MT Short Message Service Transfer
The figure to the right depicts a call flow for Mobile Terminated short message delivery. For the sake of simplicity, some of the interactions between the VMSC and VLR, and VMSC and Handset, have been omitted, and only the case when SMS home routing is not in use is shown.
When the SMSC determines it needs to attempt to deliver a short message to its destination, it will send the SMS-PP APDU containing the text message, the 'B-Party' (destination phone number) and other details to the Gateway MSC (GMSC) logical component on the SMSC. The GMSC, on receipt of this short message, needs to discover the location of the B-Party in order to be able to correctly deliver the text to the recipient (the term Gateway MSC, in this context, indicating an MSC that is obtaining routing information from the Home Location Register (HLR)). To do this, the GMSC invokes the MAP service package MAP_SEND_ROUTING_INFO_FOR_SM, which sends a sendRoutingInfoForSM (SRI-for-SM) MAP message to the destination number's HLR, requesting their present location. This SRI-for-SM message may be sent to an HLR in the same network as the SMSC, or via an interconnect to an HLR in a foreign PLMN, depending on which network the destination subscriber belongs to.
The HLR performs a database lookup to retrieve the B-Party's current location, and returns it in an acknowledgement message to the SMSC's GMSC entity. The current location may be the MSC address the subscriber is currently roaming on, the SGSN address, or both. The HLR may also return a failure, if it considers the destination to be unavailable for short messaging; see the Failed short message delivery section below.
Having obtained the routing information from the HLR, the GMSC will attempt to deliver the short message to its recipient. This is done by invoking the MAP_MT_FORWARD_SHORT_MESSAGE service, which sends a MAP mt-ForwardSM message to the address returned by the HLR, regardless of whether it is an MSC (Circuit Switched SMS delivery) or an SGSN (Packet Switched SMS delivery).
The VMSC will request the information needed for it to deliver the Short Message to its recipient by sending a Send_Info_for_MT_SMS message to the VLR. The VLR will then instigate a page request, or subscriber search, for the destination subscriber's Mobile Subscriber ISDN Number (MSISDN), and return the result to the VMSC. Since a typical deployment sees the VLR being co-located with the MSC, this message flow is usually internal to the platform. Should the page or search for the subscriber fail, the VLR will indicate the failure cause to the VMSC which will abort the Short Message delivery procedure and return the failure to the SMSC (see the Failed short message delivery section below). If the page of the handset was successful, the VMSC will then send to the SMSC indicating successful delivery. The GMSC component of the SMSC passes the result of the delivery attempt to the Service Centre. In the case of successful delivery, the delivered text message will be removed from the Store and Forward Engine (SFE) and, if requested, a delivery report sent to the text originator. If the delivery failed, the SMSC invokes a retry procedure to periodically make further attempts at delivery; additionally, it may register with the HLR to receive a notification when the B-Party becomes available for short message delivery in the future (see the Failed short message delivery section below).
Read more about this topic: Short Message Service Technical Realisation (GSM)
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