The function of short message center is to receive, store and schedule messages for delivery. There are mainly three network flows involved in the message receiving and delivery. They are as follows:

1) Mobile Origination

2) Mobile Termination

3) Alert Message Flow

Lets analyze the flows one by one.

1) Mobile Origination

Most of the times, it is referred to as MO. This flow involves the sending of message from mobile originator to the short message center. The message is sent to the air interface to the BSC which forwards it to MSC which in turn sends this short message to the signaling gateway followed by actual short message server. The following diagram shows the simple operation of the MO flow.


For simplicity, the entities involved in transporting the message to the short message center are not shown. However, I will explain them in detail later on. From the above figure, you can see that the mobile sends the short message and is acknowledged by the short message center. After the acknowledgement is sent by the SMSC, the MO flow ends for it. Before sending the ACK, the SMC checks three things:

1) If the subscriber is in the blacklist database, it is straight away rejected.

2) SMC checks whether to authenticate the user as a caller by checking its authentication database table.

2) SMC checks whether the subscriber is postpaid or a prepaid user. Generally, the postpaid subscribers are defined in the short message center or SMC judges the user type by the response code sent by the Intelligent network system. The second option is more viable since it saves the man hours for adding the postpaid users in to the system. If the user is prepaid, its balance must be enough and it is reported as an error code by the Intelligent Network.

If the message fails in any of the above mentioned checks, the message is rejected by SMC and the end user receives the note “Message Sending Fails”.

After the MO flow, SMC generates the CDRs. In case the MO is successful, the message is stored into the memory waiting for its delivery and the charge request is sent to the Intelligent Network. Whether to charge the subscriber at this point can be adjusted but it depends on which vendor short message center you are using. In most of the cases, the configuration option is available.

2) Mobile Termination:

After the MO flow is successful and the message is stored into the memory of the short message center, SMC starts the mobile termination flow where message is to be delivered to the destination mobile terminal. Since, the short message center has no knowledge as to where the destination terminal might be in the network, it sends the MAP message to the HLR in order to query the location of the terminal. This query is often called ‘SRI (Send Routing Info)’. After HLR receives the request, it checks its database and sends the destination MSC GT to the SMC. After knowing the route, SMC sends the message to the end user. Simple flow diagram is as under:


If MT flow fails even after all the number of delivery attempts have been made, the message is deleted from the memory and the user fee is rebounded.

3) Alert Message Flow:

I will explain this flow with an example. Normally, when we turn off the mobile phone, it sends a message to the VLR/HLR informing it about its state but what if my IPhone crashes without sending the message? Well, in that case, when MT message is to be delivered to my number, the HLR will respond to SMC that the mobile is still ON and hence it will send the routing information to the SMC. SMC will send the message to the corresponding MSC but meets with a timeout. So, in this case, SMC will send a message to HLR to inform it about the current terminal state. HLR will update its database. Now, SMC keeps the message stored and checks its delivery schedule (which may be different for each error). SMC will keep on checking with the HLR after the defined time about the possible routing information but if the terminal is still off, HLR will inform SMC and it goes into its waiting state again. Now, if I turn ON my mobile set, an ALERT message will be sent to the Short Message Center informing it about the mobile station coming up in the network. SMC will immediately send the message without waiting for the schedule time. The flow diagram is given as under:


Adnan Khurshid

Adnan Khurshid, the author of this article, has been working in a telecommunication sector since 2007. He has worked there as a VAS (Value Added Services) engineer and has excelled remarkably in the field. Working in this field has been his passion and he has always made efforts to keep himself up to date. Find more about him on LinkedIn

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