1)Summary on the Error indication (ERRIND) Abis interface message seen in Xian.

Two main cases of drop call condition during a failed Handover and failure to return on the old traffic channel.

Abnormal "ERRIND" found on Abis after HOCMD on Abis, as below:
Case 1
Abis A interface
HOCMD<-
HOCMD<-
ERRIND-> T200 expired (N200+1) times
CHNREL<-

Case 2
Abis A interface
HOCMD<-
HOCMD<-
ERRIND->re-establishment request (failed)
CLRREQ-> (T3103 expiry)
CHNREL<-


These two cases have normal signalling and ERRIND messages are supported by the GSM recommendations for such Data Link Layer 2 LAPDm protocol error due to Layer 1 errors such as RF UL/DL Quality issues.

All other cases of Error Indication messaging during the handover procedure did not lead to any call drop but:
-prevented the call to drop by returning to the old channel,
-helped to finally perform the successful handover after a delay of few 100 ms

-
2)Trace samples

TRACE 1: HANDOVER FAILURE: T200 EXPIRY BEFORE T3103 EXPIRY, CALL DROP

During the handover command, the target cell (not seen in the trace) was not accepting the MS handover access message. There was possibly as well no possible return on old channel on the source BTS.

This could be due to some bad UL radio quality or level which could be confirmed by decoding the MS TEMS traces. In those condition the mobile stop transmitting any frame to the BTS the Layer 1 link is release as well.

The BSC does not know the real state of the mobile. This release is based on the fact that if an MS does not receive the SACCH block for some time, it completely releases the channel.

The handover was not aborted by the T3103 expiry (usually 5s but in some Xian area at 9s or 12s) for the BSC to send a CLEAR REQUEST message to the MSC to release the radio resource.

The call was droppedafter the expiry of the T200 timer (N200+1) times as it should.
The BSC acknowledged the BTS Error Indication message by sending the DATA REQUEST CHANNEL RELEASE with cause: abnormal release, timer expired.

No COMPASS TOOL trace available in Excel report.

Here is an MA10 trace sample:

> 1- 1 0RSL32 6 DREQCONNECTProgress Inde> G16:03:12.809
< 1- 1 0RSL32 6 DINDCONNECT ACKG16:03:12.953
> 1- 1 0RSL32 6 DREQHANDO COMMANDCell Descr:NCC:> G16:03:14.813
< 1- 1 0RSL32 6 ERROR INDICATION Bm+ACCH Link Id:S> G16:03:21.977
> 1- 1 0RSL32 6 DREQCHAN RELEASERR Cause:timer > G16:03:24.008
> 1- 1 0RSL32 6 DEACTIVATE SACCH Bm+ACCHG16:03:24.026
< 1- 1 0RSL32 6 SACCH INFO MODIFY G16:03:24.050
> 1- 1 0RSL32 6 RF CHANNEL RELEASEBm+ACCHG16:03:37.006

Error Indication RLM Cause is "1 timer T200 expired (N200+1) times".


Trace1: more detail on the timer and L2 message involved:




TRACE 2: HANDOVER FAILURE: TIMER T3103, CALL DROP

During the handover command, the target cell (not seen in the trace) was not accepting the MS handover access message. There was by then a return on the old traffic channel initiated by the first Error Indication (455018) message getting for RLM cause “re-establishment request”. The five other Error Indication message (455125, 455199, 455300, 455402, 455461) are getting the same RLM cause “SABM command, multiple frame established state” which traduced the mobile initiate several SABM command messages to re-establish the Data Link Layer 2 but with no DL acknowledgement UA from the BTS. This is likely due to some bad DL radio quality or level which could be confirmed by decoding the measurement report on the source cell. In those condition the mobile stop transmitting any frame to the BTS the Layer 1 link is release as well.

The BSC does not know the real state of the mobile. This release is based on the fact that if an MS does not receive the SACCH block for some time, it completely releases the channel.

The handover was then stopped by the T3103 expiry (after 5s) when the BSC send a CLEAR REQUEST message to the MSC to release the radio resource.






Trace 2: more detail on the timer and L2 message involved:



TRACE 3: HANDOVER FAILURE: RETURN ON OLD CHANNEL, NO CALL DROP.

The target cell could not be accessed by the mobile which return on the old channel. The Error Indication “re-establishment request” is by then send by the BTS.
After the T3124 expiry the MS was then able to inform the source BTS of the handover failure on the target BTS.

The target BTS where no trace is available shall send a Connection Failure message to the BSC with cause value ”handover access failure”. This happen in case the Physical Information transmitted by the target BTS to the MS has been repeated Ny1 times without a correctly decoded frame being received from the MS (see case “Bad DL on the target BTS” in the “Trace 3: more detail on the timer and L2 message involved”).

The call was not dropped and maintained for a next possible handover.










Trace 3: more detail on the timer and L2 message involved:
Case of bad UL on target BTS:


Case of bad DL on target BTS:






TRACE 4: ERROR INDICATION “SABM COMMAND, MULTIPLE FRAME ESTABLISHED STATE” RECEIVED DURING HANDOVER.

This RLM indication is showing some DL communication problem on the target cell for establishing the data link layer correctly. An UL message from the MS is received again by the BTS while the DL BTS previous UA acknowledgement was not received correctly by the MS.

If such bad DL condition was continuing, the MS timer T3124 will expires and trigger a return on old channel.







Trace 4: more detail on the timer and L2 message involved











3)Error indication message information
In BSC Traffic Management terms, a radio link failure can be detected in several ways:
* by analysis of the reception at layer 1;
* by a data link layer failure on the data link SAPI 0.

Reception at LAYER 1
The BTS indicates this event to the BSC with a CONNECTION FAILURE INDICATION message.

Data link LAYER failure (LAYER 2)
Data link failure is detected at the BSC by receipt of an ERROR INDICATION message from BTS. This message notifies that an error has occured in the LAPDm protocol, which cannot be corrected by the data link layer. The associated parameter RLM CAUSE contains the error code.

RLM Cause
This element is used to indicate the precise protocol error or the reason for a release on the radio link layer within an ERROR INDICATION message:

The Cause Value field is coded as follows:
8 7 6 5 4 3 2 1
0 0 0 0 0 0 0 0 reserved
0 0 0 0 0 0 0 1 timer T200 expired (N200+1) times
0 0 0 0 0 0 1 0 re-establishment request
0 0 0 0 0 0 1 1 unsolicited UA response
0 0 0 0 0 1 0 0 unsolicited DM response
0 0 0 0 0 1 0 1 unsolicited DM response, multiple frame established state
0 0 0 0 0 1 1 0 unsolicited supervisory response
0 0 0 0 0 1 1 1 sequence error
0 0 0 0 1 0 0 0 U-frame with incorrect parameters
0 0 0 0 1 0 0 1 S-frame with incorrect parameters
0 0 0 0 1 0 1 0 I-frame with incorrect use ofMbit
0 0 0 0 1 0 1 1 I-frame with incorrect length
0 0 0 0 1 1 0 0 frame not implemented
0 0 0 0 1 1 0 1 SABM command, multiple frame established state
0 0 0 0 1 1 1 0 SABM frame with information not allowed in this state
All other values are reserved for future use.


From the Xian network Abis COMPASS tool trace samples provided and explain latter in the document, it has been noticed the following occurrences of RLM causes:
RLM Cause 1: timer T200 expired (N200+1) times
RLM Cause 2: re-establishment request
RLM Cause13: SABM command, multiple frame established state

The document will at first explain the COMPASS tool trace samples and those 3 main RLM causes.
These are the main occurrences as well noticed latter from the MA10 traces recorded over one hour:



Other RLM causes found, just traduced badly formated frames detected by the Data Link Layer due to Layer 1 transmission problems.


4)Error indication information from the Nortel BSS documentation:




5)Handover Failure GSM specification extracts:

Draft ETSI EN 300 940 V7.7.0 (2000-05) 68 (GSM 04.08 version 7.7.0 Release 1998)
On the mobile station side, if timer T3124 times out (only in the non- synchronized case) or if a lower layer failure happens on the new channel before the HANDOVER COMPLETE message has been sent, the mobile station deactivates the new channels, reactivates the old channels, reconnects the TCH if any and triggers the establishment of the main signaling link. It then sends a HANDOVER FAILURE message on the main signaling link and resumes normal operation as if no handover attempt had occurred. The operational parameters (e.g. ciphering mode) when returning on the old channel are those applied before the HANDOVER COMMAND message was received.
When the HANDOVER FAILURE message has been received, the network releases the new channels if they were dedicated channels and stops timers T3105 and stops T3103 in the non-synchronized case.


Handover detection
This procedure is used between the target BTS and BSC when a handed over MS accesses the new BTS.In case of an asynchronous handover, BTS builds the PHYsical INFOrmation message as specified in GSM 04.08, sends the message to MS in unacknowledged mode on the main signalling link and starts timer T3105. A HANDOver DETection message is sent to BSC. This message contains the measured delay of the access burst. If the timer expires before the reception of a correctly decoded frame from MS, BTS repeats the PHYSical INFOrmation message to MS as specified in GSM 04.08. If the PHYsical INFOrmation message has been repeated Ny1 times without a correctly decoded frame being received from MS, the BTS shall send a CONNECTION FAILURE message to BSC with the
cause value "handover access failure".

GSM 04.08 version 7.0.0 Release 1998 65
Abnormal cases
On the mobile station side, if timer T3124 times out (only in the non- synchronized case) or if a lower layer failure happens on the new channel before the HANDOVER COMPLETE message has been sent, the mobile station deactivates the new channels, reactivates the old channels, reconnects the TCHs if any and triggers the establishmentof the main signalling link


ETSI EN 300 938 V7.1.1 (2000-04) 39 (GSM 04.06 version 7.1.1 Release 1998)
Procedure after returning to the old channel (MS only)
This procedure is initiated by the DL-RECONNECT primitive. A data link layer entity (MS indeed) shall initiate a request for the multiple frame operation by transmitting the SABM command.

The DL-RECONNECT primitive is used in the mobile station by the radio resource management entity to restore multiple frame operation on the old channel after failure of the channel change.




6)Timer definition and values
T3105
Time between two PHYSICAL INFO on the radio.
Set to [60 ms] for TCH and [220 ms] for SDCCH.

Ny1
Number of PHYSCAL INFO transmitted on the radio.
Set to [16] for TCH and [4] for SDCCH.

T3124
This timer is used in the seizure procedure during a hand-over, when the two cells are not synchronized. Its purpose is to detect the lack of answer from the network to the special signal.
Its value is set to 675 ms if the channel type of the channel allocated in the HANDOVER
COMMAND is an SDCCH (+ SACCH); otherwise its value is set to 320 ms.

The mobile station starts timer T3124 at the start point of the timeslot in which the HANDOVER ACCESS message is sent the first time on the main DCCH.
When the mobile station receives a PHYSICAL INFORMATION message, it stops timer T3124

Maximum number of retransmissions (N200)
For SAPI=0 and 3, the maximum number of retransmissions N200 depends on the state and on the channel used. This ensures a common time value for layer 2 link failure on all channels when multiple frame operation is established. The N200 value for layer 2 link establishment and release is 5 (as an example).
In the state "timer recovery", N200 is set to:
- 5 for use on SACCH; 5*300=1500ms for the timeout on the link establishment
- 23 for use on SDCCH;
- 34 for use on FACCH/full rate;34*300ms=10200ms (for the timeout on the FACCH full rate)
- 29 for use on FACCH/half rate.

Nortel is implementing (and could be modified by re-building the BSC BDA):
-T200 RSL : 300ms
-N200 RSL : 3

This lead to 11700ms in case of an RSL window of 5 (but we have 3), by then we could possibly deduced that the timeout is 11700*3/5=7020ms=7s.

t3103
BSC timer triggered during the handover procedure.
It is set on transmission of HANDOVER COMMAND by the BSC and canceled on receipt of either HANDOVER COMPLETE or HANDOVER FAILURE sent by the MS
(intra-bss handover), or CLEAR COMMAND sent by the MSC (inter-bss handover). At expiry of T3103, the channel is released.
Range value : [2 to 255] seconds (t3103 < bssMapT8)
Object : bts
Default value : 5 seconds