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Bug #4976

network with no emergency calls - Is BSC still performing pre-emption?

Added by laforge about 1 month ago. Updated about 1 month ago.

Status:
Resolved
Priority:
High
Assignee:
Category:
-
Target version:
-
Start date:
01/26/2021
Due date:
% Done:

100%

Spec Reference:

Description

It is suspected by a user that the following events may take place:
  • netwokr does not permit/enable emergency calls
  • sometimes, due to the bad RACH correlation properties, fals positive RACHs are detected
  • some of those fall within the "emergency" category
  • osmo-bsc is still pre-empting/kickning out some other channel

Based on a quick look of the code, this might be possible.

In fact, I don't understand why we even bother to enqueue a RACH request in this situation. IMHO, the check should be done before enqueueing the RACH request to bts->chan_rqd_queue if it is for RACH and we don't support RACH to begin with? But I'm not an expert on the related code.

Please investigate.

Associated revisions

Revision 80184ae1 (diff)
Added by dexter about 1 month ago

abis_rsl: check if emergency calling is disabled before premption

If an emergency call arrives at the BSC while all TCH are busy, one TCH
is cleared in favor of the emergency call. However, if emergency calls
are disabled (system information), it is still possible that an MS might
try an emergency call anyway or that due to interference a regular call
might look like an emergency call (channel request reason). In those
cases the preemption must not happen and the emergency call must be
rejected.

Change-Id: I1af1f4fefcbe6a886bb5396901ce0cb2368a0e19
Related: OS#4976

History

#1 Updated by laforge about 1 month ago

#2 Updated by dexter about 1 month ago

  • Status changed from New to In Progress

#3 Updated by dexter about 1 month ago

  • % Done changed from 0 to 80

I have checked this now back and its indeed the case that the preemption takes place even though emergency calls are disabled. I have fixed that now to check earlier if emergency calls are disabled. If an emergency call is detected even though emergency calling is disabled we must reject that call.

See also: https://gerrit.osmocom.org/c/osmo-bsc/+/22455 abis_rsl: check if emergency calling is disabled before preemption

#4 Updated by dexter about 1 month ago

I have moved up the check further up before the enqueuing of the request. This shouldn't make much of a difference, but it makes more sense.

#5 Updated by laforge about 1 month ago

  • Status changed from In Progress to Resolved
  • % Done changed from 80 to 100

dexter wrote:

I have moved up the check further up before the enqueuing of the request.

thanks, patch merged.

This shouldn't make much of a difference, ...

tell that to the callers whose calls get interrupted for no reason :/

On a busy network without emergency calls I would expect this to happen relatively often.

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