SMS - Protocol Layers

The SMS protocol stack is composed of four layers: the application layer, the transfer layer, the relay layer, and the link layer. The application layer is implemented in SMEs in the form of software applications that send, receive, and interpret the content of messages (e.g., message editor, games, etc.). The application layer is also known as SM-AL for Short-Message-Application-Layer. At the transfer layer, the message is considered as a sequence of octets containing information such as message length, message originator or recipient, date of reception, etc. The transfer layer is also known as the SM-TL for Short-Message-Transfer-Layer. The relay layer allows the transport of a message between various network elements. The relay layer is also known as the SM-RL for Short-Message-Relay-Layer. The link layer allows the transmission of the message at the physical level. For this purpose, the message is protected to cope with low-level channel errors. The link layer is also known as the SM-LL for Short-Message-Link-Layer. For transport purposes, an application maps the message content and associated delivery instructions onto a Transfer Protocol Data Unit (TPDU) at the transfer layer. ATPDU is composed of various parameters indicating the type of the message, specifying whether or not a status report is requested, containing the text part of the message, etc.

SMS Interworking between Mobile Networks
For this purpose, operators have commercial agreements. Each mobile network counts the number of messages being sent from another network. After a given period of time, these counts are compared and there is a commercial settlement between operators.

Message Structure
A message refers to the subscriber’s perception of the message composed of text and/or elements such as pictures, melodies, etc. For transport purposes and due to limitations at the transfer layer, an application may need to segment the message into several pieces called message segments.


SME-SMSC Transactions/Submit, Deliver, Report, and Command
At the transfer layer, six types of transactions can occur between an SME and an SMSC:

• SMS-SUBMIT: this transaction corresponds to the submission of a message segment from the SME to the SMSC. Upon submission of the message segment, the SMSC acknowledges the submission with the SMS-SUBMIT-REPORT transaction.

• SMS-DELIVER: this transaction corresponds to the delivery of a message segment from the SMSC to the SME. Upon delivery of the message segment, the SME acknowledges the delivery with the SMS-DELIVER-REPORT transaction.

• SMS-STATUS-REPORT: this transaction corresponds to the transfer of a status report from an SMSC back to an SME.

• SMS-COMMAND: this transaction corresponds to the request from an SME, usually an external SME, for the execution of a specific command by the SMSC.