SMS MO and MT Call Flow Explained

This article covers the following aspects of SMS:

• SMS basics and types
• SMS network and elements
• SMS MO MT call flow
• SMS versus MMS

As SMS has become a daily necessity for most people, this article will focus on the MO (Mobile Originated) and MT (Mobile Terminated) SMS call flows. Let’s assume MS-A is sending an SMS to MS-B, as illustrated in the figure below.

The SMS delivery process is divided into two phases before reaching its final destination. The phase where the SMS travels from the originating mobile subscriber A to the SMSC (Short Message Service Center) is known as the MO call. Conversely, the phase from the SMSC to the mobile subscriber B is referred to as the MT call.

SMS Mobile Originated and Terminated Call Flow

The diagram below illustrates the message flow and signaling between various network elements when an SMS is sent from MS-A to MS-B.

SMS MO MT call GSM

Behind the scenes, SMS relies on fundamental protocols like SS7 (Signaling System No.7) and SMPP (Short Message Peer to Peer Protocol). SS7 facilitates communication between core network elements, while SMPP is employed in the IP domain for communication between the SMSC and ESME (External Short Messaging Entity, e.g., a service or content provider).

Here’s a breakdown of the steps involved in the entire SMS process, from the originating mobile (MO) to the terminating mobile (MT):

• The SMS-SUBMIT is the MO FSM (Mobile Originating Forward Short Message) message, exchanged between the originating MSC (Mobile Switching Center) and the originating SMSC.
• Upon receiving the message, the SMSC acknowledges it to the originating subscriber, MS-A, via an SMS-SUBMIT REPORT. MS-A then receives a “message sent” indication from the originating SMSC.
• The originating SMSC retrieves necessary information from the HLR (Home Location Register) regarding the destination MSC/VLR (Visitor Location Register) to route the SMS. This information is obtained through an SRI SM (Send Routing Information for Short Message) request. The HLR responds with the required information in a RESP message.
• After obtaining the routing information, the originating SMSC delivers the message to the appropriate destination MSC/VLR, which, in turn, delivers the SMS to MS-B. This process is represented by MT-FSM and MT-FSM ACK in the figure.
• Once the SMS is delivered to MS-B, MS-A receives a “message delivered” or “message successfully delivered” indication. This is indicated by the SMS-STATUS REPORT in the figure.

Note that both MS-A and MS-B connect to the BTS (Base Transceiver Station), which connects to the BSC (Base Station Controller). The BSC then interfaces with the MSC.

Testing Mobile Originated and Mobile Terminated SMS using Agilent 8960

MO SMS TEST from mobile subscriber

AT+CMGS=20

0001000491214300150B48692074686572652E2E2E <PRESS ctrl+shift+Z>

The Agilent 8960 will then display the received message.

### Following are the steps to perform MT SMS TEST from Agilent 8960

*   Click "short message service"
*   Click "point to point"
*   Click "send message" (it will display "sending")
*   Finally, the Agilent 8960 should get the message status as "Acknowledged by DUT"

For more detailed information, please download the Agilent manual for reference.