In previous article, I’ve already explained basic overview of SMS messaging services in an operator. Now we’ll see deeper for SMS flow and how it works.
Just see in the picture below :
sms diagram network
You can see at that picture that SMS have two domains : IP domain and SS7 domain. So we need protocol conversion between that domain in our SMS Center. Commonly it is using SS7 card or other external devices to do same function.
SMS delivery can be divided into 2 parts, they are Mobile Originated (MO) and Mobile Terminated (MT).
We can say easily that MO is the process of sending SMS from a number until it reaches the SMSC. The indicator in mobile subscriber is they get notification ” message sent ” in their phone. So the message which is sent is just arrived in SMSC of his operator, not yet reached destination number. Next we can say that if message already delivered to destination number, the sender will get the notification which we call as delivery report. Commonly delivery report message is like ” message delivered / message successfully delivered“, etc.
Hot about MT? MT is process of message delivery which already reached SMSC to destination number . It is the way how SMS Center send message to destination number.
Lets see further in the flow basic sms below :
sms flow
For MO process we need action message : SMDPP-SUBMIT, AUT-ACC-RES ( A number ) until aut-acc-reserver and SMDPP-SUBMIT (response ) again. Easily, if SMDPP-SUBMIT from SMSC is OK, MSC will send back it to sender. The sender will see in his phone with message ‘ message sent‘. For MT we need message flow SMS-REQ (SMS Request) to HLR until SMDPP-SUBMIT and process sending delivery report.
We continue our discuss. Lets call that mobile subcriber sender is subscriber A / MS A, and the destination or receiver is subscriber B / MS B.
Let say subscriber A is in MSC / VLR A coverage want to send message or SMS to subscriber B who is in MSC/ VLR B location where status B number is active ( ON ).
Status ON or OFF for a subscriber is defined by status sent by MSC / VLR which cover that subscriber to HLR using message REGNOT. If subscriber is active, MSC / VLR will tell HLR about the location and MSC/VLR address according to addressing PC or SSN in protocol SS7 format. I suggest you to read basicly about addressing SS7 protocol.
When Mobile subscriber A ( MS A ) send message to Mobile subscriber B ( MS B), MSC will send message SMDPP ( Short Message Delivery Point to Point) to SMSC ( SMS Center ). SMSC will check the status of MS A, if need authetication about charging in case prepaid subsriber, SMSC will as IN ( Intelligent Network ) about permission to do sending SMS include check balance, status account etc using message AUT-ACC-RES. IN will reply it and answer about permission of that subsriber. If OK, SMSC will return about the authentication to MSC A with message SMDPP-SUBMIT ( response ). MS A will see ” message sent” in their phone after MSC A send that message to him. Until here, the process MO ( Mobile Originated ) end.
We will continue discuss for MT process in next article… 
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salam kenal sahabat
wah info yg bagus meski rada mbulet ga paham tapi membuat saya lebih mengerti thnxs n good luck ya
thanks maz… sama2 belajar aja.. ini didedikasiin buat yg mau kenal lebih dalam.. kalo overview “how is sms works” part I dah cukup hihihi
[...] How SMS works ? (part II) [...]
I just wanted to drop you a line and let you know that I really have enjoyed your well-written articles. I have bookmarked this site and will definitely be checking back for new posts.
thanks karmen