GSM Handover Call Flow

Inter BSC - Intra MSC Handover Call Flow (GSM Inter BSC - Intra MSC Handover Call Flow)Highway GSM Coverage GSM

EquipmentGSM Mobile Bethesda Rockville MSC VLRs

Mobile Bethesda Cell Bethesda BSC Rockville Cell Rockville BSC Maryland MSCVLR

EventHelix.com/EventStudio 2.5

04-Dec-04 11:46 (Page 1)

How does a GSM mobile phone maintain a call even when moving from one cell to another?

The calls are maintained by handing over a call from one cell to another. This call flow covers a simple case of call handover in GSM. Here auser has an active call and is moving from the Rockville Cell to the Bethesda Cell. As the user moves, the call will be handed over by theRockville Cell to the Bethesda Cell.

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The GSM Mobile has an active call in the Rockville Cell.

RR MEASUREMENT REPORTSignal Quality = GOOD

When a call is active, the mobile periodicallyreports the signal quality to the network viathe Measurement Report message. Thismessage is sent in every SACCH frame witha periodicity of 480 ms. The measurementreport also includes the signal qualitymeasurements for neighboring cells.

RR MEASUREMENT REPORTSignal Quality = GOOD

The mobile is reporting good signal quality,so no further action is taken.

The user reaches the boundary between the Rocville Cell and Bethesda cell.

RR MEASUREMENT REPORTSignal Quality = POOR

The mobile is at the edge of the Rockvillecell and it reports that it is seeing a muchweaker signal from the Rockville cell.

RR MEASUREMENT REPORTSignal Quality = POOR

The Rockville BSC decides to initiate ahandover as the mobile will be better servedby another cell.

Check if handover is needed The BSC analyses the measurement reportsto determine that the mobile will be bestserved by the Bethesda Cell.

BSSMAP HANDOVER REQUIREDTarget Cells, Origin Cell

The BSC decides to request a handover. Alist of target cells is provided to the MSC.The Bethesda Cell is included in the list oftarget cells.

T7 The T7 timer is started to wait for thehandover command from the MSC.

BSSMAP HANDOVER REQUESTTarget Cells, Origin Cell

The MSC passes on the handover request tothe Bethesda BSC. (The Rockville BSCidentified this BSC as as a target cell forhandover.)

T101 The MSC-VLR starts a timer to await theresponse from the Bethesda BSC.

Allocate TCH The handover request is treated as a newcall. A traffic channel (TCH) is allocated forthe call that will be handed-in.

Build RR HANDOVER COMMANDmessage

At this point the Bethesda BSC prepares thehandover command that needs to be sent tothe mobile. This message contains all theinformation the mobile will need tohandover to this cell.

BSSMAP HANDOVER REQUEST ACKNOWLEDGERR HANDOVER COMMAND

The Bethesda BSC includes the RRHANDOVER COMMAND message as apayload in the HANDOVER REQUEST ACKthat is sent back to the MSC. The RRHANDOVER COMMAND will be delivered tothe mobile via the Rockville BSC.

T101 The MSC has heard back from thedestination BSC, thus the T101 timer isstopped.





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BSSMAP HANDOVER COMMANDRR HANDOVER COMMAND

The MSC delivers the handover commandto the Rockville BSC. This commandencapsulates the RR HANDOVERCOMMAND from the destination BSC.

T102 T102 is started to track the completion ofthe handover.

T7 The handover command has been received.So the T7 timer can now be stopped.

RR HANDOVER COMMANDHandover Reference, TCH

The Rockville BSC extracts the RRHANDOVER COMMAND message from theBSSMAP message and sends it to themobile.

T8 T8 is started to await the clear of this callfrom the MSC. If the handover to the targetcell is successful, the MSC will initiate aresource release to the source BSC.

Tune to the channelspecified in the handover

command

The extracts the destination channelinformation from the message and tunes tothe assigned channel.

RR HANDOVER ACCEPTAccess Burst, Handover Reference

After tuning to the assigned channel, themobile starts sending the handover acceptmessage. Note that this message is sent asan access burst as the mobile is notcompletely synchronized to send normalbursts.

T3124 The T3124 timer is started to await thePHYSICAL INFORMATION message fromthe network.

RR HANDOVER ACCEPTAccess Burst, Handover Reference

The BSC receives the HANDOVER ACCEPTfrom the terminal. The actual call isidentified using the handover reference.(The handover reference was send in theencapsulated HANDOVER COMMANDmessage. )

BSSMAP HANDOVER DETECTED The BSC informs the MSC that thehandover has been detected. At this pointthe MSC can switch the voice path.

Switch Call to new Path The MSC switches the voice path.

RR PHYSICAL INFORMATIONHandover Reference

The BSC sends the PHYSICALINFORMATION message to the mobile. Thismessage contains a time and frequencycorrection.

T3105 T3105 is started to await the receipt of theSABM for the signaling connection.

RR PHYSICAL INFORMATIONHandover Reference

The mobile applies the received correctionsand can now send TCH bursts on thechannel. TCH bursts contain the speechfrom the user.

T3124 T3124 is stopped as PHYSICALINFORMATION message has been received.

RR SABM Mobile sends a SABM to establish thesignaling connection.

T3105 Receipt of SABM stops the T3105 timer.

RR UA The BSC replies with a UA message.

RR HANDOVER COMPLETE The mobile uses the signaling connection toindicate that the handover has beencompleted.





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BSSMAP HANDOVER COMPLETE The BSC forwards the handover completionevent to the MSC.

T102 Handover has been completed, so T102 isstopped.

Release call resources in Rockville BSC.

BSSMAP CLEAR COMMAND Call release has been completed, now theRR connection is released by the MSC.

T8 The T8 timer is stopped as the handed overcall's resources in the source BSC arereleased.

Release TCH

BSSMAP CLEAR COMPLETE The BSS informs the the MSC that the RRconnection has been released.

Documents

GSM Handover Call Flow