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

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.

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

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  1. 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.

  2. - 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.

  3. 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.

  4. Um A MS BSC INTRA-BSS INTER BTS HANDOVER BSS Signal Strength Measurement 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. 1 Handover Command SACCH 2 Handover Access SACCH 3 Handover Complete SACCH 4 Handover Performed 5

  5. INTRA-MSC HANDOVER Um A OLD BSS A MS MSC New BSS Signal Strength Measurement SACCH 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. 1 Handover Required 2 Handover Requested 3 Handover Requested Acknowledge (hrn.) 4 Handover command (hrn) 5 Handover command (hrn) SACCH 6

  6. INTRA-MSC HANDOVER Um A OLD BSS A MS MSC New BSS Handover Access 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. 7 Handover Detected 8 Handover Complete SACCH 9 10 Handover Complete Release Radio Channel 11 Release Channel Released 12

  7. 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)

  8. BSS A MSC A MSC B BSS B VLR B Um A E A MS Signalling Strength Measurement SACCH 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. 1 Handover Required 2 Perform Handover 3 Allocate Handover Number 4 Handover Number Allocated (hrn) 5

  9. BSS A MSC A MSC B BSS B VLR B Um A E A MS Handover Request 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. 6 HO Request Acknowledge (TN) 7 Perform HO Acknowledge 8 Network Setup 9 Setup Complete 10

  10. BSS A MSC A BSS A BSS B VLR B MS 10A 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). Handover Command 10B MS send handover access message (hrn) 10C BSS sends Handover Detect message 10D 10E MS sent handover complete Message to BSS-B Handover Complete message to MSC-B 10E Release TRK & Radio channel 10F 10H

  11. BSS A MSC A MSC B BSS B VLR B A E A MS Handover Command 11 Handover Command SACCH 12

  12. BSS A MSC A MSC B BSS B Um A E A MS Handover Access SACCH 13 Handover Delected 14 Handover Complete SACCH 15 Handover Complete 16

  13. BSS A MSC A MSC B BSS B Um A E A MS Send END Signal 17 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. Answer 18 Release Radio Released 19 Radio Channel Released 20

  14. E B MSC B MSC A VLR B End Signal SACCH 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. 21 Network Release SACCH 22 Release Handover Number 23

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