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.