Feature #3277

handover: for intra-cell HO, use RR Assignment Command, not RR Handover Command

Added by neels 8 months ago. Updated 2 months ago.

Target version:
Start date:
Due date:
% Done:


Spec Reference:


Handover Decision algorithms may decide to do a re-assignment within the same cell.
In practical tests, some time ago, I noticed that using the RR Assignment Command wasn't working here.
Hence we currently send an RR Handover Command referencing the same cell, which is kind of weird.

Now I saw one reason why Assignment doesn't work for handover: in bsc_api.c, in handle_ass_compl(),
we receive the RR Assignment Complete message and dispatch an SS_LCHAN signal S_LCHAN_ASSIGNMENT_COMPL.
However, throughout osmo-bsc, there is not a single handler for S_LCHAN_ASSIGNMENT_COMPL.

This might be another casualty from the split-off from osmo-nitb.

Anyway, it doesn't make sense to me that we send an S_LCHAN_ASSIGNMENT_COMPL,
while the remaining code triggers bsc_subscr_conn_fsm.c GSCON_EV_* events directly.

Fix: Actually handle RR Assignment Complete during handover,
and send RR Assignment Command instead of RR Handover Command for intra-cell HO.

This is not high priority, and should wait until after intra-BSC handover has rippled through the lchan allocation code.

Related issues

Has duplicate OsmoBSC - Feature #2949: use RR Assignment for Handover within a cellNew2018-02-16


#1 Updated by neels 8 months ago

  • Blocked by Bug #2283: Inter-BSC hand-over is missing (BSC side) added

#2 Updated by neels 5 months ago

The new FSM implementations have been merged to osmo-bsc master, but we are still using an RR Handover Command to re-assign an lchan within the same cell; it works ok, but an RR Assignment Command would make more sense. The code is now ready to change this and use the Assignment FSM (?) or use RR Assignment Command from handover FSM (?) for handovers where source and target cells are identical.

#3 Updated by neels 5 months ago

  • Blocked by deleted (Bug #2283: Inter-BSC hand-over is missing (BSC side))

#4 Updated by neels 5 months ago

  • Has duplicate Feature #2949: use RR Assignment for Handover within a cell added

#5 Updated by neels 5 months ago

  • Tracker changed from Bug to Feature
  • Subject changed from handover: intra-cell HO using Assignment cannot possibly work, "RR Assignment Complete" is factually ignored to handover: for intra-cell HO, use RR Assignment Command, not RR Handover Command

#6 Updated by neels 2 months ago

actually, now I found a spec snippet that might be interpreted to suggest to rather use the Handover Command:


Internal Intra-Cell Handover Procedure
The definition of internal intra cell handover is given in sub-clause 5.
It is optional that a BSS support internal intra-cell handover. However if it is supported, it should be as follows:
It should be possible to inhibit internal intra-cell handover at an BSS that supports it by operation and maintenance
Internal intra-cell handover occurs between channels on the same cell. It is decided and executed autonomously by the
BSS, so that no message is generated at the BSS-MSC interface, until the completion of the handover execution, when
the BSS sends a HANDOVER PERFORMED message over the SCCP and terrestrial resources that are presently
assigned to that call. Changes in type of resources (for instance channel rate change, speech version change, ciphering
algorithm change) are indicated in the HANDOVER PERFORMED message.

Compare to describing the successful Assignment operation:

If an intra-BSS cell change has
occurred during the assignment, the new cell identity is included in the ASSIGNMENT COMPLETE message and a
HANDOVER PERFORMED message is not required.

The first snippet suggests that a Handover procedure using a Handover Command does exist for changing lchans within the same cell.
The second snippet suggests that during Assignment, no Handover Performed message is needed; but at the same time, the second snippet suggests that even during an Assignment, the BSS may decide to change the cell.

In summary, it seems to me that both are possible and we can choose not to bother and just use RR Handover Command.

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