MO Message Flow

The letters MO stands for "Mobile Originated". So MO message is the message which originates from the mobile phone. And even though it is not indicated in the name, destination of the MO messages will be the "Short Message Service Center" (SMSC). From that point onwards SMSC takes care of the message performing routing accordingly.

The message flow of the MO messages is shown in the picture below.

Before subscriber sends the message, access request and authentication is done using the information available on VLR (Visitor location register)

Step 1

In this step, message is sent to the MSC after following the process explained above. MSC (Mobile Switching Center) is responsible for handling all the subscriber events, who are in a particular geographical area.

Step 2

Then the VLR is updated with the relevant information about the MO message. This will be used to obtain the location information about the subscriber who sent the message as well.

Step 3

In this step three, the message is transfered to the SMSC, which handles the rest of the message routing process.

Step 4

SMSC will take relevant routing actions and will be sent to further processing. MO message flow ends up here.

Step 5

The delivery report is sent to the MSC from SMSC, once the SMSC obtains the delivery status of the message.

Step 6

From there onwards MSC will handle the delivery report. Will access VLR and sends relevant delivery status to the subscriber who sent the message. If he's not in his area then will inform back to the SMSC.

The point to note here is that the HLR is not accessed at any point in the MT flow.