While doing some background reading I stumbled over the following optional Mobile Terminated Call procedure for a race condition:
The scenario: Just when the mobile network receives an incoming call for a user, the user's mobile changes to a cell which is controlled by a different mobile switching center. This results in a race condition, i.e. the previous MSC receives the call while the mobile is already performing a location update via the new MSC. If this is not treated, the mobile will not see the paging in the old cell and the call establishment fails.
This is where the "Mobile Terminating Roaming Retry Call" feature comes into play: If implemented, the previous MSC which has sent out the paging message to contact the mobile is informed of the location update by a "Cancel Location" message from the HLR. This is standard practice so far. However, instead of failing the paging procedure, e.g. after a timeout, the Cancel Location message is used as a trigger to signal to the Gateway MSC that the subscriber is no longer with this MSC. The Gateway MSC then releases the speech path to the previous MSC, runs another subscriber location search with the Home Location Register and then forwards the call to the new MSC. All quite elegant.
For details see 3GPP TS 23.018, chapter 5.2.1
I wonder, if this feature is widely implemented and used today? If you know, please let me know.
One thought on “Mobile Terminating Roaming Retry Call”
As far as I know it isn’t widely used today. Used more often in a CDMA network is a function called Inter-System Page. If the previous MSC last saw the mobile near a handoff border, the ISPAGE logic is triggered to send a page across the SS7 network to all neighboring MSCs and they will page the mobile. If the mobile responds on a different MSC, the call is bridged between the previous and current MSC in the same manner that a hard-handoff is done using IS-41.
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