GSM Channels

Slide 1 : GSM Channels

Slide 2 : Contents Channel Concepts Call Setup Burst & Multi Frames Mapping of Logical Channels Definitions of SDCCH Call Set up

Slide 3 : Logical Channels

Slide 4 : Broadcast Channels (BCH) Frequency Correction Channel (FCCH) Downlink Channel BTS: Transmits a carrier frequency (Pure sine wave of 67.7 Khz) This Solve 2 purpose : a> Make sure that this is BCCH Carrier b> To allow the MS to synchronize to the frequency MS: After Switch on MS Scan for this channel, since it has no information to which frequency to use. FCCH carrier enables a mobile to tune its frequency to that being broadcast by the BTS.

Slide 5 : Broadcast Channels (BCH) Syncronization Channel (SCH) Downlink Channels BTS: Transmits TDMA Frame number + Base Station Identity Code (BSIC= NCC + BCC ) MS: MS decodes the BSIC if the chosen BTS is GSM Base station within a cell

Slide 6 : Broadcast Channels (BCH) Broadcast Control Channel (BCCH) Downlink Channels BCCH contains the detailed Network and cell specific information such as : Frequency used by Cell and its Neighboring cells. Frequency HSN Paging Groups LAI Max output power allowed in the cell

Slide 7 : Common Control Channels (CCCH) Paging Channel (PCH) Downlink Channels BTS: Broadcast the paging message to indicate the Incoming Calls or Incoming SMS. Paging message also includes the MS’s identity number IMSI/TMSI MS: MS listens to the PCH. If it identifies its own mobile subscriber identity number on the PCH, it will respond.

Slide 8 : Common Control Channels (CCCH) Random Access Channel (RACH) RACH is transmitted Uplink only When mobile is paged , it replies on RACH requesting a signaling channel. RACH can also used if the MS wants to make a contact the NW/ Originating calls

Slide 9 : Common Control Channels (CCCH) Access Grant Channel (AGCH) Downlink channel AGCH is answer to the RACH NW assigns a signaling channel (SDCCH) to the MS. This assignment is performed on the AGCH

Slide 10 : Dedicated Control Channels (DCCH) Stand Alone Dedicated Control Channel (SDCCH) SDCCH is bi-directional Channel System Signaling Call Setup Authentication Location Update Assignment of Traffic channels and Transmission of Short messages

Slide 11 : Dedicated Control Channels (DCCH) Slow Associated Control Channel (SACCH) SACCH is transmitted in both Uplink and Downlink directions SACCH is associated with each SDCCH and also with TCH Uplink : MS Sends the averaged measurement on its own BTS and neighboring BTS’s Downlink: MS receives information regarding information concerning Transmit power to use Instructions on Timing Advance

Slide 12 : Dedicated Control Channels (DCCH) April 25, 2007 Fast Associated Control Channel (FACCH) While Calls in progress and HO is required FACCH is used FACCH works in Stealing mode meaning that one 20ms segment of speech is exchanged for signaling information necessary for the HO Cell Broadcast Channel (CBCH) CBCH is used in Downlink only It is used to carry Short Message Service Cell Broadcast (SMSCB) and uses the same physical channel as the SDCCH

Slide 13 : Traffic Channels (TCH) Traffic channels are Bi-directional logical channel that transfer the user speech or data. Full Rate TCH ( TCH/F) : This channel carries information at a gross rate at 13Kbit/s * * Now it is 22.8 Kbit/s with latest R9.1 Half Rate TCH (TCH/H) : This Channel carried information at a gross rate at 6.5Kbit/s* * Now it is 11.4 Kbit/s with latest R9.1 Enhance Full Rate : The speech coding in EFR is still done at 13Kbit/s, but the coding mechanism is different that is used for normal FR. EFR gives better speech quality at the same bit rate than normal FR.

Slide 14 : Call to an MS More..

Slide 15 : Relationship Between Burst & Frame Burst : Physical content of a TS is called Burst.There are 5 types of Bursts each having 15/26 ms duration and 156.25 Bits. Hyperframe: In GSM system every TDMA frame is assigned a fixed number, which repeats itself in a time period of 3 HOURS 28 MINUTES 53 SECONDS 760 MILLISECONDS. This time period is referred to as Hyperframe. Superframe : =51x26 Multiframes. So, Duration =51x26x8x15/26=6Sec 120ms Multiframe :There are two types of multiframe. 26 TDMA Frame Multiframe :Used to carry TCH, SACCH and FACCH Duration =26 x 8 x 15/26 =120ms 51 TDMA Frame Multiframe : Used to carry BCCH,CCCH,SDCCH and SACCH. Duration =51 x 8 x 15/26 =235.38ms

Slide 16 : TDMA frames with Common and Dedicated Channels

Slide 17 : Bursts Normal Bursts: This burst is used to carry information on : Traffic channel SDCCH Channel Broadcast Control Channel Paging Channel Access Grant Channel SACCH & FACCH Channel

Slide 18 : Bursts Frequency Bursts: All 148 bits(142+6) are coded with 0. The output of GMSK Modulator is a fixed frequency signal exactly 67.7 Khz above the BCCH carrier frequency. Thus the MS on receiving this fixed frequency signal fine tunes to the BCCH frequency and waits for the Sync burst to arrive after 1 TDMA Frame .i.e =15/26*8=4.615ms

Slide 19 : Bursts Synchronization Bursts: This burst is used for time synchronization of the MS 39Bit x 2=78 Bits :Are decoded to arrive 25-SCH control bits and that contains the information of the NCC ,BCC & TDMA FN 64 Bits : Long training seq. of 64 Bits are identical for all BTS

Slide 20 : Bursts Access Bursts: This burst is used only for initial access by the MS to the BTS which applies 2 cases : For connection setup when idle state where a CHAN_REQ message is sent using access burst For HO when MS send HND_ACC message. 36Bit Contains : BSIC+CHAN_REQ or HND_ACC 41Bit Contains : Fixed bit sequence allow BTS to recognize it is Access Burst 86.25 bits GP: Long GP enables BTS to get Propagation delay information.

Slide 21 : Bursts Dummy Bursts: To enable the BCCH frequency to be transmitted with a constant power level, dummy burst are inserted. This burst is transmitted on CHGR=0 when no other type of burst is to be sent. Thus it makes possible for MS to perform the power measurement on the BTS in order to determine which BTS to use for initial access or which to use for HO CCCH is replaced by the dummy page, when there is no paging message to transmit. 58Bits: Coded with pseudo random bit seq. to prevent confusion with Freq correction burst

Slide 22 : Call Setup-Mobile Terminating Call 1>Paging(LAI+IMSI/TMSI) 2>Paging Command Imsi/Tmsi+PG+TRX+CG+TN 3>Paging Req(Imsi on PCH) 4>Channel Req(On RACH) 5>Channel Reqd (Access Delay) 6>Channel Actn (MSPwr,BSPwr,TA) 7>Channel Activation Ack 8>Imm Assign Cmd(On AGCH , Freq +TS+ SDCCH SubChannel No+TA 8>Immediate Assign 9>Estblish Ind (Paging Resp) IMSI+MS Class 9>Conn Req (Paging Resp: BSC add CGI) 10>Auth Req (128 bit RAND+ 3bit CKSN) 10>Auth Req (128 bit RAND+3bitCKSN) 10>Auth Req (128 bit RAND+3bitCKSN) 11>Auth Response (MS Calculate SRES & Kc with its own Ki stored in SIM by appling algorithm A3&A8) 11>Auth Response (SRES) 11>Auth Response (SRES) SABM (Paging Resp:IMSI/MS Class) UA(Paging Resp) Unnumbered Ack Frame which confirms only 1 MS is using Sig Channel PCH RACH AGCH SDCCH SDCCH Next..

Slide 23 : Call Setup-Mobile Terminating Call 14>Setup (Req for Services I.e. Speech/Data/Fax etc) 15>Call Confirmed 17>Channel Activation (BSC Allocated Idle TS for Traffic) 20>Assign Comp (MS tune to TCH and send Ind that Chan is Seized) 14>Setup 14>Setup 15>Call Confirmed 15>Call Confirmed 16>Assignment Req (MSC send CIC to BSC) 18>Channel Activation Ack 19>Assignment Cmnd (BSC send message on SDCCH to MS telling to go TCH) 19>Assignment Cmnd (BSC send message on SDCCH to MS telling to go TCH) 20>Assign Comp (MS tune to TCH and send Ind that Chan is Seized) 20>Assign Comp (MS tune to TCH and send Ind that Chan is Seized) 21>RF Chann Realease 21>RF Chann Realease Ack 22>Alert (MS Send Alert to MSC as soon as the ringing is started in MS) 22>Alert (MS Send Alert to MSC as soon as the ringing is started in MS) 22>Alert (MS Send Alert to MSC as soon as the ringing is started in MS) 23>Connect (When MS Sub Answer the Conn message sent to MSC) 23>Connect (When MS Sub Answer the Conn message sent to MSC) 23>Connect (When MS Sub Answer the Conn message sent to MSC) 24>Connect Ack 24>Connect Ack 24>Connect Ack SDCCH SDCCH SDCCH TCH TCH TCH TCH Exit..

Slide 24 : Thank you Know More - SDCCH