Wednesday, 9 July 2014

GSM Traffic Cases

GSM Traffic Cases

  u   MS can either be in detached mode or it can be in attached mode.

  u  When the MS is in detached mode, there is no traffic case involved with that.

  u  While in attached mode, the MS can either in idle state or in active state.

  u  Changing from idle to active state will involve different types of traffic cases:

1  .     Location updating.
2  .     Call setup.
3  .     Handover.

     1. Location updating

Need for Location updating:

l  It should be possible for the MS to receive a call from the network at any time at any location.

l  While the MS moves freely within a network, the network should know about its whereabouts, in terms of its location and the cell with which it is attached.

l  The MS should update the network whenever it changes the location and is called Location Updating.

Three types of location updating:
l  Location updating normal.
l  IMSI attach.
l  Periodic Registration.

n  A location area is the area handled by one or more BTSs where the MS can move around without updating the network.
n  A location area is controlled by one or more BSCs but strictly by one MSC.
n  MS gets the location area identity of the serving cell, through listening the BCCH.

n  MS compares the Location Area Identity (LAI) to the one stored in the MS through the SIM card & if LAI differs from the one stored in the SIM card, the MS decides to do a location update, type normal.

Location Updating, type normal. MS already registered in VLR

1. The MS listens to the system information on BCCH Channel, compares the LAI with the one stored in SIM card. When it finds the difference, it decides to do an location update.

2a. The MS sends a channel request message through RACH channel.
2b. The message received by the BTS is forwarded to the BSC. The BSC allocates a SDCCH, if there is one idle, and tells the BTS to activate it.
2c. BTS give acknowledgement on AGCH channel.

3.  The MS sends a location updating request message which contains the identity of the MS, the identity of the old location area and the type of updating.

4a. The authentication parameter is sent to the MS. If the MS is not already registered in this   MSC/VLR the appropriate HLR or the previously used MSC/VLR must be contacted to retrieve MS subscriber data and authentication parameters.
4b. MS sends an answer calculated using the received authentication parameter.

5.   If authentication is successful, the VLR is updated.

6.   The MS receives an acceptance of the location updating.

7a. The BTS is told to release the SDCCH.
7b. The MS is told to release the SDCCH and switches to idle mode.

During the location change, if the MS is busy in call, it receives the information about the new LAI on the SDCCH. The location updating (above said procedure) takes place after the call is released.

(B) IMSI attach:

n  IMSI attach is used by the MS to notify the system that it was powered on, provided it is still in the same location area as it was when it entered the detached state.
n  This procedure is to be used only when the IMSI detach flag is set in the VLR.
If the flag is set in the HLR, switching to active mode requires a normal location updating of the  MS.

1. The MS requests a SDCCH ( The point number 1 , 2a, 2b , 2c all are same as in Normal Updating).
2. The system receives the IMSI attach message from the MS.
3. The MSC sends the IMSI attach message to the VLR. The VLR removes the IMSI detached flag and resumes normal call handling for the MS.
4. The VLR returns the IMSI attach acknowledge message to the MSC.
5. The MS also receives an acknowledge message.

NOTE: - This procedure is to be used only when the IMSI detach flag is set in the VLR. If the flag is set in the HLR, switching to active mode requires a normal location updating of the MS.

(C) Type periodic registration:

 Periodic registration is a type of location updating procedure that is used to avoid unnecessary paging of the mobile in cases where the MSC never receives the IMSI detach message and also to prevent damage in case of database failure.

     1.  The MS is informed via the system information if periodic registration is used in the cell. If periodic registration is used, the MS is told how often it must register. The time is set by a counter where the values are 0 to 255 deci-hours (a unit of six minutes). If the parameter is set to zero means periodic registration is not used, if the parameter is set to twenty for example, then MS must register every two hours.

2.   The procedure is controlled by a timer both in the MS and in MS.

3.  When the timer in the MS expires, the MS performs a location updating, type periodic registration and the timers in MS and MSC restart.

4.  In the MSC there is a time scanning function for the MS, if the MS does not register within the determined interval plus a guard time, then the scanning function in MSC detects this and the MS is flagged as detached .

2. Call Setup

Call set up can be classified into two major categories, they are

(i)             Call From MS (Mobile Originated call)  (MOC)
(ii)            Call to MS (Mobile Terminated call) ( MTC)

(i) Call From MS (Mobile Originated Call) (MOC):-

Mobile Originated call is possible when the MS is attached with the MSC/VLR and is listening to the system information.

   Mobile Originating call establishment

1. The MS requests a SDCCH using RACH ( Point 1, 2a, 2b, 2c of Normal Location Updating all  are  also occured internally or in background.)

2. The MS indicates that it wants to set up a call. The identity of the MS, IMSI is analyzed and the MS is marked as busy in the VLR.

3.  Authentication is performed.
4.  Ciphering may be initiated.
5.  The MSC receives a setup message from the MS. This includes the kind of service the MS wants and the B-number. MSC verify that the MS doesn’t have services like barring of outgoing calls. If the MS is not barred, the setup of call proceeds.

6. Between the MSC and the BSC a link is established and a PCM time slot is seized. The MSC sends a request to the BSC to assign a TCH. The BSC assigns a TCH to the call (if there is an idle TCH available) and tells the BTS to activate the channel. The BTS sends an acknowledgement when the activation is completed and the BSC orders the MS to switch over to the TCH. The BSC informs the MSC when the assignment is complete.

7. An alert message is sent to the MS indicating that a ringing tone has been generated on the other side. The ringing tone generated in the exchange on the B- subscriber side is sent t the MS via the group switch in the MSC.

8. When the B subscriber answers, the network sends a connect message to the MS  indicating that the call is accepted . The MS returns a connect acknowledgement which completes the call set-up.

(ii)  Call to MS (Mobile Terminated Call) ( MTC):-

Call to MS from PSTN.

1.  The number dialed by the calling party is called the Mobile station ISDN number (MSISDN). If the call is being made from a PSTN exchange, the exchange analyses the number and decides that the call is for a GSM subscriber

2.     From PSTN, the call is routed to the GMSC in the home PLMN of the called MS.

3.  By analyzing the MSISDN, the GMSC finds out which HLR the subscriber is registered in. GMSC asks the HLR for information so the call can be routed to the MSC/VLR where the MS is temporarily registered.

4.   The HLR contacts the VLR and gets the roaming number.

5.   The HLR forwards the roaming number to the GMSC.

6. With the help of the roaming number (MSRN) , the GMSC can   route the call to the appropriate MSC. The call is routed to the MSC.

7.  The MSC knows which location area the MS is located in and sends a paging message to the BSCs handling this location area.

8.   The BSC distributes the paging message to the BTSs in the LAI.

9.  The BTSs page the MS using IMSI .

Please checkout my other blogs