GSM and Similar Architectures Lesson 08 GSM Traffic and Control Data Channels

Transcription

1 GSM and Similar Architectures Lesson 08 GSM Traffic and Control Data Channels 1

2 Four Types of Control Data Bursts Access burst The call setup takes place when setting the initial connection using a burst The channel in which this burst is sent is called AGCH (access grant channel) a part of CCCH (common control channel) 2

3 Four Types of Control Data Bursts Synchronization burst Synchronization burst of 64 TR bits helps in synchronizing the transmitter and receiver time slots and in timing advance The data bits after header and before tail in the burst are [(142 64)/2] 1 = 38 bits in place of 57 bits SCH (synchronization channel) the channel used of this burst, a part of BCCH (broadcast control channel) 3

4 Need for synchronization The TR bits help the receiver in correcting path changes All the MSs that are communicating with the BTS must be synchronized The total time durations of forward and return paths vary, as some MSs are closer than the others 4

5 Need for synchronization A timing advance required for synchronization when a BTS receives a signal from a far off MS compared to a short distance MS The advance is of maximum 0.24 ms ( µs period for 63 bits, because each bit is transmitting in a µs interval 5

6 Four Types of Control Data Bursts Frequency correction burst Corrects the carrier frequency In place of the TR, S, and user data bits, a 142-bit sequence between H and T is deployed FCCH (frequency correction channel) The channel for this burst sent, a part of BCCH 6

7 Need for frequency correction A deviation in the frequency of a radio carrier possible Interference with the neighbouring channel frequency possible During synchronous data transmission), the receiver must synchronize the clock rate according to the incoming data bits 7

8 Four Types of Control Data Bursts Dummy burst When no useful burst being transmitted from an MS or BTS after a connection setup 8

9 Traffic Data Channel Voice coding Using a codec (coder decoder) A circuit that codes analog signals into digital signals and decodes digital signals into analog according to various coding and decoding algorithms 9

10 Traffic Data Channel CRC and redundant bits for FEC The error correction bits (cycle redundancy check (CRC) and redundant bits) appended and data interleaving is performed 10

11 Application of data interleaving Introducing noise in idle state to prevent user uneasiness in periods of silence 11

12 Three type of voice traffic TCH/FS (traffic channel/full rate set for transmission) TCH/HS (traffic channel/half rate set for transmission) TCH/EFR (traffic channel/enhanced full rate set for transmission) 12

13 Three type of voice traffic TCH/F14.4 Traffic channel/full rate at 14.4 TCH/F9.6 Traffic channel/half rate at 9.6 kbps TCH/F4.8 Traffic channel/half rate at 4.8 kbps 13

14 Three type of voice traffic Due to large number of subscribers at a base station, the GSM specifications provide for the traffic rates of 14.4 kbps, 9.6 kbps and 4.8 kbps also 14

15 TCH/FS Voice coded with a codec (coder decoder) at 13 kbps Additional bits appended after coding, the data rate is enhanced to 22.8 kbps when transmitting at full speed 15

16 TCH/HS Coded with a codec at 5.6 kbps and after the error correction bits the data rate is enhanced to 11.4 kbps and transmission takes place at half speed The available data rate is 22.8 kbps 16

17 TCH/HS advantage Double voice signals can now be transmitted However this sort of voice data results in degradation of voice quality 17

18 TCH/EFR Coded with another enhanced coding technique employing a codec EFR gives at enhanced voice quality but has limited error correction bits because the data rate is limited to 12.8 kbps 18

19 TCH/EFR advantage The voice quality upgraded in those cases where the transmission error rate is small A codec may function in automatic mode and code the voice as TCH/FS TCH/EFR depending on the transmission error rate detected in the bursts 19

20 Control Data Channels The 184-bit packet from data link layer Formatted for the data burst bits The 184-bits added with 40 parity bits, 4 tail bits, and 224 half-convolution coding bits Total result in the 456-bit packet (Multiple of 114 bit in a data burst) 20

21 DCCH (dedicated control channels) An MS sends TCH traffic only after a call setup A bi-directional communication channel present between the BTS and MS before the TCH traffic starts Called standalone DCCH (SDCCH) 21

22 SADCCH (slow associated dedicated control channel) Used for the registration, authentication, and other requirements Total 782 bits sent as dedicated control channel data in 1 s in case of slow associated standalone DCCH (SADCCH) 950 bps can be sent as a control data slot in a traffic multiframe 22

23 FACCH (fast associated control channel) with TCH When more than 782 bits are to be sent per second, then the TCH part of the data bursts can be used Then DCCH is called FACCH (fast associated control channel) 23

24 BCCH (broadcast control channel) A BTS needs to broadcast the frequency and cell identity A BTS needs to broadcast the information regarding frequencies and sequence options for hopping that can be assigned to the MSs in the cell to all the MSs BCCH used for that 24

25 BCCH (broadcast control channel) Enables an MS to get an available radiocarrier frequency channel and transmit with different frequencies on different hops and synchronize with the BTS The synchronization and frequency correction bursts also use the BCCH 25

26 CCCH (common control channel) A BTS (when granting access to an MS so that MS can use either SDCCH or TCH) uses a channel called AGCH (access grant channel) After the access is granted, the call setup or call forwarding can take place The control channel used for such purposes is called a CCCH 26

27 CCCH When call setup requirements are transmitted from the MS, CCCH called RACH (random access channel) 27

28 RACH Data burst format during 577 µs in place of the (H, user data, S, TR, S, user data, and T) sequence, a 145-bit sequence is modified as 8 H bits, 41 synchronization bits, 36 bits user data and 3 T bits (total 88 bits). The guard-space time intervals are now equal to ( )/2 = 126 µs before H and after T bits 28

29 PCH (paging channel) When call-forwarding information transmits from the BTS, the CCCH is called PCH 29

30 Paging Example Transmission of information to a select target MS for example, the identity of the caller of an incoming call to the MS to which the call is to be forwarded 30

31 AGCH (access grant channel). When access granting information is transmitted from the BTS, the CCCH is called AGCH 31

32 Control Multiframes A control multiframe 51 data frames A control multiframe transmit in ms = ms A traffic multiframe transmits in 120 ms 32

33 Superframe After formatting, a super frame can have 51 traffic multiframes of 120 ms each or 26 control multiframes of ms each A super frame, therefore, transmits in ms or ms, both equal 6.12 s 33

34 Hyperframe Consists of 2048 superframes Transmits in s = s 3½ hours 34

35 Use of sequence number s in Hyperframe s = 0 to ( ) 1 slots in a hyperframe) to each ms data frame s encrypted along with the data so that after decryption, the original frame number can be recovered for sequential arrangement of data 35

36 Use of sequence number in Hyperframe Since the frames sequentially transmitted through the 8 TDMA time slots, the s also helps in identifying the original time slot of a given data burst 36

37 Summary Three type of traffic data frames TCH/FS, TCH/HS and TCH/EFR Control data frame DCCH, SADCCH and FADCCH BCCH CCCH Superframe and hyperframe 37

38 End of Lesson 08 GSM Traffic and Control Data Channels 38