Cellular Network Infrastructure » Radio Access Network » OsmoBSC

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.

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

48.008

3.1.6
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
command.
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 3.1.1.1 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.

• the TDMA timing of the old and new channel are identical (guaranteed within one BTS, but implementation option if other BTS have synchronized TDMA clocks
• the timing advance to be used when establishing the new channel (known within same BTS as it is identical to the old; typically not known for other BTSs unless you happen to be handing over between two sectors of the same site and hence the distance is identical)

For all other cases, HANDOVER must be performed, because the MS will re-sync to the frame clock and send access bursts to establish the timing advance.

Implemented in
commit 69def1f97e427fa65d242f21110f01cb5d0a2818
hodec 2: do intra-cell congestion resolution by Assignment
Change-Id: Id56a890106b93fcee67ac9401b890e7b63bba421