11-Handover failure
leads to call dropped due to bad radio conditions from serving cell
NOTE: H.O failure [from system KPI point of view] means that you [MS] did sent H.O Access to new target H.O cell
but you didn't receive physical channel information so handover fails.
When you fail to perform H.O from your serving cell [old] to a better level neighbor target cell [new], so you make
ROC [Return to Old Channel] on your serving old cell to continue your call and you hear handover failure on TEMS,
this may occur due to 1 of the following reasons:
-Bad radio conditions [quality] or a H.W [hardware] problem on targeted H.O cell.
-Bad radio conditions [quality] on MS handset.
-Data link failure = Layer 2 failure.
1-Bad quality or a H.W problem on targeted H.O cell:
[Means that targeted H.O cell suffered from UL Up-Link interference ICM [Idle Channel Measurement external interference] at the time MS was sending its UL H.O access command to it, so targeted H.O cell couldn't decode MS UL message due to UL interference or a H.W problem on targeted cell.
As shown in the following snaps, H.O failure occurred due to target H.O cell C06063 BCCH: 718 suffer from UL interference from an external resource causing bad quality on targeted H.O cell so it couldn't decode the UL H.O access command sent by MS on UL due to interference and as so targeted H.O cell didn't sent the DL Physical channel information to MS.
2-Bad quality on MS handset:
[Means that targeted H.O cell suffered from DL Down-Link interference [Co-Adjacent channel interference] at the time new targeted H.O cell was sending its DL Physical channel information to MS, so MS couldn't decode targeted H.O cell DL message due to DL interference.
As shown in the following snaps, H.O failure occurred due to target H.O cell 12593 BCCH: 67 suffer from DL Cochannel interference on BCCH: 67 from a neighboring cell 6403 BCCH: 67 causing bad quality on MS handset so it couldn't decode the DL Physical channel information sent by target H.O cell 12593 on DL.
NOTE: Any H.O failure occurred due to [Protocol error – Physical channel failure – or any other messages, etc.] in layer 3 messages on TEMS is considered to be a H.O failure due to a 1 of the 2 radio problems that we have just mentioned.
3-Data link failure:
MS connection with the mobile network is established upon layers connection, so in order for MS to establish connection with the new target H.O cell to complete handover process, MS should establish a transmission layer 2 [Data link layer] with the new BTS. When MS fails to any reason to establish layer 2 with new BTS, H.O fails and MS returns to old serving BTS ROC and you hear H.O failure on TEMS.
NOTE: Any H.O failure occurred due to [Data link failure- Layer 2 failure- N200.T200 timer expiry] in layer 3
messages on TEMS is considered to be a H.O failure due to a transmission problem which is not RF [Radio]
problem.
NOTE: Bad radio conditions [quality] on serving cell [old] in H.O process is not a reason for handover failure from
network KPI point of view!!!! HOW???
Handover is a process between all 3 of:
1- Serving cell. 2-Mobile station. 3-Target H.O cell.
Where H.O Scenario on TEMS is:
1-H.O command: Sent on DL FACCH from BSC to MS containing info. About new target H.O cell.
2-H.O Access: Sent on UL FACCH from MS to new target H.O cell requesting a TCH to continue call when handover
occurs.
3-Physical channel information: Sent on DL FACCH from targeted H.O cell to MS informing the MS about the new
hold TCH to take it.
4-H.O complete: After H.O process completed successfully MS sent H.O complete to BSC through target new H.O
cell to inform that H.O is completed.
So, from network KPI point of view:
H.O is considered to be failed when MS send H.O Access in UL to new target cell and didn't receive physical channel information from it in DL due to any of the 2 before possible reasons. But from TEMS point of view:
H.O is considered to be failed when any of the following happens;
1-BSC sends H.O command in DL to MS and didn't receive H.O complete message in UL from MS within timer
T3103, this issue is considered as a H.O drop or call drop in the system not H.O failure.
2- MS send H.O Access in UL to new target cell and didn't receive physical channel information from it in DL within
timer T3124, this issue is considered as a H.O failure on both TEMS and network.
3-Target H.O cell sends physical channel information in DL to MS and didn't receive SABM message to establish data link [layer 2] in UL from MS within timer T3105, this issue is considered as a H.O drop or call drop in the system not H.O failure.
So, sometimes TEMS counts events as H.O failures however these events are not counted as failures in network KPI's but they are counted as H.O drops or call drops, but we normally analysis these types of events however they are not counted as H.O failures from system point of view, that's why sometimes in reports you find H.O failures due to bad radio conditions on serving cell whether level or quality where MS couldn't decode the DL H.O command sent by BSC on the serving old cell due to the bad radio conditions between your MS and the network back then.
So we will add a 4th reason for H.O failures however it is not correct as these failures are not counted as H.O failures in the system but as H.O drops.
-Bad radio conditions [quality] or H.W [Hardware] problem on serving cell.
4-Bad quality or a H.W problem on serving cell:
[Means that serving cell suffered from bad radio conditions [level or quality] from DL Down-Link interference [Co-Adjacent channel interference] at the time it was sending BSC's DL H.O command to MS, so MS couldn't decode serving cell DL message due to DL interference. As shown in the following snaps, H.O failure occurred due to serving cell 19101 BCCH: 79 suffer from bad radio conditions causing bad quality on MS handset so it couldn't decode the DL H.O command sent by BSC on serving cell.
NOTE: As a conclusion, H.O failure [from system KPI point of view] means that you did sent H.O Access to new target H.O cell but you didn't receive physical channel information so handover fails and MS ROC to old BTS and send H.O failure as an indication to the network.
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Prepared by:
Eng. Abdelrahman Ashraf Mostafa
abdelrahman_ashraf@hotmail.com