Handover

- The Process of Changing cells during a call is called handover

- MS & BTS perform measurements

- MS measures signal strength and quality on its cell and signal strength on the BCCH carries of neighbouring cells.

- Measurement are made on down link when MS is in active mode.

- Measurement result are sent to the BTS at regular intervals.

- Measurement from BTS & MS are sent to BSC in form of measurement reports.

- Based on reports received BSC decides if a handover is necessary and to which cell. This is called Locating.

- As soon as a neighbouring cell is considered to be better than the serving cell, a handover is attempted.

- Other reason for handover is TA. When MS exceeds TA than a threshold value set by the operator. This happens when MS is moving over the cell border to another cell.

- When the MS has changed cells the new BTS informs the MS about new BCCH carrier so measurement can be taken again.

- If the MS has switched to a new LA, a location updating type normal takes place after the call has finished.

- Handover can be used for load balancing between cells.

- During a call setup in a congested cell, MS can be transferred to a cell with less traffic, if the quality of new connection is acceptable.

- Handover is also used during maintenance. Channel can be released for BTS maintenance. This is called forced handover.

Types of Handover:-

1. Intra - cell handover

2. Handover between cells controlled by the same BSC.

3. Handover between cells controlled by different BSC but same MSC-/VLR.

4. Handover between cells controlled by different MSC/VLRs.

For each handover, the decision to perform a handover has already been made and a target cell has been identified

Intra cell - Handover:- It is when BSC considers the quality of connection is too low but receives no indication from the measurements that another cell would be better, BSC identifies another channel in the same cell which may offer better qualify, and MS is ordered to re-tune to it.

Signal Strength Measurement

SACCH

MS BSS

1

2

3

5

Um A

Handover Access

SACCH

1. MS listen to BCCHs, (Current) and neighbouring channels and reporting results to BSS

2. BSC allocates a new radio channel and instructs the MS to re-tune to it.

3. MS tunes to new radio channel.

4. MS sends a handover complete message to BSS.

5. BSS informs the MSC about new location of the MS.

BSC

Handover Command

SACCH

Handover Performed

4Handover Complete

SACCH

INTRA-BSS INTER BTS HANDOVER

1. MS listen is own BCCH & neighbouring BCCH.

2. Old BSS determines that the MS requires to be handed over, it will send a Handover Required message with a list of cells to which the MS can be handed over to MSC.

3. MSC send a handover Request Message to new BSS.

4. New BSS sends Handover Request-ACK message to MSC.(It contains Handover Reference Number - hrn.)

5. MSC informs Old BSS for Handover.

6. Old BSS send command to MS, to take to new radio channel.

Handover Requested

Handover Required

MSOLDBSS MSC New BSS

1

2

3

4

Um AA

Handover RequestedAcknowledge (hrn.)

Signal Strength Measurement

SACCH

Handover command (hrn)5

6 Handover command (hrn)SACCH

INTRA-MSC HANDOVER

7. MS across the new radio resourse by sending the message Handover Access containing the (hrn.) The (hrn.) is checked by new BSS to ensure that MS captured is correct.

8. If correct new BSS sends Handover detect message to MSC

9. When MS communicating with new BSS, it (MS) sends a Handover Complete Message .

10. New BSS forwards this message to MSC.

11. Trunk between MSC & Old BSS is released.

12. Radio Traffic Channel is released.

Handover Detected

MSOLDBSS MSC New BSS

7

8

9

10

Um AA

Handover Complete SACCH

Handover Access

Handover Complete

11

12

Release Radio Channel

INTRA-MSC HANDOVER

Release ChannelReleased

• BASIC HAND-OVER

CALL IS HANDED OVER FROM HOME MSC

(MSC-A) TO A 2nd MSC (MSC-B)

• SUBSEQUENT HAND-OVER

1. CALLS IS HANDED OVER FROM THE 2nd MSC BACK TO THE HOME MSC.

2. CALL IS HANDED OVER FROM THE 2nd MSC TO ANOTHER MSC (3rd MSC)

1. MS is listening to it own BCCH & neighbouring BCCH.

2. Old BSS (BSS-A) determines that MS requires to be handover it will

send a Handover Required message to MSC-A, with a list of cells to which the MS can be handed over.

3. MSC-A send message to MSC-B for handover, this message contain

information to allocate radio channel and identity of cell, to which the call to be handed over.

4. MSC-B ask its VLR to allocate a (hrn.)

5. MSC-B retrieves the (hrn.) which will be used for routing the connection of the call to MSC-A.

Perform

Handover

HandoverRequired

MSBSS

A

1

2

3

Um EA

Signalling StrengthMeasurement

SACCH

AllocateHandoverNumber

4

5 HandoverNumber

Allocated (hrn)

MSCA

MSCB

VLRB

BSSB

A

6. If no traffic channel is available in MSC-B, it ask the BSS-B to allocates one

7. BSS-B return the allocated traffic channel (TN).

8. An ACK-message containing the new radio resources definition sent by MSC-B to MSC-A.

9. MSC-A asks MSC-B to establish a communication circuit.

10. MSC-B establishes a circuit with MSC-A using the signalling procedures supported by the N/W.

HO Request

Acknowledge (TN)

Perform HO Acknowledge

MSBSS

A

6

7

8

Um EA

Handover Request

Network Setup

9

10Setup

Complete

MSCA

MSCB

VLRB

BSSB

A

MS send handover access message (hrn)

BSS sends Handover Detect

message

MSBSS

A

10A

Handover Command

MS sent handover complete Message to BSS-B

HandoverComplete message

to MSC-B

MSCA

BSSA

VLRB

BSSB

Handover Command

10A MSC-A send handover command to Old BSS-A.

10B Old BSS-A send handover command to MS.

10C MS access the new radio resources by sending the message. Handover access containing hrn. The hrn is checked by new BSS to ensure it, as expected and the correct MS has been captured.

10D If this is correct the new BSS sends a handover detect message to MSC-B

10E When the MS is successfully communicating with BSS-B,it (MS) sends a handover complete message to BSS-B.

10F BSS-B forward the message to MSC-B.

10G MS is now served by BSS-B the trunk between MSC-A & BSS-A can be released.

10H Consequently the radio channel (between MS & BSS-A is released).

10B

10C

10D

10E

10E

10F

10H

Release TRK& Radio channel

MSBSS

AEA

HandoverCommand

11

12

MSCA

MSCB

VLRB

BSSB

A

HandoverCommandSACCH

MSBSS

AE Um A

Handover AccessSACCH

13

14

MSCA

MSCB

BSSB

A

HandoverDelected

Handover CompleteSACCH

15

16

HandoverComplete

17. MSC-B inform , MSC-A that connection can be ended.

18. MSC-A acknowledge the receipt of this message.

19. Link between MSC-A & BSS-A is released.

20. Radio Channel between BSS-A to MS is released.

Release Radio

Released

Send END Signal

MSBSS

A

17

E Um A MSCA

MSCB

BSSB

A

18 Answer

19

20 Radio Channel

Released

End Signal

SACCH

MSCA

21

E B

21. MSC-A inform MSC-B that the old resources used by the MS are released.

22. Network connection between MSC-A & MSC-B to released.

23. hrn between MSC-A & MSC-B is now needed and can be de-allocated.

VLRB

Network Release

SACCH

MSCB

22

23Release Handover

Number