Alert Service Center of Short Message Service in the view of VLR, SGSN and HLR

Alert-SC or Alert Service Center is one of the flow in Short Message Service Systems under ETSI GSM Specification or later 3GPP. It is used to inform the Service Center (SMSC) of the availability of the handset (Mobile Station) for the readiness of recieving the Mobile Termination (MT) SMS. A particular Mobile Station can be undergoes different state e.g. Idle, Deatch From Network, Busy etc. The Alert-SC feature in return to help the Service Center(es) to deliver/terminate the MT SMS efficiently.

The Mobile Application Part protocol (MAP) operation Alert-SC is triggered from HLR to Service Center Address and there can be more than 1 service centers in a PLMN(Public Land Mobile Network).

Each of the MT termination will need 2 MAP operation which is SRI-SM to obtain the routing information for Short Message which include the current MSC, IMSI, LMSI or SGSN Address that is required. Followed by MO-FSM operation which terminate the SMS content to the targeted MSC/SGSN.

There is few assumption for the Alert-SC process:-

• It is optional for HLR to have the Alert-SC Feature; In case does,'t have the SMSC will need to have retry mechanism.
• MT SMS termination can be done in 2 approach; via MSC and via SGSN using GPRS barrier service.

The Alert Service Center flow will need the cooperation or integration of different network node/element in the 2G GSM network(HLR, VLR(MSC), SGSN(GPRS).

• MNRF flag - Mobile Station Not Reachable Flag (Mandatory for VLR) in VLR is to indicate whether that particular MS can be approached. This flag will be set if the MT SMS can not be terminated by the MSC after the SendRoutingInfoForMT query. MNRF Flag in VLR is also important to determine whether the operation ReadyForSM need to be triggered from VLR to HLR informing the availability of the Mobile Station whether is Memory available or MS present. The same flag will be appear in the Waiting Message Data(WMD) in HLR.

• MNRG flag - Mobile Station not Reachable Flag For GPRS the same purpose as MNRF and it is used for MT-SMS that terminate using GPRS

• MCEF flag - Mobile Station capacity Exceed Flag in HLR which indicate the particular MS is not able to received MT SMS due to the memory capacity full.

• MNRR - Mobile Station Not Reachable Reason in HLR will store the reason for the MS being absent. The reason can be:- No paging response from MSC or SGSN, IMSI detach etc and other AbsentSubscriber Diagnostic information which can refer GSM 09.02 for the MAP operation error code parameter.

MNRF, MNRG in VLR flag will be cleared or set depending on the MSC and SGSN termination of the MT SMS via BSC. While the MNRF, MNRG, MCEF and MNRR and Service Center Address state/flag changed will rely on Service Center's Report-SM-Deliver operation after the MO-FSM Operation to the MSC/SGSN depending on the result of the MO-FSM. Besides, certain MSC will also inform the HLR for the status of MNRF, MNRG, MECF and MNRR ahead of Service Center normal operation.

There is few Alert-SC condition that able to obtain from the ETSI specification:=

• When either the HLR or VLR detects that the MS has recovered operation the HLR will invoke operations to alert the SCs within the MWD Once the Alert SC operations have been invoked, the MNRF and MNRR via the MSC are cleared. After each SC is alerted by the HLR, the address for that SC is deleted from the MWD. If the MCEF is set in the HLR, the HLR clears the MNRF and MNRR via the MSC, but does not invoke operations to alert the SCs.

• When the HLR receives a notification that the MS has memory capacity available to receive one or more short messages, the HLR will invoke operations to alert the SCs within the MWD Once the Alert SC operations have been invoked, the MNRF is cleared in the VLR and the MCEF, MNRF and MNRR via the MSC are cleared in the HLR. After each SC is alerted by the HLR, the address for that SC is deleted from the MWD.
  

• When the HLR receives from the SMS-GMSC a notification that a SMS has been successfully delivered from an SC to an MS via the MSC for which the MCEF is set and the MWD are not empty, the HLR will invoke operations to alert other SCs within the MWD Once the Alert SC operations have been invoked, the MCEF, MNRF and MNRR via the MSC are cleared in the HLR. After each SC is alerted by the HLR, the address for that SC is deleted from the MWD. The SC which successfully delivered the message is also deleted fromthe MWD, if present.
  

• When the HLR receivesa notification that the MS has memory capacity available to receive SMS but the MCEF is not set and the MWD are empty, the HLR acknowledges the notification but does not alert any service centre.
  

Hope this post will help in understanding the SMSC relation with the MSC, HLR and VLR to deliver a quality Data Messaging Service in GSM Network.