11.1 Restart of the SGSN
After an SGSN restart, the SGSN deletes all MM and PDP contexts affected by the restart. SGSN storage of data is volatile except as specified in this subclause. The SGSN maintains in volatile memory a GGSN Restart counter for each GGSN with which the SGSN is in contact, and in non-volatile memory an SGSN Restart counter. The SGSN Restart counter shall be incremented and all the GGSN Restart counters cleared immediately after the SGSN has restarted.
The GGSN performs a polling function (echo request and echo response) towards the SGSNs with which the GGSN is in contact. The SGSN Restart counter shall be included in the echo response. If the value received in the GGSN differs from the one stored for that SGSN, the GGSN will consider that the SGSN has restarted (see GSM 09.60). The GGSN Restart counters shall be updated in the SGSN to the value received in the first echo message coming from each GGSN after the SGSN has restarted.
When the GGSN detects a restart in an SGSN with which it has any PDP context activated, it shall delete all these PDP contexts. Also, the new value of the SGSN Restart counter received in the echo response from the SGSN restarted shall be updated in the GGSN.
11.2 Restoration Procedures
11.2.1 Mobile terminated user data transmission
When the SGSN receives a tunnel PDU for which no PDP context exists, it discards the tunnel PDU and sends an Error indication message to the originating GGSN. The SGSN may search for the MS by paging with the IMSI in the SGSN area if there is no MM context for the MS. An MS that is paged with the IMSI performs a GPRS attach procedure.
11.2.2 Mobile terminated services requested by the MSC/VLR
When the SGSN receives a request for CS paging from an MSC/VLR for an IMSI unknown by the SGSN, if the “SGSN-Reset” indicator is set to “true”, the SGSN sends the paging request with the location information provided by the VLR. If no such location information is provided, the SGSN should page for the MS in all the routeing areas corresponding to that SGSN.
If the “SGSN-Reset” indicator is set to “false” and the IMSI is unknown or the MS is marked as GPRS or non-GPRS detached by the SGSN, the paging request is rejected.
If the “SGSN-Reset” indicator is set to “false” and the IMSI is known and the MS is marked as GPRS and is non-GPRS attached by the SGSN, the paging request shall be sent to the MS.
11.2.3 Mobile terminated SMS over GPRS
a) Send Routing Information for MT SMS (SMS-GMSC -> HLR)
The HLR returns the SGSN number as for normal operation.
b) Send Information for MT SMS
When the SGSN receives a mobile terminated SMS for an unknown MM context for the MS, or if the SGSN indicator "Subscriber Data Confirmed by HLR" is marked “Not Confirmed” it rejects the SMS request and returns a failure report with cause value “Unidentified Subscriber” to the SMS gateway MSC indicating unsuccessful delivery of the SMS. The Gateway MSC sends a "Report SM Delivery Status" request, with a cause of "Absent Subscriber", to the HLR. This causes the HLR to set the "Mobile Station Not Reachable for GPRS Flag" for the MS, as described in the Technical Specifications GSM 03.40 and GSM 09.02.
If the SGSN has the indicator "Subscriber Data Confirmed by HLR" set to “Confirmed”, the SGSN handles the SMS request in the normal way.
The state of the indicator "Location Information Confirmed in HLR" does not affect the Mobile Terminated SMS procedure.
11.2.4 Mobile originated Routeing Area Updating or Attach
If the MS is unknown in the SGSN (i.e. the SGSN has no MM context for the MS) the SGSN creates an MM context for the MS and sets the indicators "Location Information Confirmed in HLR" and "Subscriber Data Confirmed by HLR" to "Not Confirmed". If authentication is required, the SGSN retrieves authentication data. The SGSN then performs an "Update GPRS Location" to the HLR. If this is successful, the SGSN sets the indicators "Location Information Confirmed in HLR" and "Subscriber Data Confirmed by HLR" to "Confirmed".
If the SGSN has an MM context for the MS, and the indicators "Location Information Confirmed in HLR" or "Subscriber Data Confirmed by HLR" is set to “Not Confirmed” the SGSN performs an "Update GPRS Location" to the HLR. If this is successful, the SGSN sets the indicators "Location Information Confirmed in HLR" and "Subscriber Data Confirmed by HLR" to "Confirmed".
If the SGSN has an MM context for the MS with the indicator "Subscriber Data Confirmed by HLR" marked “Confirmed” the originated transmission is handled in the normal way.
The SGSN retrieves subscriber data from the HLR by sending an “Update GPRS Location” request, which triggers one or more "Insert Subscriber Data" operations from the HLR.
11.2.5 Mobile originated LLC frame
If an SGSN receives an LLC frame for which no MM context exists in the SGSN, and if the LLC frame does not contain an Attach Request or a Routeing Area Update Request signalling message, then the LLC frame shall be discarded. The MS may determine that the network is not responding and attempt to re-attach or eventually a periodic Routing Area Update message is sent by the MS which initiates the attach procedures.
11.3 Use of TLLI
After the SGSN has restarted but before the next authenticated radio contact the P-TMSI and TLLI known by the MS are invalid, as the P-TMSI was allocated before the SGSN restarted. The SGSN may request the MS to identify itself with the IMSI in order to make a relationship between the IMSI and the received old TLLI. The SGSN shall allocate a new P-TMSI for that MS.
If an MS identifies itself by a TLLI in an MS originating transmission, the SGSN proceeds as follows:
a) The SGSN checks the routing area identity (RAI) of the previous routing area sent by the MS. If this previous RAI belongs to a different SGSN, the request is handled in the normal way.
b) If the previous RAI belongs to the current SGSN, the status of the TLLI is checked.
– If the P‑TMSI derived from the TLLI was allocated after the SGSN restarted, and corresponds to a valid IMSI record, then the request is handled in the normal way.
– If the P‑TMSI derived from the TLLI was allocated before the SGSN restarted, or does not correspond to a valid IMSI record, then the SGSN requests the IMSI from the MS. If the MS returns an IMSI the SGSN proceeds in the normal way. If the MS does not return an IMSI the network aborts the originating transmission request or location registration procedure.
11.4 VLR associations
All associations with VLRs affected by the restart of an SGSN are marked as unreliable and may be deleted. Based on configuration data, “Reset” messages are sent on the Gs-interface to the VLRs served by the SGSN. The VLRs mark all associations with the SGSN as unreliable by setting the restoration indicator “Confirmed by radio contact” to “Not Confirmed” for the MSs served by that SGSN. The associations will be re-initiated one by one by the SGSN at the next Routing Area update, or combined RA/LA update from each MS.