Concepts of Channels in GSM


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Concepts of Channels in GSM:

Concepts of Channels in GSM A company vehicle is used for several purposes in a day Similarly in GSM, the timeslots are used for different purposes at different times

Frames and Multiframes:

Frames and Multiframes 0 6 5 4 3 2 1 7 3 Data 1 Midamble 1 Data 3 8.25 bits 156.25 bits 576.92 micro sec 4.615 ms Time Slot Frame 0 50 0 25 Control Channel Multiframe Traffic Channel Multiframe

Channels in GSM Air interface:

Channels in GSM Air interface Logical Channels : ARFCN –x, Timeslot – y, Frame Number - N Logical Channels Control Channels Traffic Channels

Control Channels:

Control Channels Control Channels BCH – Broadcast Control Channel CCCH – Common control Channels DCCH – Dedicated control Channels FCH BCCH SCH PCH RACH AGCH SDCCH FACCH SACCH

Traffic Channels:

Traffic Channels Traffic Channels TCH - F TCH - H

BCH (Broadcast Channel):

BCH (Broadcast Channel) One ARFCN, On all the time, in every cell Uses Timeslot 0 on a channel, in Downlink FCH – Frequency correction channl (allows mobile to tune to BTS frequency. This channel carries a 142 bit zero sequence and repeats once in every 10 frames on BCH) SCH – Synchronization channel (carries the Frame number and BSIC). A midamble of 64 bit helps mobiles to synchronize. It also repeats once in 10 Frames

BCH (Broadcast Channel):

BCCH – Broadcast Control CHannel. It has information encoded on it which identifies the network (LAC). It also carries list of the channels in use in the cell (CA tables) BCH (Broadcast Channel)

Common Control Channel:

Common Control Channel CCCH shares Timeslot 0 with BCH on a Multiframe CCCH consists of PCH, RACH and AGCH PCH – Paging channel is used to alert mobiles on incoming calls. PCH carries IMSI to page for Mobiles in the cell. PCH is Downlink channel. RACH – Random Access Channel is a short burst sent by mobile to BTS, to initiate a call request. RACH uses timeslot 0 on reverse BCH channel on uplink

RACH – Random Access Channel:

RACH – Random Access Channel 8 start bits 41 Synchronization bits 36 Encrypted bits 3 stop bits 68.25 Extended Guard Period 88 bits Used by mobile to get attention from Base Station Several mobiles might originate RACH simultaneously Mobile doesn’t know path delay So RACH has to be a special short burst Mobile sends normal burst after getting timing advance on downlink SACCH

DCCH – Dedicated Control Channels:

DCCH – Dedicated Control Channels SDCCH – Standalone Dedicated Common Control Channel. It is used as a interim channel before final assignment of TCH. SDCCH is used for signaling and Authentication message transfers. It can be called the stepping stone between BSC and TCH The SDCCH, by using less of the cells resource of physical channels, improves efficiency, and provides a useful holding channel for the mobile until speech data needs to be exchanged.

Dedicated Control Channels (cont..):

Dedicated Control Channels (cont..) FACCH – Fast Associated Control Channel Interrupts TCH on uplink and downlink Rapid message exchange for handovers Control bits on either side of midamble – 0 indicates TCH and 1 indicates FACCH Bothways – uplink and downlink

SACCH – Slow associated Control Channel:

SACCH – Slow associated Control Channel Downlink (BTS – MS) Mobile Tx Power Commands Mobile Timing Advance Cell’s Channel Configuration Uplink (MS - BTS) Received signal quality report (RXQual) Received signal level report (RXLev) Adjacent BCH power measurement

Common Channel Downlink:

Common Channel Downlink F A A A A B B B B S P P P P P P P P S F D D D D S F D D D D D S F D D D H H H H S F D D D D I H H H H H Time Slots 0 1 3 2 0 1 3 2 Frame Numbers 0 3 2 1 25 26 50 F – FCH S – SCH B – BCCH A – AGCH P – PCH D – SDCCH H – SACCH

Common Channel Uplink:

Common Channel Uplink D A A A A R R D D D R R R R R R A A A A R R R R R R R R R R R R R R R R R D D D D D D D D R I D D D D R Time Slots 0 1 3 2 0 1 3 2 Frame Numbers 0 3 2 1 25 26 50 R – RACH D- SDCCH A- SACCH

TCH – Traffic Channel:

TCH – Traffic Channel Traffic Channel carries the voice data Two blocks of 57 bits contain voice data One TCH is allocated for every active call. While call is in progress if there is degradation in quality of current channel, BTS may shift the communication to another TCH on a different Carrier and/or Timeslot A full rate TCH carries 13 kbps voice data, and Half rate TCH carries a 6.5 kbps voice data 3 Data 1 Midamble 1 Data 3 8.25 bits

TCH Multiframe:

TCH Multiframe T T T T T T T T T T T T T T T T T A T T - T T T T T Frame Numbers 0 3 2 1 25 26 Frames – 120 ms 24 carry speech, 1 idle, 1 SACCH

Framing Structure:

Framing Structure 1 Frame = 8 bursts = 8*0.57692=4.616 ms 1 Control Ch. Multiframe = 51 TF = 234.6 ms 1 TCH Multiframe = 26 TF = 120 ms 1 Superframe = 51 * 26 CCH MF = 1326 TF = 6.12 sec 1 Superframe = 26 * 51 TCH MF = 1326 TF = 6.12 sec 1 Hyperframe = 2048 superframes = 2715648 TDMA frames = 3 hrs. 28 mins. 760 ms.

Framing Structure:

Framing Structure Multiframe is used for distribution of logical channels Superframe is used for Mobile synchronization Hyperframe is used for Signalling procedures and Ciphering

Mobile Maximum Range:

Mobile Maximum Range Range = (Timing advance * bit period * velocity) / 2 Range = (63 * (3.693 * 10e-6) * (3 * 10e5)) /2 So range is 34.9 Kms.

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