HANDOVER FAILURE DUE TO CONGESTION

[MCBEP]

Hello,
Does it possible that handover failure due to congested cell? It seems strange to me because as i know BSC/RNC decides to handover and before sending handover command to relevant cell ,it reserves required resource for incoming mobile. So if handover command comes , it means resource already reserved.So how can be possible handover failure due to capacity?Which point am I missing?

Best regards.

[khurrambilal01]

Yes it is possible, Because once BSC decides that handover is going to take place and sends HO request, till that time BSC didn't check whether or not resources on target cells are available or not, Once BSC checks the resources on target side & if congestion is found then it is HO failure.

[MCBEP]

Yes it is possible, Because once BSC decides that handover is going to take place and sends HO request, till that time BSC didn't check whether or not resources on target cells are available or not, Once BSC checks the resources on target side & if congestion is found then it is HO failure.

But in this document , it explains like this

The successful operation of the procedure is as follows. When the

BSS (BSS-A), currently su
pporting the
MS, determines that the MS requires to be handed over it w
ill send an A-HANDOVER-REQUIRED
message to the MSC (MSC-A). The A-HANDOVER-REQUIRED message shall contain a list of cells, or a
single cell, to which the MS can be handed over. The list of cells shall be given in order of preference
based upon operator determined criteria (These criteria are not addressed within this technical
specification and are operator dependent). When the MSC-A receives the A-HANDOVER-REQUIRED
message it shall begin the process of handing over the MS to a new
BSS (BSS-B). (NOTE: BSS-A and
BSS-B maybe the same BSS). The MSC-A shall generate an A-HANDOVER-REQUEST mess
age to the
selected BSS (BSS-B). When BSS-B receives the A-HANDOVER-REQUEST mess
age it shall take the
necessary action to allow the MS to access the radio resource of BSS-B, this is detailed in GSM 08.58
and the GSM 05 series of Technical Specifications. The switching of the radio resource through the
necessary terrestrial resources is detailed in GSM 04.08 and GSM 08.08.
Once resource allocation has been completed by

BSS-B it shall return an A-HANDOVER-REQUEST-ACK.
to MSC-A. When this message is received by MSC-A it shall begin the process of instructing the MS to
tune to a new dedicated radio resource. An A-HANDOVER-COMMAND w
ill be sent by the MSC-A to BSS-
A. On receipt of the A-HANDOVER-COMMAND message
BSS-A will send the radio interface mess
age RI-
HANDOVER-COMMAND, containing a Handover Reference number previously allocated by BSS-B, to the
MS. The MS will then access the new radio resource using the Handover Reference number contained in
the RI-HANDOVER-ACCESS mess
age. The number w
ill be checked by BSS-B to ensure it is as expected
and the correct MS has been captured. If this is the correct MS then the
BSS-B shall send an A-
HANDOVER-DETECT to MSC-A. When the MS is successfully communicating with the BSS-B a RI-
HANDOVER-COMPLETE message w
ill be sent by the MS to BSS-B. The BSS-B will then send an A-
HANDOVER-COMPLETE message to MSC-A

http://www.etsi.org/deliver/etsi_gts/03/0309/05.01.00_60/gsmts_0309v050100p.pdf

[MCBEP]

Any idea? Which conditions may trigger this kind of handover failure? I couldnt find any relation between site load and handover failure.

[Syed Fahad]

Well, If BSC is unable to secure TCH Resource in the Target Cell due to congestion, the counter of HO Attempts in not incremented in HO Statistics. The difference of HO Requests and HO Attempts will give you the "HO TCH Block Times". So as per my humble opinion it will a" Failure to perform a HO" due to Capacity Issue in Target Cell, but not a HO Failure due to Quality Related Issues, highlighted in Post Processing tools. If your HSR Formula Takes HO Request in Denominator, than HO Blocking will impact the HSR, but incase HO Attempts are used in Denominator, HO Blocking is not Impacting.

[MCBEP]

Well, If BSC is unable to secure TCH Resource in the Target Cell due to congestion, the counter of HO Attempts in not incremented in HO Statistics. The difference of HO Requests and HO Attempts will give you the "HO TCH Block Times". So as per my humble opinion it will a" Failure to perform a HO" due to Capacity Issue in Target Cell, but not a HO Failure due to Quality Related Issues, highlighted in Post Processing tools. If your HSR Formula Takes HO Request in Denominator, than HO Blocking will impact the HSR, but incase HO Attempts are used in Denominator, HO Blocking is not Impacting.
Thank you for answer and opinion.
As i understand this scenario happens between BSC and MSC. Bsc decides to handover from serving cell to target cell and ask resources from Msc for the target cell but if msc gives not ok to bsc bsc will never send to MS/UE handover command.Because as i know there is not any MS originated handover request between MS/UE and BSC .So i can not see your scenario in drive test equipment.
So if i see handover failure in DRIVE TEST EQUIPMENT, Can i say "it can not related with lack of time slot resource". Because if handover command comes it means resources allocated already by bsc.
What do you think?

[Syed Fahad]

Dear, Kindly note that MSC is only involved in Inter BSC HOs, Inter MSC HO's, and IRAT HO's if I am not missing any thing. However for Intra BSC Internal HO's, BSC is the sole entity for checking TCH Congestion on the target cells. Please, its not the MSC, as in case of internal BSC HO's, MSC is just informed at the end of HO by the BSC, by sending a HO Completion Message to MSC. You can Google the Signaling Flow from Google as it will clear out every thing. Drive Test Equipment like TEMS only shows the HO Failures after the HO Attempt from MS towards target cell, so you cannot judge HO Failures due to congestion in TEMS.

[MCBEP]

Dear, Kindly note that MSC is only involved in Inter BSC HOs, Inter MSC HO's, and IRAT HO's if I am not missing any thing. However for Intra BSC Internal HO's, BSC is the sole entity for checking TCH Congestion on the target cells. Please, its not the MSC, as in case of internal BSC HO's, MSC is just informed at the end of HO by the BSC, by sending a HO Completion Message to BSC. You can Google the Signaling Flow from Google as it will clear out every thing. Drive Test Equipment like TEMS only shows the HO Failures after the HO Attempt from MS towards target cell, so you cannot judge HO Failures due to congestion in TEMS.

Ok , let me explain our case again. We are performing drive test with test equipment and our MS get HANDOVER COMMAND from cell/Bsc so than handover failure comes. So if handover command send to ms by bsc, it means that all resources are ready , and if it goes to failure after handover command, we can say it could not happen due to target cell congestion? What do you think?

I understand from your explanations that you are explaining handover steps which on going in BSC part , bsc is request target cell resources and if it can not find or reserve resource it dont make attempt also. So if not attemt i can understand that bsc ill never send to ms handover command... So if i see from test equipment handover command, it means that bsc requested for resource allocation and secure it than send to ms for starting handover process...m i right?

[Syed Fahad]

Now we are talking:D, yes in TEMS, HO Failures observed are the ones after MS Attempt to the target and there is no Congestion on Target. You are also right by the fact that when there is HO Congestion on the target , BSC Never send the HO Command to MS for making a HO Attempt.