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GSM and Similar Architectures

Lesson 08GSM Traffic and Control Data Channels


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)─ apart of CCCH (common control channel)


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)


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


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 (63 × 3.692 µs period for 63 bits, because each bit is transmitting in a 3.692 µs interval


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


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


Four Types of Control Data Bursts─Dummy burst

• When no useful burst being transmitted from an MS or BTS after a connection setup


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


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


Application of data interleaving

• Introducing noise in idle state to prevent user uneasiness in periods of silence


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)



• 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



• 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


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


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


TCH/HS advantage

• Double voice signals can now be transmitted

• However this sort of voice data results in degradation of voice quality


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


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


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)


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)


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


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)


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


BCCH (broadcast control channel)

• Enables an MS to get an available radio-carrier 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


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


CCCH

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


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 (68.25 × 3.692)/2 = 126 µs before H and after T bits


PCH (paging channel)

• When call-forwarding information transmits from the BTS, the CCCH is called PCH


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


AGCH (access grant channel).

• When access granting information is transmitted from the BTS, the CCCH is called AGCH


Control Multiframes

• A control multiframe 51 data frames• A control multiframe transmit in 51× 4.615

ms = 235.4 ms• A traffic multiframe transmits in 120 ms


Superframe

• After formatting, a super frame can have 51 traffic multiframes of 120 ms each or 26 control multiframes of 235.4 ms each

• A super frame, therefore, transmits in 26 ×235.4 ms or 51 × 120 ms, both equal 6.12 s


Hyperframe

• Consists of 2048 superframes• Transmits in 2048 × 6.12 s = 12533.76 s ≈

3½ hours


Use of sequence number s in Hyperframe

• s = 0 to (2048 × 26 × 51) – 1 slots in a hyperframe) to each 4.615 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


Use of sequence number in Hyperframe

• Since the frames sequentially transmitted through the 8 TDMA time slots, the s alsohelps in identifying the original time slot of a given data burst


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


End of Lesson 08GSM Traffic and Control Data Channels

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GSM Traffic and Control Data Channels GSM and … and Similar Architectures ... used of this burst, a part of BCCH (broadcast ... GSM Traffic and Control Data Channels. Created Date: