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Know More - SDCCH




Nov 17, 2003        S. Rajshekhar Deshraj
Contents

• Channel Concepts

• Call Setup

• Burst & Multi Frames

• Mapping of Logical Channels

• Definitions of SDCCH

• Usage of SDCCH in the GSM network (BSC)

• Possible SDCCH Configuration

• SDCCH Holding Time

   Nov 17, 2003          S. Rajshekhar Deshraj
Contents

• SDCCH Traffic Estimations

• SDCCH Congestion

• Reasons for SDCCH congestion

• How to detect SDCCH congestions

• Preventive actions to avoid SDCCH congestions

• SDCCH Dimensioning

• Parameters for SDCCH Dimensioning

• Counters & Report Analysis

   Nov 17, 2003         S. Rajshekhar Deshraj
Logical Channels




Nov 17, 2003      S. Rajshekhar Deshraj
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.




     Nov 17, 2003
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




  Nov 17, 2003
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




    Nov 17, 2003
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.




   Nov 17, 2003
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




   Nov 17, 2003
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




   Nov 17, 2003
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

 Nov 17, 2003
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



 Nov 17, 2003
Dedicated Control Channels (DCCH)

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




 Nov 17, 2003
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.


  Nov 17, 2003
Call to an MS




                                         More..

Nov 17, 2003     S. Rajshekhar Deshraj
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
         Nov 17, 2003                 S. Rajshekhar Deshraj
TDMA frames with Common and Dedicated Channels




    Nov 17, 2003
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
            1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )

         TB Encrypted bits       Training Sequence       Encrypted bits TB     GP
                             F                     F
          3      57                      26                   57         3     8.25

         F : One Stealing Bit:=0 Indicates 57bit packet contains user data or speech
                             :=1 Indicates burst stolen for FACCH Signalling

            RxQual derived from the 26 bit midable from the TDMA frame



       Nov 17, 2003
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
            1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )

       TB                                                             TB   GP
                              ALL ZERO 142 BITS
        3                                                              3   8.25




       Nov 17, 2003
Bursts

Synchronization Bursts:
This burst is used for time synchronization of the MS


         1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )

    TB      SCH DATA         Extended Training Sequence       SCH DATA   TB   GP
     3       39 Bits                  64 Bits                  39 Bits    3   8.25


    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




         Nov 17, 2003
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.


        1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )

   TB     SCH Sequence                 RACH Data              TB    Guard Band
    8        41 Bits                    36 Bits                3    68.25 Bits
        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.


        Nov 17, 2003
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.
             1 Time slot = 156.25 bits durations (15/26 = 0.577 ms )

        TB       Mixed Bits            Training Sequence          Mixed Bits    TB Guard Band
         3          58                         26                    58          3    8.25

         58Bits: Coded with pseudo random bit seq. to prevent confusion with Freq correction burst




       Nov 17, 2003
Mapping of Logical Channel
Method of transmitting logical channels onto physical channel is called Mapping


FCCH+SCH+BCCH+CCCH: An idle MS search for the FCH. When MS finds
the frequency correction burst it knows that this is TS 0 on CHGR=0
The cycle means F,S,B repeats after the Idle frame I.e. at Frame no 50. Cycle=51TS
       TS=0 / CHGR=0       DOWNLINK

        0   1          7      0      1           7      0     1        7




       F0 F1 F2-F5 F6-9      F10    F11 F12-F15 F16-19 F20-23 F24 F25 F26-29 F30 F31 F32-F35 F36-F39 F40 F41 F42-F45 F46-49 F50
  TS-0 F S BCCH CCCH          F      S   BCCH CCCH CCCH F          S CCCH F       S   CCCH CCCH F         S   CCCH CCCH      I


            F: FCCH 1 TS ( Use 4 Slots in each 51 TDMA Frame)
            S: SCH 1 TS ( Use 4 Slots in each 51 TDMA Frame)
            I: IDLE 1 TS
               BCCH 4 TS
               CCCH 4 TS (PCH or AGCH) Paging Block
                         51 TDMA Frame = 9 Paging Blocks

       TS=0 / CHGR=0       UPLINK

       F0 F1    F3    F3     F4     F5    F6     F7     F8   F9 F10                                 F46 F47    F48    F49   F50
  TS-0 R    R    R     R      R      R    R      R      R     R   R                                  R   R      R      R     R


            R: RACH 1 TS


            Nov 17, 2003
Mapping of Logical Channel

SDCCH+SACCH: Cycle=102 TS This sequence is repeated after last idle frame.
The Uplink & Downlink pattern are time shifted, so SDCCH sub channel is sent in frame 0-3 on downlink
and in frame 15-18 on uplink. The reason for this is to achieve efficient communication, by giving MS time to
calculate its answer to the request received on down link SDCCH
         TS=2 / CHGR=0                                     DOWNLINK

     1         2         3                         1         2         3                  1    2         3              7




          F0-F3      F4-F7      F8-F11       F12-F15       F16-F19   F20-F23 F24-F27 F28-F31 F32-F35 F36-F39 F40-F43 F44-F47 F48 F49 F50
         SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH0 SACCH1 SACCH2 SACCH3                                  I   I   I
         SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH4 SACCH5 SACCH6 SACCH7                                  I   I   I

         TS=2 / CHGR=0                       UPLINK
                         I:     IDLE   1 TS
     1     2        3
                                SDCCH 4 TS
                                     1    2            3               1      2       3              7
                                SACCH 4 TS



          F0-F3    F4-F7      F8-F11   F12 F13 F14 F15-F18 F19-F22 F23-F26 F27-F30 F31-F34 F35-F38 F39-F42 F43-F46 F47-F50
         SACCH5 SACCH6 SACCH7            I     I       I   SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH0
         SACCH1 SACCH2 SACCH3            I     I       I   SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH4


                    I:        IDLE  1 TS
                              SDCCH 4 TS
                              SACCH 4 TS



          Nov 17, 2003
Mapping of Logical Channel
TCH+SACCH: 0&2 used by control channels. This leaves TS-1and 3-7 free for the
use by TCHs.
Repetition time (Cycle) =26 Frames .i.e. =26*8*(15/26)=120ms
Every 13th TS contains SACCH. Downlink contains TA value and Uplink contains
measuring report.



       TS=1 / CHGR=0 DOWNLINK

   0    1   2         7    0   1       0   1   2       7




       F0   F1   F2   F3   F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26
       T    T    T    T    T   T   T   T   T   T   T   T   A   T   T   T   T   T   T   T   T   T   T   T   T   T   I


            T: TCH   1 TS
            A: SACCH




        Nov 17, 2003
Usage of SDCCH

The SDCCH are used in some different ways in the GSM network:


• Registrations: Periodic Location Updates, IMSI Attach/Detach
• Call Setup: Immediate Assignment -> Assignment.
• SMS point-to-point: SMS messages to/from MS in Idle mode.
• Fax Setup
• Optional: USSD (Unstructured Supplementary Service Data) data
              transfer. MS<->Network. Similar to SMS. Controlled by the
              MSC.




     Nov 17, 2003
Possible Configurations
SDCCH/8 : 8 Sub channels for signaling are mapped on 1 TS, this results 1 less TCH
for the cell.
SDCCH/4 (Combining BCCH and SDCCH) :4 Sub channels for signaling are mapped
on BCCH TS. As a result Paging capacity on BCCH is reduced by 1/3.
CBCH : If CBCH is active 1 sub channel of SDCCH is replaced by 1 CBCH channel.


Limitations : SDCCH/8 in a cell = Number of TRX’s
                  CONFIGURATIONS              HYD007A          HYD006A
                                               1 TRX            2 TRX
                                          SDCCH      TCH   SDCCH      TCH
         SDCCH/4                            4         7      4         15
         SDCCH/4+CBCH                       3         7      3         15
         SDCCH/8                            8         6      8         14
         SDCCH/8+CBCH                       7         6      7         14
         SDCCH/4+SDCCH/8                    12        6      12        14
         SDCCH/4+SDCCH/8+CBCH               11        6      11        14
         SDCCH/4+SDCCH/8+SDCCH/8            X         X      20        13
         SDCCH/4+SDCCH/8+SDCCH/8+CBCH       X         X      19        13


       Nov 17, 2003
Possible Configurations -BCCH COMBC+CBCH

                CELL      BCCH        CBCH        SDCCH       NOOFTCH
                HYD007A   1           1           3            7




                CHGR      BPC    CHANNEL      CHRATE SPV     STATE ICMBAND
                CHBAND
                0         4590 SDCCH-28646            BUSY
                                SDCCH-28645           IDLE
                                CBCH-34305            BUSY
                                 SDCCH-28644          IDLE
                                 BCCH-34797           BUSY
                          4496 TCH-4629    FR   1,2   IDLE
                          4497 TCH-4630    FR   1,2   IDLE
                          4499 TCH-4631    FR   1,2   IDLE
                          4505 TCH-4654    FR   1,2   IDLE
                          4506 TCH-4655    FR   1,2   IDLE
                          4509 TCH-4661    FR   1,2   IDLE
                          4575 TCH-4670    FR   1,2   IDLE




 Nov 17, 2003
Possible Configurations -BCCH NCOMB+CBCH
                CELL            BCCH            CBCH                SDCCH          NOOFTCH
                HYD006A         1               1                   7              14
                CHGR   BPC     CHANNEL          CHRATE SPV STATE ICMBAND CHBAND 64K
                0      6899 SDCCH-27810                         IDLE
                              SDCCH-27809                       IDLE
                              CBCH-34619                        BUSY
                              SDCCH-27808                       IDLE
                              SDCCH-27807                       IDLE
                              SDCCH-27806                       IDLE
                              SDCCH-27805                       IDLE
                              SDCCH-27804                       IDLE
                       6900   BCCH-34804                        BUSY
                       6887   TCH-1242          FR     1,2   IDLE           NONE
                       6888   TCH-1243          FR   1,2     IDLE           NONE
                       6891   TCH-1246          FR   1,2     IDLE           NONE
                       6893 TCH-1248            FR   1,2     IDLE           NONE
                       6895 TCH-1250            FR   1,2     BUSY           NONE
                       6897 TCH-1252            FR   1,2     IDLE           NONE
                CHGR   BPC     CHANNEL      CHRATE SPV STATE ICMBAND CHBAND 64K
                1   6885 TCH-1240               FR   1,2     BUSY           NONE
                    6886 TCH-1241               FR   1,2     IDLE           NONE
                    6889 TCH-1244               FR   1,2     IDLE           NONE
                    6890 TCH-1245               FR   1,2     IDLE           NONE
                    6892 TCH-1247               FR   1,2     IDLE           NONE
                    6894 TCH-1249               FR   1,2     BUSY           NONE
                    6896 TCH-1251               FR   1,2     IDLE           NONE
                    6898 TCH-1253               FR   1,2     IDLE           NONE


 Nov 17, 2003
SDCCH Congestion

SDCCH congestion: All SDCCH in a cell are at the same time busy for a
period of time which leads to rejection for new MS.

Reasons :
• Hanging SDCCH: Are SDCCH that are hanging busy and unusable for a
 long time (many minutes or indefinite).
 Hanging SDCCH are generally caused by SW faults.

• Heavily used SDCCH: SDCCH seen as continuously busy even though
 they are used by different MS and thus carries traffic.
    It may cause due to :
    > Extreme end-user behaviors: Sport event ending, festivals or celebrations.
    > Increased mean hold time of the SDCCH: Mean hold time increase from
       2 to 10 seconds can give SDCCH congestion.



     Nov 17, 2003
SDCCH Holding Time
Normal Location Updating = 3.5 Sec
Periodic Registration   = 3.5 Sec
IMSI Attach             = 3.5 Sec
IMSI Detach             = 2.9 Sec (IMSI detach Indication message sent to
                          NW, no authentication is performed (which
                          normally takes 0.6Sec) & no ack is sent to MS.)

Call Setup              = 2.7 Sec (MOC)
                        = 2.9 Sec (MTC)
Short Message Service(SMS) = 6.2 Sec (Vary depending the length of SMS)

Fax Transmission        = 2.7 Sec (MOC)
                        = 2.9 Sec (MTC)
False Access            = 1.8 Sec (when Channel req is rec’d by system ,as SDCCH
                          is allocated by sending Imm Ass message, and the system
                          waits a certain time before performing disconnection.)
      Nov 17, 2003
SDCCH Traffic Estimations

Normal Location Update:           Mean Holding time         =3.5 Sec
                                  No of Location update     =1.0 per sub and BH

AC: Avg No of Location updates             =1000X(1 X 3.5)/3600=0.972 mE/Sub
IC : No Location updating at all           =1000X(0 X 3.5)/3600=0 mE/Sub
BC: 3 times the Avg no of Location updates =1000X(3 X 3.5)/3600=2.916mE/Sub

SMS:                              Mean Holding time         =6.2 Sec
                                  No of SMS submitted       =1.0 per Sub and BH

Traffic : 1000 X (1.0 X 6.2) / 3600 =1.7 mE/sub

Call Setup:       Mean Holding time        =2.7 Sec(MOC) & 2.9 Sec(MTC)
                  Mob Originating Traffic (Incl B Ans)    =0.8 BHCA
                  Mob Terminating Traffic (Incl B Ans)    =0.4 BHCA

Traffic : 1000 X {(0.8 X 2.7)+(0.4 X 2.9)} / 3600 =0.9 mE/sub

       Nov 17, 2003
Reasons for SDCCH congestion

Increased mean hold time of SDCCH can be caused by:

• Too low signal strength at access to the system
    Due to LSS MS will be lost communication with the system, This will lead to timeout in the BSC
    (RLINKT ), and thus the SDCCH is used until timeout. Increase ACCMIN.

• Congestion of TCH/TRA/RALT
   Will increase CP execution time before rejecting Imm Ass. Minor increase in mean hold time expected.


• C7 problems to MSC (or TRC)
    In case of C7 signalling problems (SCCP) towards MSC (and/or TRC) can lead to timeout on SCCP
    connection setup. This will lead to more than 2 minutes hold time for SDCCH.

• Handover on SDCCH
    Handover on SDCCH will in itself increase mean hold time on SDCCH. However minor increase is
    expected.



       Nov 17, 2003
Reasons for SDCCH congestion


• Congestion on Air-interface
    Congestion on Air-interface leads to delay in communication to the MS. Can give timeout in BSC
    during Imm Ass.Increases SDCCH mean hold time with more than 2 seconds.


• Congestion on Abis (LAPD link)
    Congestion on Abis leads to delay in communication with BTS and MS. Can give timeout in BSC
    during channel activation (TCHACTIVE). Increase SDCCH mean hold time with more than 5 seconds..


• Congestion on A-interface
    Congestion on the A-interface will lead to increased mean hold time on SDCCH. Increase is unknown.


• High load in MSC/VLR or HLR
    High load in MSC/VLR and/or HLR will lead to increased mean hold time on SDCCH. Increase is
    unknown.




       Nov 17, 2003
How to detect SDCCH congestion in the BSC
There is no good way to detect SDCCH congestion in real time in the
BSC!


A number of symptoms to look for:
• Increased CP Load.
• Decreased usage of TRA devices.
• Location Updates fails => Mobile terminating calls does not reach the subscriber.
• Subscriber complaints. Can not access the system.
• STS counters . Object type: CLSDCCH and CLSDCCHO.
• Seizure Supervision of LCHs (but only for Hanging SDCCHs!)



       Nov 17, 2003
Preventive actions to avoid SDCCH congestion
• Avoid combined BCCH in cells with many SDCCHs

• Use the optional feature Adaptive Configuration of SDCCHs (ACLC)
• Use USSD (Unstructured Supplementary Service data) with care, can cause long
 meanhold time on SDCCH.
• Avoid Handover on SDCCH
• Proper Dimensioning of the TCHs and TRA devices in the BSC.
• Use recommended values for Periodic Location Updates
    Avoid unnecessary Periodic Location Updates :
    BSC: T3212 (RLSBC)           = 40 (4 hours)
    MSC: BTDM (MGIDI)            = 240 (4 hours)
    MSC: GTDM (MGIDI)            = 6 (6 minutes)
• Use Immediate Assignment on TCH.
• Increases the no of SDCCH in a Cell where SDCCH load is high


       Nov 17, 2003
SDCCH Dimensioning
SDCCH Dimensioning is a compromise between SDCCH blocking
rate and TCH Capacity. In order to have a successful call setup
there has to be an available SDCCH as well as available TCH.

Basic SDCCH configuration:

   It is recommended to choose
   1 SDCCH/8 as the basic configuration for all the cells,
   If LA> 2100 Erlang (500 TRX)

   1 SDCCH/4 as the basic configuration for all the cells,
   If LA< 2100 Erlang (500 TRX)




     Nov 17, 2003
SDCCH Dimensioning
Automatic SDCCH dimensioning:
This can be done with optional Adaptive Configuration of Logical
Channel feature.
This feature will add extra SDCCH/8 by reconfiguring idle TCH
when SDCCH load is high, and revert back to TCH if SDCCH load
goes down.

Manual SDCCH Dimensioning :
•Monitoring SDCCH / TCH Traffic in a cell
•SDCCH/TCH load ratio
•SDCCH Grade of Service:- Max allowed TCH GOS % = 2 %
 The rule of Thumb says:
 SDCCH/4 : Max. SDCCH GOS =1/2 * 2= 1 %
 SDCCH/8 : Max. SDCCH GOS =1/4 * 2= 0.5%


     Nov 17, 2003
SDCCH Dimensioning
Immediate Assignment on TCH:
In case of this feature on the channel administration assigns TCH for
signaling instead of an SDCCH, based on 7 different channels
allocation strategies (CHAPs)

TCH first strategy :
      Decreases the SDCCH load and enable to use SDCCH/4 in all

     the cells Traffic load on TCH will in this case increases
     substantially so this strategy is not recommended.
SDCCH first strategy :
     SDCCH is always allocated first if available, otherwise
     signaling is performed on TCH.



      Nov 17, 2003
SDCCH Dimensioning
Example :
ASSUME: CELL=3TRX ,SDCCH Traffic =4 Erlang, Bcchtype=NCOMB.No CBCH
GOS: SDCCH/4 = 1%, SDCCH/8 =0.5% & TCH % =2%


1> When Immediate Assignment on TCH not used
Erlang B Table :To maintain 0.5 % GOS & 4 Erlang min 13 Subchannels reqd
2SDCCH/8 => TCH=(3TRX*8TS) –(1BCCH)-(2SDCCH/8)=21TCH’s
21TCH’s with 2 % GOS => 14.04 Erlang


2> When Immediate Assignment on TCH used
If we use 1SDCCH/8 only than 4 Erlangs with 8 sub channels GOS=>3 %
Congested Erlang will be =4 Erlang x 3 %=0.12Erlang
1SDCCH/8 => TCH=(3TRX*8TS) –(1BCCH)-(1SDCH/8)=22TCH’s
22TCH’s with 2 % GOS => 14.9 Erlang
Actual TCH Capacity=14.9 – 0.12 = 14.7 Erlang

TCH Capacity in the cell is increased with 0.7 Erlang
       Nov 17, 2003
SDCCH Dimensioning
Immediate Assignment on TCH not used




     Nov 17, 2003
SDCCH Dimensioning
Recommended SDCCH Configurations




    Nov 17, 2003
SDCCH Dimensioning
HALF RATE CAPACITY CALCULATION
Half rate will affect SDCCH dimensioning since more signaling will be
req’d when no of TCH is increased.
Important dimensioning factor is therefore the Half rate penetration
.i.e.the % of Half rate mobile in a NW.
Example: 2 TRX Cell, Half Rate Penetration =10 %, 1 SDCCH/8

TCH/F =14 support 14 Subscriber
Penetration 10% of 14 is 1.4 = 2 Subscribers (Req’d 2 Half Rate TCH/H)


Total 13 TCH/F+2TCH/H = 15 TCH required
Capacity :
Erlang B Table: 15 TCH @ 2% GOS => 9.0096 Erlang


     Nov 17, 2003
SDCCH Dimensioning
Max Allowed SDCCH/TCH Load Ratio (Half Rate)


                                               AC : Average Cells
                                               BC : Border Cells
                                               IC : Inner Cells




     Nov 17, 2003
SDCCH Dimensioning
Dimensioning based on STS data
More accurate dimensioning is achieved by using cell statistics .i.e. STS
counters

   •SDCCH Traffic
   •TCH Traffic
   •SDCCH/TCH Load Ratio %
   •SDCCH Congestion
   •TCH Congestion
   •SDCCH Mean Holding Time
   •Availability of SDCCH Channels
   •Availability of TCH Channels




      Nov 17, 2003
Parameters
MFRMS :This parameter defines period of transmission for PAGING REQUEST messages to the same
paging subgroup.

T3212 : Is the periodic registration timer

ACCSTATE : Activate/Deactivate the Adaptive config of logical channel feature per cell basis

CHAP :Selecting Channel Allocation profile per cell.

SLEVEL: The attempt to increase the number of SDCCH/8 will take place when allocation of an
SDCCH has failed due to congestion.

STIME :Parameter STIME determines how long the system waits before the number of SDCCH/8
added by this function is decreased when the demand for signalling channels has returned to a low
level

BTDM :Implicit detach supervision should be equal or longer than T3212 in BSC.

GTDM : is an extra Gurad time in minutes before the subscriber is set to detach.

RLINKT: Radio link time-out This parameter defines the time before an MS disconnects a call due to failure
in decoding SACCH messages. The parameter is given as number of SACCH periods (480ms).


         Nov 17, 2003
Parameters for SDCCH Dimensioning




Nov 17, 2003
STS Counters
CCALLS             : Call attempt counter
CCONGS             : Congestion counter
CTRALACC : Traffic level accumulator.
CNSCAN             : Number of accumulations of SDCCH traffic level


                    counter.
CNDROP             : Dropped connections due to Failure.
CNUCHCNT : Number of defined channels.
CAVAACC            : Available channels accumulator.
CMSESTAB : Successful MS channel establishment on SDCCHs.
CTCONGS            : SDCCH congestion time ( Sec)
    Nov 17, 2003
Formulas
No. of SDCCH Attempts       CCALLS


SDCCH Congestion (%)        100 X { CCONGS / CCALLS }


No. of SDCCH Connections    CMSESTAB


SDCCH Establishments No     100 X {CMSESTAB / (CCALLS - CCONGS)}
Congestion (%)

SDCCH Time Congestion (%)   100 X { CTCONGS / (RPL * 60)}


SDCCH Drop (%)              100 X { CNDROP / CMSESTAB }

SDCCH Mean Holding Time     RPL X 60 X {(CTRALACC/CNSCAN) / CMSESTAB }
(Sec.)

SDCCH Traffic (Erlang)      CTRALACC / CNSCAN


SDCCH/TCH Ratio             (CTRALACC/CNSCAN) / ((TFTRALACC/TFNSCAN)+
                            (THTRALACC/THNSCAN))



  Nov 17, 2003
Quality Report : Call Setup Analysis Report
     Measuring Period : 03 Nov 2003 to 09 Nov 2003 (7Days)
                      (Weekly Avg Stats)                                                                                                            W-6
                                         QUALITY REPORT :Call Setup Analysis Report ( W-6)                                                                               23.09.03
                  S DRP(HYD)              S DRP (ROAP)           SCSR %(HYD)        SCSR % (ROAP)              CONR% (HYD)                   CONR% (ROAP)                   ALL
                                                                                                                                                                       SDRP>1%
                                                                                                                                                                       SER <96%
                                                                                                                                                                       CONR >0.5 %
15Sep-21Sep              134   Cells              275   Cells        175   Cells        357   Cells            68   Cells                   109    Cells                   72Cells
22Sep-29Sep              127   Cells              295   Cells        192   Cells        378   Cells            98   Cells                   174    Cells                  116Cells
29Sep-5Oct               112   Cells              309   Cells        187   Cells        400   Cells            71   Cells                   135    Cells                   97Cells
06Oct-12Oct               25   Cells              154   Cells        178   Cells        382   Cells            55   Cells                   109    Cells                   81Cells
03Nov-09Nov               89   Cells              295   Cells        206   Cells        372   Cells            59   Cells                     78   Cells                   60Cells
   Sr No       CELL     NOD    >1%      CELL     NOD    >1%      CELL      < 96 %    CELL     < 96 %    CELL        >0.5% NOC    CT      CELL      >0.5% NOC      CT    Worst Cells
                                                                                                                                 (S)                              (S)
     1        HYD001B    633    1.08   ADO001B    109    1.52   HYD012C     73.35   KOL001B    65.95   HYD201A       3.31 3081   1952   HIN002C     2.37    90    326 NIZ001A
     2        HYD004B    305    1.03   ADO002B     54    1.33   HYD117C     78.45   JGP001C    72.07   HYD176A       3.19 2862   8177   KAD001B     2.35   602   1989 NIZ001B
     3        HYD005A    359    1.54   AKI001A     29    1.04   HYD155B     81.57   HIN002C    73.01   HYD176C       2.98 1695   4397   GUN009B     2.19   641   2410 CHL001A
     4        HYD007B     79    1.34   AKI001B     21    1.04   HYD161B     82.57   RAJ001B    73.06   HYD176B        2.3  942   3095   VIZ027C     1.91   267    638 BHI001A
     5        HYD012C    432    6.51   AKI001C     37    2.15   HYD168A     83.77   VIZ020C    73.67   HYD137C       1.94  559   1423   KAD001C     1.72   392   1702 MAH002B
     6        HYD018A    433    1.18   AMA001A      3    1.54   HYD058A     83.99   JGP001A     75.2   HYD168A       1.75  207    656   HIN001B     1.59   183    664 KHA001B
     7        HYD018B    653    1.13   AMA001A     82    2.66   HYD124B        84   VJA018B    75.81   HYD005B        1.6  725   2465   ANA001B     1.42   275   1084 TEN002C
     8        HYD020B    298    1.02   AMA001B     37    1.08   HYD146B     86.41   CHI001C    77.68   HYD155A       1.39  152    403   TEN002C      1.4   372   1434 TIR008A
     9        HYD025C    481    1.28   AMA001C      5    2.94   HYD012A     86.52   EMG001C    78.01   HYD161B       1.36  138    262   KAD001A     1.33   268   1127 AKI001A
    10        HYD026C    653     1.7   AMA001C     72    2.94   HYD093B     86.86   RAZ001B    78.98   HYD137B        1.3  257    993   VIZ023C     1.32   183    790 BPL001B
    11        HYD028A    413    1.03   ANK001B    144    1.11   HYD170B        87   NID001C    79.41   HYD054B       1.28  386   1654   ADO001C     1.32   190    860 TVR001C
    12        HYD028C    601    1.23   ANN001A     14    3.29   HYD155A     87.29   CHL004C    79.47   HYD170B       1.27  139    375   GOD001B     1.31   372   1099 DHA001C
    13        HYD030C    373    1.26   ANN001B     30    2.15   HYD161C     87.59   RJM007B    80.13   HYD171B       1.23  323   1219   SUL001A      1.3    71    169 MAH002A
    14        HYD031C    581    1.35   ANN001C     96    5.64   HYD089B     87.82   VJA020A    81.21   HYD168C       1.22   82    320   MAH002A     1.23   104    390 HIN001B
    15        HYD038A    327    1.11   ANP001A     18    1.06   HYD011B     87.85   ONG004B    81.78   HYD137A        1.1  129    346   STP001B     1.05   374   1865 BPL001C
    16        HYD040A    279    1.07   ANP001B     29    2.19   HYD121B     87.97   ANN001A    81.92   HYD177B       1.05  206    791   VSP001C     1.03   135    270 TAD001C
    17        HYD040B    522     1.3   ANP001C     20     1.7   HYD146C     88.01   CHI003B    82.55   HYD142A       1.02  147    701   STP001A     1.01   329   1542 CHL002C
    18        HYD042A    505    1.14   AVA001A     49    1.95   HYD145B     88.23   VSP001B     82.8   HYD053C       1.01  161    496   TVR001C     1.01   258    924 MAC001A
    19        HYD045B   1632    1.28   AVA001B     26    1.13   HYD108C     88.49   ANN001C    83.19   HYD161C          1   95    147   RJM008B     0.98    15      65 DHA001B
    20        HYD051C    229    1.01   BHI001A    258    1.45   HYD101C     88.71   VJA019B    83.22   HYD153C       0.99  191    541   SAN001B     0.98   134    318 ADO001B


                              LEGENDS                                      NOC         SDCCH Congestion Counter
               SDRP           SDCCH Drop Rate %                            CT(S)      Cong. Time in Secs (Week Tot)
                NOD        Week Total Dropped SDCCH's                      ALL            SDCCH Drop Rate %
               SCSR        SDCCH Call Success Rate %                                  SDCCH Estab Success Rate %
               CONR         SDCCH Congestion rate %                                     SDCCH Congestion rate %




              Nov 17, 2003
Know More - SDCCH




                   Thank you




Nov 17, 2003        S. Rajshekhar Deshraj
Call Setup-Mobile Terminating Call
    MS                                 BTS                                   BSC                             MSC
                                                                                   1>Paging(LAI+IMSI/TMSI)
  PCH                                         2>Paging Command
        3>Paging Req(Imsi on PCH)             Imsi/Tmsi+PG+TRX+CG+TN

RACH
        4>Channel Req(On RACH)               5>Channel Reqd (Access Delay)

                                               6>Channel Actn (MSPwr,BSPwr,TA)

                                             7>Channel Activation Ack
AGCH
                                              8>Imm Assign Cmd(On AGCH , Freq
           8>Immediate Assign
                                              +TS+ SDCCH SubChannel No+TA
         SABM (Paging Resp:IMSI/MS Class)

         UA(Paging Resp) Unnumbered Ack      9>Estblish Ind (Paging Resp)          9>Conn Req (Paging Resp: BSC
         Frame which confirms only 1 MS is   IMSI+MS Class                         add CGI)
         using Sig Channel
SDCCH                                                                              10>Auth Req (128 bit RAND+
        10>Auth Req (128 bit                 10>Auth Req (128 bit RAND+3bitCKSN)      3bit CKSN)
        RAND+3bitCKSN)

SDCCH
        11>Auth Response (MS Calculate      11>Auth Response (SRES)
        SRES & Kc with its own Ki stored in                                        11>Auth Response (SRES)
        SIM by appling algorithm A3&A8)
                                                                                                         Next..

    Nov 17, 2003
Call Setup-Mobile Terminating Call
      MS                                     BTS                                        BSC                              MSC
                                                                                          14>Setup (Req for Services I.e.
SDCCH                                                14>Setup
           14>Setup                                                                       Speech/Data/Fax etc)
SDCCH
         15>Call Confirmed                       15>Call Confirmed                         15>Call Confirmed
                                                  17>Channel Activation                     16>Assignment Req
                                                  (BSC Allocated Idle TS for Traffic)       (MSC send CIC to BSC)
                                                 18>Channel Activation Ack
SDCCH 19>Assignment Cmnd (BSC send               19>Assignment Cmnd (BSC send
      message on SDCCH to MS telling to          message on SDCCH to MS telling to go
      go TCH)                                    TCH)




  TCH
         20>Assign Comp (MS tune to TCH
                                                 20>Assign Comp (MS tune to TCH           20>Assign Comp (MS tune to TCH
         and send Ind that Chan is Seized)       and send Ind that Chan is Seized)        and send Ind that Chan is Seized)
                                                  21>RF Chann Realease
  TCH                                             21>RF Chann Realease Ack
         22>Alert (MS Send Alert to MSC as
                                                  22>Alert (MS Send Alert to MSC as       22>Alert (MS Send Alert to MSC as
         soon as the ringing is started in MS)    soon as the ringing is started in MS)
  TCH                                                                                     soon as the ringing is started in MS)
         23>Connect (When MS Sub Answer          23>Connect (When MS Sub Answer
         the Conn message sent to MSC)                                                     23>Connect (When MS Sub Answer
                                                 the Conn message sent to MSC)             the Conn message sent to MSC)
  TCH                                                                                       24>Connect Ack
             24>Connect Ack                             24>Connect Ack                                               Exit..

      Nov 17, 2003

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GSM Channel concept and SDCCH

  • 1. Know More - SDCCH Nov 17, 2003 S. Rajshekhar Deshraj
  • 2. Contents • Channel Concepts • Call Setup • Burst & Multi Frames • Mapping of Logical Channels • Definitions of SDCCH • Usage of SDCCH in the GSM network (BSC) • Possible SDCCH Configuration • SDCCH Holding Time Nov 17, 2003 S. Rajshekhar Deshraj
  • 3. Contents • SDCCH Traffic Estimations • SDCCH Congestion • Reasons for SDCCH congestion • How to detect SDCCH congestions • Preventive actions to avoid SDCCH congestions • SDCCH Dimensioning • Parameters for SDCCH Dimensioning • Counters & Report Analysis Nov 17, 2003 S. Rajshekhar Deshraj
  • 4. Logical Channels Nov 17, 2003 S. Rajshekhar Deshraj
  • 5. 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. Nov 17, 2003
  • 6. 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 Nov 17, 2003
  • 7. 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 Nov 17, 2003
  • 8. 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. Nov 17, 2003
  • 9. 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 Nov 17, 2003
  • 10. 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 Nov 17, 2003
  • 11. 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 Nov 17, 2003
  • 12. 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 Nov 17, 2003
  • 13. Dedicated Control Channels (DCCH) 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 Nov 17, 2003
  • 14. 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. Nov 17, 2003
  • 15. Call to an MS More.. Nov 17, 2003 S. Rajshekhar Deshraj
  • 16. 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 Nov 17, 2003 S. Rajshekhar Deshraj
  • 17. TDMA frames with Common and Dedicated Channels Nov 17, 2003
  • 18. 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 1 Time slot = 156.25 bits durations (15/26 = 0.577 ms ) TB Encrypted bits Training Sequence Encrypted bits TB GP F F 3 57 26 57 3 8.25 F : One Stealing Bit:=0 Indicates 57bit packet contains user data or speech :=1 Indicates burst stolen for FACCH Signalling RxQual derived from the 26 bit midable from the TDMA frame Nov 17, 2003
  • 19. 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 1 Time slot = 156.25 bits durations (15/26 = 0.577 ms ) TB TB GP ALL ZERO 142 BITS 3 3 8.25 Nov 17, 2003
  • 20. Bursts Synchronization Bursts: This burst is used for time synchronization of the MS 1 Time slot = 156.25 bits durations (15/26 = 0.577 ms ) TB SCH DATA Extended Training Sequence SCH DATA TB GP 3 39 Bits 64 Bits 39 Bits 3 8.25 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 Nov 17, 2003
  • 21. 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. 1 Time slot = 156.25 bits durations (15/26 = 0.577 ms ) TB SCH Sequence RACH Data TB Guard Band 8 41 Bits 36 Bits 3 68.25 Bits 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. Nov 17, 2003
  • 22. 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. 1 Time slot = 156.25 bits durations (15/26 = 0.577 ms ) TB Mixed Bits Training Sequence Mixed Bits TB Guard Band 3 58 26 58 3 8.25 58Bits: Coded with pseudo random bit seq. to prevent confusion with Freq correction burst Nov 17, 2003
  • 23. Mapping of Logical Channel Method of transmitting logical channels onto physical channel is called Mapping FCCH+SCH+BCCH+CCCH: An idle MS search for the FCH. When MS finds the frequency correction burst it knows that this is TS 0 on CHGR=0 The cycle means F,S,B repeats after the Idle frame I.e. at Frame no 50. Cycle=51TS TS=0 / CHGR=0 DOWNLINK 0 1 7 0 1 7 0 1 7 F0 F1 F2-F5 F6-9 F10 F11 F12-F15 F16-19 F20-23 F24 F25 F26-29 F30 F31 F32-F35 F36-F39 F40 F41 F42-F45 F46-49 F50 TS-0 F S BCCH CCCH F S BCCH CCCH CCCH F S CCCH F S CCCH CCCH F S CCCH CCCH I F: FCCH 1 TS ( Use 4 Slots in each 51 TDMA Frame) S: SCH 1 TS ( Use 4 Slots in each 51 TDMA Frame) I: IDLE 1 TS BCCH 4 TS CCCH 4 TS (PCH or AGCH) Paging Block 51 TDMA Frame = 9 Paging Blocks TS=0 / CHGR=0 UPLINK F0 F1 F3 F3 F4 F5 F6 F7 F8 F9 F10 F46 F47 F48 F49 F50 TS-0 R R R R R R R R R R R R R R R R R: RACH 1 TS Nov 17, 2003
  • 24. Mapping of Logical Channel SDCCH+SACCH: Cycle=102 TS This sequence is repeated after last idle frame. The Uplink & Downlink pattern are time shifted, so SDCCH sub channel is sent in frame 0-3 on downlink and in frame 15-18 on uplink. The reason for this is to achieve efficient communication, by giving MS time to calculate its answer to the request received on down link SDCCH TS=2 / CHGR=0 DOWNLINK 1 2 3 1 2 3 1 2 3 7 F0-F3 F4-F7 F8-F11 F12-F15 F16-F19 F20-F23 F24-F27 F28-F31 F32-F35 F36-F39 F40-F43 F44-F47 F48 F49 F50 SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH0 SACCH1 SACCH2 SACCH3 I I I SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH4 SACCH5 SACCH6 SACCH7 I I I TS=2 / CHGR=0 UPLINK I: IDLE 1 TS 1 2 3 SDCCH 4 TS 1 2 3 1 2 3 7 SACCH 4 TS F0-F3 F4-F7 F8-F11 F12 F13 F14 F15-F18 F19-F22 F23-F26 F27-F30 F31-F34 F35-F38 F39-F42 F43-F46 F47-F50 SACCH5 SACCH6 SACCH7 I I I SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH0 SACCH1 SACCH2 SACCH3 I I I SDCCH0 SDCCH1 SDCCH2 SDCCH3 SDCCH4 SDCCH5 SDCCH6 SDCCH7 SACCH4 I: IDLE 1 TS SDCCH 4 TS SACCH 4 TS Nov 17, 2003
  • 25. Mapping of Logical Channel TCH+SACCH: 0&2 used by control channels. This leaves TS-1and 3-7 free for the use by TCHs. Repetition time (Cycle) =26 Frames .i.e. =26*8*(15/26)=120ms Every 13th TS contains SACCH. Downlink contains TA value and Uplink contains measuring report. TS=1 / CHGR=0 DOWNLINK 0 1 2 7 0 1 0 1 2 7 F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15 F16 F17 F18 F19 F20 F21 F22 F23 F24 F25 F26 T T T T T T T T T T T T A T T T T T T T T T T T T T I T: TCH 1 TS A: SACCH Nov 17, 2003
  • 26. Usage of SDCCH The SDCCH are used in some different ways in the GSM network: • Registrations: Periodic Location Updates, IMSI Attach/Detach • Call Setup: Immediate Assignment -> Assignment. • SMS point-to-point: SMS messages to/from MS in Idle mode. • Fax Setup • Optional: USSD (Unstructured Supplementary Service Data) data transfer. MS<->Network. Similar to SMS. Controlled by the MSC. Nov 17, 2003
  • 27. Possible Configurations SDCCH/8 : 8 Sub channels for signaling are mapped on 1 TS, this results 1 less TCH for the cell. SDCCH/4 (Combining BCCH and SDCCH) :4 Sub channels for signaling are mapped on BCCH TS. As a result Paging capacity on BCCH is reduced by 1/3. CBCH : If CBCH is active 1 sub channel of SDCCH is replaced by 1 CBCH channel. Limitations : SDCCH/8 in a cell = Number of TRX’s CONFIGURATIONS HYD007A HYD006A 1 TRX 2 TRX SDCCH TCH SDCCH TCH SDCCH/4 4 7 4 15 SDCCH/4+CBCH 3 7 3 15 SDCCH/8 8 6 8 14 SDCCH/8+CBCH 7 6 7 14 SDCCH/4+SDCCH/8 12 6 12 14 SDCCH/4+SDCCH/8+CBCH 11 6 11 14 SDCCH/4+SDCCH/8+SDCCH/8 X X 20 13 SDCCH/4+SDCCH/8+SDCCH/8+CBCH X X 19 13 Nov 17, 2003
  • 28. Possible Configurations -BCCH COMBC+CBCH CELL BCCH CBCH SDCCH NOOFTCH HYD007A 1 1 3 7 CHGR BPC CHANNEL CHRATE SPV STATE ICMBAND CHBAND 0 4590 SDCCH-28646 BUSY SDCCH-28645 IDLE CBCH-34305 BUSY SDCCH-28644 IDLE BCCH-34797 BUSY 4496 TCH-4629 FR 1,2 IDLE 4497 TCH-4630 FR 1,2 IDLE 4499 TCH-4631 FR 1,2 IDLE 4505 TCH-4654 FR 1,2 IDLE 4506 TCH-4655 FR 1,2 IDLE 4509 TCH-4661 FR 1,2 IDLE 4575 TCH-4670 FR 1,2 IDLE Nov 17, 2003
  • 29. Possible Configurations -BCCH NCOMB+CBCH CELL BCCH CBCH SDCCH NOOFTCH HYD006A 1 1 7 14 CHGR BPC CHANNEL CHRATE SPV STATE ICMBAND CHBAND 64K 0 6899 SDCCH-27810 IDLE SDCCH-27809 IDLE CBCH-34619 BUSY SDCCH-27808 IDLE SDCCH-27807 IDLE SDCCH-27806 IDLE SDCCH-27805 IDLE SDCCH-27804 IDLE 6900 BCCH-34804 BUSY 6887 TCH-1242 FR 1,2 IDLE NONE 6888 TCH-1243 FR 1,2 IDLE NONE 6891 TCH-1246 FR 1,2 IDLE NONE 6893 TCH-1248 FR 1,2 IDLE NONE 6895 TCH-1250 FR 1,2 BUSY NONE 6897 TCH-1252 FR 1,2 IDLE NONE CHGR BPC CHANNEL CHRATE SPV STATE ICMBAND CHBAND 64K 1 6885 TCH-1240 FR 1,2 BUSY NONE 6886 TCH-1241 FR 1,2 IDLE NONE 6889 TCH-1244 FR 1,2 IDLE NONE 6890 TCH-1245 FR 1,2 IDLE NONE 6892 TCH-1247 FR 1,2 IDLE NONE 6894 TCH-1249 FR 1,2 BUSY NONE 6896 TCH-1251 FR 1,2 IDLE NONE 6898 TCH-1253 FR 1,2 IDLE NONE Nov 17, 2003
  • 30. SDCCH Congestion SDCCH congestion: All SDCCH in a cell are at the same time busy for a period of time which leads to rejection for new MS. Reasons : • Hanging SDCCH: Are SDCCH that are hanging busy and unusable for a long time (many minutes or indefinite). Hanging SDCCH are generally caused by SW faults. • Heavily used SDCCH: SDCCH seen as continuously busy even though they are used by different MS and thus carries traffic. It may cause due to : > Extreme end-user behaviors: Sport event ending, festivals or celebrations. > Increased mean hold time of the SDCCH: Mean hold time increase from 2 to 10 seconds can give SDCCH congestion. Nov 17, 2003
  • 31. SDCCH Holding Time Normal Location Updating = 3.5 Sec Periodic Registration = 3.5 Sec IMSI Attach = 3.5 Sec IMSI Detach = 2.9 Sec (IMSI detach Indication message sent to NW, no authentication is performed (which normally takes 0.6Sec) & no ack is sent to MS.) Call Setup = 2.7 Sec (MOC) = 2.9 Sec (MTC) Short Message Service(SMS) = 6.2 Sec (Vary depending the length of SMS) Fax Transmission = 2.7 Sec (MOC) = 2.9 Sec (MTC) False Access = 1.8 Sec (when Channel req is rec’d by system ,as SDCCH is allocated by sending Imm Ass message, and the system waits a certain time before performing disconnection.) Nov 17, 2003
  • 32. SDCCH Traffic Estimations Normal Location Update: Mean Holding time =3.5 Sec No of Location update =1.0 per sub and BH AC: Avg No of Location updates =1000X(1 X 3.5)/3600=0.972 mE/Sub IC : No Location updating at all =1000X(0 X 3.5)/3600=0 mE/Sub BC: 3 times the Avg no of Location updates =1000X(3 X 3.5)/3600=2.916mE/Sub SMS: Mean Holding time =6.2 Sec No of SMS submitted =1.0 per Sub and BH Traffic : 1000 X (1.0 X 6.2) / 3600 =1.7 mE/sub Call Setup: Mean Holding time =2.7 Sec(MOC) & 2.9 Sec(MTC) Mob Originating Traffic (Incl B Ans) =0.8 BHCA Mob Terminating Traffic (Incl B Ans) =0.4 BHCA Traffic : 1000 X {(0.8 X 2.7)+(0.4 X 2.9)} / 3600 =0.9 mE/sub Nov 17, 2003
  • 33. Reasons for SDCCH congestion Increased mean hold time of SDCCH can be caused by: • Too low signal strength at access to the system Due to LSS MS will be lost communication with the system, This will lead to timeout in the BSC (RLINKT ), and thus the SDCCH is used until timeout. Increase ACCMIN. • Congestion of TCH/TRA/RALT Will increase CP execution time before rejecting Imm Ass. Minor increase in mean hold time expected. • C7 problems to MSC (or TRC) In case of C7 signalling problems (SCCP) towards MSC (and/or TRC) can lead to timeout on SCCP connection setup. This will lead to more than 2 minutes hold time for SDCCH. • Handover on SDCCH Handover on SDCCH will in itself increase mean hold time on SDCCH. However minor increase is expected. Nov 17, 2003
  • 34. Reasons for SDCCH congestion • Congestion on Air-interface Congestion on Air-interface leads to delay in communication to the MS. Can give timeout in BSC during Imm Ass.Increases SDCCH mean hold time with more than 2 seconds. • Congestion on Abis (LAPD link) Congestion on Abis leads to delay in communication with BTS and MS. Can give timeout in BSC during channel activation (TCHACTIVE). Increase SDCCH mean hold time with more than 5 seconds.. • Congestion on A-interface Congestion on the A-interface will lead to increased mean hold time on SDCCH. Increase is unknown. • High load in MSC/VLR or HLR High load in MSC/VLR and/or HLR will lead to increased mean hold time on SDCCH. Increase is unknown. Nov 17, 2003
  • 35. How to detect SDCCH congestion in the BSC There is no good way to detect SDCCH congestion in real time in the BSC! A number of symptoms to look for: • Increased CP Load. • Decreased usage of TRA devices. • Location Updates fails => Mobile terminating calls does not reach the subscriber. • Subscriber complaints. Can not access the system. • STS counters . Object type: CLSDCCH and CLSDCCHO. • Seizure Supervision of LCHs (but only for Hanging SDCCHs!) Nov 17, 2003
  • 36. Preventive actions to avoid SDCCH congestion • Avoid combined BCCH in cells with many SDCCHs • Use the optional feature Adaptive Configuration of SDCCHs (ACLC) • Use USSD (Unstructured Supplementary Service data) with care, can cause long meanhold time on SDCCH. • Avoid Handover on SDCCH • Proper Dimensioning of the TCHs and TRA devices in the BSC. • Use recommended values for Periodic Location Updates Avoid unnecessary Periodic Location Updates : BSC: T3212 (RLSBC) = 40 (4 hours) MSC: BTDM (MGIDI) = 240 (4 hours) MSC: GTDM (MGIDI) = 6 (6 minutes) • Use Immediate Assignment on TCH. • Increases the no of SDCCH in a Cell where SDCCH load is high Nov 17, 2003
  • 37. SDCCH Dimensioning SDCCH Dimensioning is a compromise between SDCCH blocking rate and TCH Capacity. In order to have a successful call setup there has to be an available SDCCH as well as available TCH. Basic SDCCH configuration: It is recommended to choose 1 SDCCH/8 as the basic configuration for all the cells, If LA> 2100 Erlang (500 TRX) 1 SDCCH/4 as the basic configuration for all the cells, If LA< 2100 Erlang (500 TRX) Nov 17, 2003
  • 38. SDCCH Dimensioning Automatic SDCCH dimensioning: This can be done with optional Adaptive Configuration of Logical Channel feature. This feature will add extra SDCCH/8 by reconfiguring idle TCH when SDCCH load is high, and revert back to TCH if SDCCH load goes down. Manual SDCCH Dimensioning : •Monitoring SDCCH / TCH Traffic in a cell •SDCCH/TCH load ratio •SDCCH Grade of Service:- Max allowed TCH GOS % = 2 % The rule of Thumb says: SDCCH/4 : Max. SDCCH GOS =1/2 * 2= 1 % SDCCH/8 : Max. SDCCH GOS =1/4 * 2= 0.5% Nov 17, 2003
  • 39. SDCCH Dimensioning Immediate Assignment on TCH: In case of this feature on the channel administration assigns TCH for signaling instead of an SDCCH, based on 7 different channels allocation strategies (CHAPs) TCH first strategy : Decreases the SDCCH load and enable to use SDCCH/4 in all the cells Traffic load on TCH will in this case increases substantially so this strategy is not recommended. SDCCH first strategy : SDCCH is always allocated first if available, otherwise signaling is performed on TCH. Nov 17, 2003
  • 40. SDCCH Dimensioning Example : ASSUME: CELL=3TRX ,SDCCH Traffic =4 Erlang, Bcchtype=NCOMB.No CBCH GOS: SDCCH/4 = 1%, SDCCH/8 =0.5% & TCH % =2% 1> When Immediate Assignment on TCH not used Erlang B Table :To maintain 0.5 % GOS & 4 Erlang min 13 Subchannels reqd 2SDCCH/8 => TCH=(3TRX*8TS) –(1BCCH)-(2SDCCH/8)=21TCH’s 21TCH’s with 2 % GOS => 14.04 Erlang 2> When Immediate Assignment on TCH used If we use 1SDCCH/8 only than 4 Erlangs with 8 sub channels GOS=>3 % Congested Erlang will be =4 Erlang x 3 %=0.12Erlang 1SDCCH/8 => TCH=(3TRX*8TS) –(1BCCH)-(1SDCH/8)=22TCH’s 22TCH’s with 2 % GOS => 14.9 Erlang Actual TCH Capacity=14.9 – 0.12 = 14.7 Erlang TCH Capacity in the cell is increased with 0.7 Erlang Nov 17, 2003
  • 41. SDCCH Dimensioning Immediate Assignment on TCH not used Nov 17, 2003
  • 42. SDCCH Dimensioning Recommended SDCCH Configurations Nov 17, 2003
  • 43. SDCCH Dimensioning HALF RATE CAPACITY CALCULATION Half rate will affect SDCCH dimensioning since more signaling will be req’d when no of TCH is increased. Important dimensioning factor is therefore the Half rate penetration .i.e.the % of Half rate mobile in a NW. Example: 2 TRX Cell, Half Rate Penetration =10 %, 1 SDCCH/8 TCH/F =14 support 14 Subscriber Penetration 10% of 14 is 1.4 = 2 Subscribers (Req’d 2 Half Rate TCH/H) Total 13 TCH/F+2TCH/H = 15 TCH required Capacity : Erlang B Table: 15 TCH @ 2% GOS => 9.0096 Erlang Nov 17, 2003
  • 44. SDCCH Dimensioning Max Allowed SDCCH/TCH Load Ratio (Half Rate) AC : Average Cells BC : Border Cells IC : Inner Cells Nov 17, 2003
  • 45. SDCCH Dimensioning Dimensioning based on STS data More accurate dimensioning is achieved by using cell statistics .i.e. STS counters •SDCCH Traffic •TCH Traffic •SDCCH/TCH Load Ratio % •SDCCH Congestion •TCH Congestion •SDCCH Mean Holding Time •Availability of SDCCH Channels •Availability of TCH Channels Nov 17, 2003
  • 46. Parameters MFRMS :This parameter defines period of transmission for PAGING REQUEST messages to the same paging subgroup. T3212 : Is the periodic registration timer ACCSTATE : Activate/Deactivate the Adaptive config of logical channel feature per cell basis CHAP :Selecting Channel Allocation profile per cell. SLEVEL: The attempt to increase the number of SDCCH/8 will take place when allocation of an SDCCH has failed due to congestion. STIME :Parameter STIME determines how long the system waits before the number of SDCCH/8 added by this function is decreased when the demand for signalling channels has returned to a low level BTDM :Implicit detach supervision should be equal or longer than T3212 in BSC. GTDM : is an extra Gurad time in minutes before the subscriber is set to detach. RLINKT: Radio link time-out This parameter defines the time before an MS disconnects a call due to failure in decoding SACCH messages. The parameter is given as number of SACCH periods (480ms). Nov 17, 2003
  • 47. Parameters for SDCCH Dimensioning Nov 17, 2003
  • 48. STS Counters CCALLS : Call attempt counter CCONGS : Congestion counter CTRALACC : Traffic level accumulator. CNSCAN : Number of accumulations of SDCCH traffic level counter. CNDROP : Dropped connections due to Failure. CNUCHCNT : Number of defined channels. CAVAACC : Available channels accumulator. CMSESTAB : Successful MS channel establishment on SDCCHs. CTCONGS : SDCCH congestion time ( Sec) Nov 17, 2003
  • 49. Formulas No. of SDCCH Attempts CCALLS SDCCH Congestion (%) 100 X { CCONGS / CCALLS } No. of SDCCH Connections CMSESTAB SDCCH Establishments No 100 X {CMSESTAB / (CCALLS - CCONGS)} Congestion (%) SDCCH Time Congestion (%) 100 X { CTCONGS / (RPL * 60)} SDCCH Drop (%) 100 X { CNDROP / CMSESTAB } SDCCH Mean Holding Time RPL X 60 X {(CTRALACC/CNSCAN) / CMSESTAB } (Sec.) SDCCH Traffic (Erlang) CTRALACC / CNSCAN SDCCH/TCH Ratio (CTRALACC/CNSCAN) / ((TFTRALACC/TFNSCAN)+ (THTRALACC/THNSCAN)) Nov 17, 2003
  • 50. Quality Report : Call Setup Analysis Report Measuring Period : 03 Nov 2003 to 09 Nov 2003 (7Days) (Weekly Avg Stats) W-6 QUALITY REPORT :Call Setup Analysis Report ( W-6) 23.09.03 S DRP(HYD) S DRP (ROAP) SCSR %(HYD) SCSR % (ROAP) CONR% (HYD) CONR% (ROAP) ALL SDRP>1% SER <96% CONR >0.5 % 15Sep-21Sep 134 Cells 275 Cells 175 Cells 357 Cells 68 Cells 109 Cells 72Cells 22Sep-29Sep 127 Cells 295 Cells 192 Cells 378 Cells 98 Cells 174 Cells 116Cells 29Sep-5Oct 112 Cells 309 Cells 187 Cells 400 Cells 71 Cells 135 Cells 97Cells 06Oct-12Oct 25 Cells 154 Cells 178 Cells 382 Cells 55 Cells 109 Cells 81Cells 03Nov-09Nov 89 Cells 295 Cells 206 Cells 372 Cells 59 Cells 78 Cells 60Cells Sr No CELL NOD >1% CELL NOD >1% CELL < 96 % CELL < 96 % CELL >0.5% NOC CT CELL >0.5% NOC CT Worst Cells (S) (S) 1 HYD001B 633 1.08 ADO001B 109 1.52 HYD012C 73.35 KOL001B 65.95 HYD201A 3.31 3081 1952 HIN002C 2.37 90 326 NIZ001A 2 HYD004B 305 1.03 ADO002B 54 1.33 HYD117C 78.45 JGP001C 72.07 HYD176A 3.19 2862 8177 KAD001B 2.35 602 1989 NIZ001B 3 HYD005A 359 1.54 AKI001A 29 1.04 HYD155B 81.57 HIN002C 73.01 HYD176C 2.98 1695 4397 GUN009B 2.19 641 2410 CHL001A 4 HYD007B 79 1.34 AKI001B 21 1.04 HYD161B 82.57 RAJ001B 73.06 HYD176B 2.3 942 3095 VIZ027C 1.91 267 638 BHI001A 5 HYD012C 432 6.51 AKI001C 37 2.15 HYD168A 83.77 VIZ020C 73.67 HYD137C 1.94 559 1423 KAD001C 1.72 392 1702 MAH002B 6 HYD018A 433 1.18 AMA001A 3 1.54 HYD058A 83.99 JGP001A 75.2 HYD168A 1.75 207 656 HIN001B 1.59 183 664 KHA001B 7 HYD018B 653 1.13 AMA001A 82 2.66 HYD124B 84 VJA018B 75.81 HYD005B 1.6 725 2465 ANA001B 1.42 275 1084 TEN002C 8 HYD020B 298 1.02 AMA001B 37 1.08 HYD146B 86.41 CHI001C 77.68 HYD155A 1.39 152 403 TEN002C 1.4 372 1434 TIR008A 9 HYD025C 481 1.28 AMA001C 5 2.94 HYD012A 86.52 EMG001C 78.01 HYD161B 1.36 138 262 KAD001A 1.33 268 1127 AKI001A 10 HYD026C 653 1.7 AMA001C 72 2.94 HYD093B 86.86 RAZ001B 78.98 HYD137B 1.3 257 993 VIZ023C 1.32 183 790 BPL001B 11 HYD028A 413 1.03 ANK001B 144 1.11 HYD170B 87 NID001C 79.41 HYD054B 1.28 386 1654 ADO001C 1.32 190 860 TVR001C 12 HYD028C 601 1.23 ANN001A 14 3.29 HYD155A 87.29 CHL004C 79.47 HYD170B 1.27 139 375 GOD001B 1.31 372 1099 DHA001C 13 HYD030C 373 1.26 ANN001B 30 2.15 HYD161C 87.59 RJM007B 80.13 HYD171B 1.23 323 1219 SUL001A 1.3 71 169 MAH002A 14 HYD031C 581 1.35 ANN001C 96 5.64 HYD089B 87.82 VJA020A 81.21 HYD168C 1.22 82 320 MAH002A 1.23 104 390 HIN001B 15 HYD038A 327 1.11 ANP001A 18 1.06 HYD011B 87.85 ONG004B 81.78 HYD137A 1.1 129 346 STP001B 1.05 374 1865 BPL001C 16 HYD040A 279 1.07 ANP001B 29 2.19 HYD121B 87.97 ANN001A 81.92 HYD177B 1.05 206 791 VSP001C 1.03 135 270 TAD001C 17 HYD040B 522 1.3 ANP001C 20 1.7 HYD146C 88.01 CHI003B 82.55 HYD142A 1.02 147 701 STP001A 1.01 329 1542 CHL002C 18 HYD042A 505 1.14 AVA001A 49 1.95 HYD145B 88.23 VSP001B 82.8 HYD053C 1.01 161 496 TVR001C 1.01 258 924 MAC001A 19 HYD045B 1632 1.28 AVA001B 26 1.13 HYD108C 88.49 ANN001C 83.19 HYD161C 1 95 147 RJM008B 0.98 15 65 DHA001B 20 HYD051C 229 1.01 BHI001A 258 1.45 HYD101C 88.71 VJA019B 83.22 HYD153C 0.99 191 541 SAN001B 0.98 134 318 ADO001B LEGENDS NOC SDCCH Congestion Counter SDRP SDCCH Drop Rate % CT(S) Cong. Time in Secs (Week Tot) NOD Week Total Dropped SDCCH's ALL SDCCH Drop Rate % SCSR SDCCH Call Success Rate % SDCCH Estab Success Rate % CONR SDCCH Congestion rate % SDCCH Congestion rate % Nov 17, 2003
  • 51. Know More - SDCCH Thank you Nov 17, 2003 S. Rajshekhar Deshraj
  • 52. Call Setup-Mobile Terminating Call MS BTS BSC MSC 1>Paging(LAI+IMSI/TMSI) PCH 2>Paging Command 3>Paging Req(Imsi on PCH) Imsi/Tmsi+PG+TRX+CG+TN RACH 4>Channel Req(On RACH) 5>Channel Reqd (Access Delay) 6>Channel Actn (MSPwr,BSPwr,TA) 7>Channel Activation Ack AGCH 8>Imm Assign Cmd(On AGCH , Freq 8>Immediate Assign +TS+ SDCCH SubChannel No+TA SABM (Paging Resp:IMSI/MS Class) UA(Paging Resp) Unnumbered Ack 9>Estblish Ind (Paging Resp) 9>Conn Req (Paging Resp: BSC Frame which confirms only 1 MS is IMSI+MS Class add CGI) using Sig Channel SDCCH 10>Auth Req (128 bit RAND+ 10>Auth Req (128 bit 10>Auth Req (128 bit RAND+3bitCKSN) 3bit CKSN) RAND+3bitCKSN) SDCCH 11>Auth Response (MS Calculate 11>Auth Response (SRES) SRES & Kc with its own Ki stored in 11>Auth Response (SRES) SIM by appling algorithm A3&A8) Next.. Nov 17, 2003
  • 53. Call Setup-Mobile Terminating Call MS BTS BSC MSC 14>Setup (Req for Services I.e. SDCCH 14>Setup 14>Setup Speech/Data/Fax etc) SDCCH 15>Call Confirmed 15>Call Confirmed 15>Call Confirmed 17>Channel Activation 16>Assignment Req (BSC Allocated Idle TS for Traffic) (MSC send CIC to BSC) 18>Channel Activation Ack SDCCH 19>Assignment Cmnd (BSC send 19>Assignment Cmnd (BSC send message on SDCCH to MS telling to message on SDCCH to MS telling to go go TCH) TCH) TCH 20>Assign Comp (MS tune to TCH 20>Assign Comp (MS tune to TCH 20>Assign Comp (MS tune to TCH and send Ind that Chan is Seized) and send Ind that Chan is Seized) and send Ind that Chan is Seized) 21>RF Chann Realease TCH 21>RF Chann Realease Ack 22>Alert (MS Send Alert to MSC as 22>Alert (MS Send Alert to MSC as 22>Alert (MS Send Alert to MSC as soon as the ringing is started in MS) soon as the ringing is started in MS) TCH soon as the ringing is started in MS) 23>Connect (When MS Sub Answer 23>Connect (When MS Sub Answer the Conn message sent to MSC) 23>Connect (When MS Sub Answer the Conn message sent to MSC) the Conn message sent to MSC) TCH 24>Connect Ack 24>Connect Ack 24>Connect Ack Exit.. Nov 17, 2003