 1 bit).
3. 8k E1 frames per second pass through the interface, that is, 8K * 256=2048kbps.
4. Each timeslot occupies 8 bits in E1 frame, 8 * 8K=64K, that is, an E1 contains 32 64K.
Illustration
8 (bit) * 32 (time slot)=256 (bit)=1 frame (F)
If the transmission speed is 2Mbps (2M bits per second), that is, 8k frames per second (2M (bps)/256 (bpF)=8k (Fps), then the transmission amount per time slot is 8K (Fps) * 8 (bpF)=64k (bps);
TS0=64kbps；
TS1=64kbps；
……；
Similarly, each time slot is reconstructed into a frame to get
32*64Kbps=2Mbps
E1 has three modes of framing, multi framing and non framing. In the framed E1, the 0 th slot is used to transmit frame synchronization data, and the remaining 31 slots can be used to transmit effective data;In the E1 of multiple frames, the 16th time slot is used for transmitting signaling except the 0th time slot, and only the 1st to 15th, 17th to 31st time slots can be used for transmitting effective data;In unframed E1, all 32 time slots can be used to transmit valid data.[1]
E1 Basics
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Illustration
Brief description of frame structure of E1 channel In E1 channel, 8 bits form a time slot (TS), 32 time slots form a frame (F), and 16 frames form a multiple frame (MF).In one frame, TS0 is mainly used to transmit frame positioning signal (FAS), CRC-4（Cyclic redundancy check）And peer alarm indication. TS16 mainly transmits path following signaling (CAS), multi frame positioning signal and multi frame peer alarm indication. There are 30 time slots from TS1 to TS15 and from TS17 to TS31 to transmit voice or data and other information.We call TS1 to TS15 and TS17 to TS31 "net load", and TS0 and TS16 "overhead".If out of band common channel signaling (CCS) is used, TS16 will lose 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.[2]
Coding introduction E1
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According to the time slot characteristics of E1 in PCM code, E1 is divided into 32 time slots TS0-TS31.Each time slot is 64K, where TS0 is the frame synchronization code.TS16 is a signaling timeslot. When signaling (common path signaling or random path signaling) is used, this timeslot is used to transmit signaling, and users cannot use it to transmit data.So the PCM code type of 2M has
① PCM30: PCM30 users can use 30 timeslots, TS1-TS15, TS17-TS31. TS16 transmits signaling, noneCRC check。
② PCM31: PCM30 users can use 31 timeslots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling, and there is no CRC check.
③ PCM30C: The available timeslots for PCM30 users are 30, TS1-TS15, TS17-TS31. TS16 transmits signaling with CRC check.
④ PCM31C: PCM30 users can use 31 timeslots, TS1-TS15, TS16-TS31. TS16 does not transmit signaling, but has CRC verification.
interrelation
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Time slot can be understood as a channel. Multiple people share one resource. Time sharing method is adopted. One time slot is equivalent to one channel
The channel can be compared to the RJ45 network cable. There are 11 available channels.Considering that there is a signal overlapping area between two adjacent wireless APs, in order to ensure that the signal channels used in this area cannot cover each other, specifically, the wireless APs whose signals cover each other must use different channels, otherwise it is easy to cause interference between the signals of various wireless APs, which will lead to the overall performance of the wireless network degradation.[3]
