Here we continue about SMS flow until sending message to destination number. Please see previous articles for overview SMS and how to send SMS to SMS Center.
In the previous articles, we discussed how SMS arrive to SMS Center. This flow mentioned as Mobile Originated (MO). Now we try to discuss about Mobile Terminated ( MT ). What is MT message ?
MT message is the way how SMS Center sending the message to destination number (MS B) based on what source number ( MS A ) request to SMSC.
After MS A got authentication and permission from IN ( Intelligent Network), and SMS Center send back SMDPP-Submit response to MSC A, SMSC ( SMS Center ) will do some steps to send the message to destination number.
The most important step is that SMSC will ask HLR ( Home Location Register ) as database master of subscriber about the status and condition of destination number. SMSC will have message flow ” SMS-Req or SMS-Request” for asking that status to HLR. Here we have conditions :
- HLR see that status MS B / destination number is active and registered. HLR will reply message SMS-Req from SMSC by giving address of MSC / VLR of destination number, and also MIN number of MS B.
Then, SMSC will do message flow SMDPP to MSC / VLR given by HLR. The MSC / VLR which receive SMDPP message will do paging to lookup location of MS B. After message successfully delivered to MS B, MSC / VLR will report back to SMSC that message already reached destination number using message flow SMDPP response to SMSC.
SMS then will tell the MSC / VLR which cover MS A with the delivery report message. - HLR see that status MS B / destination number is inactive based on last report from MSC / VLR which cover B number. When MSC give status MS B with inactive, HLR will response the report with message MSINACT.
When HLR got SMS-Req from SMSC and the status of MS B is inactive, HLR will response message SMS-Req with parameter ’SMSAccessDeniedReason‘ which show that Mobile terminated (MT) process cannot be done at the moment. HLR will mark this situation with flag SMS Delivery Pending ( SMDPF).
How about SMSC ? SMSC will store the pending message into a sore and will see the retry mechanism, scheduling time for next sending SMS based on error code return by HLR or others network element.
Every SMSC of each operator will have different retry mechanism based on strategy taken by the planning team which see the condition of network first.
When MS A going active again, MSC / VLR B will do powerup registeration process by using REGNOT message. HLR will notify SMSC with message SC_alert. SMSC will response it by sending the pending message immedietly without see retry mechanism anymore.
From this articles we see that MT process of message is depend on SMSC action and other network element. Not only SMSC have a role for this action. SMSC do in application layer of SS7 layer, but need also action from other network element like MSC, HLR, etc.
So if something happen in your SMS, we have to check all, not only in SMSC. cheers ! 
nice info bro…
thanks bro… ^^
makasih atik
kalau saya tahunya
sms ngurangin pulsa aja wkwkwkw
nah kalo dah tau ini kan bisa complain ke customer service lebih galak bro kalo ada apa2 hihihi