The Short Message Service (SMS) provides a means of sending text messages containing up to 160 alpha numerical characters to and from MS’s. SMS makes use of a SMS Center (SMS-C), which acts as a store and forward center for short messages. SMS consists of two basic services:
• Mobile terminated SMS: from an SMS–C to an MS
• Mobile originated SMS: from an MS to an SMS–C In the two cases described below, the MS is in idle mode. If the MS is in active mode, short message is transmitted on the SACCH. No paging, call set-up, authentication, etc. needs to be performed in that case.
MOBILE ORIGINATED SMS
Mobile originated SMS transfers a short message submitted by the MS to an SMS-C. It also provides information about the delivery of the short message, either by a delivery report or failure report.
Mobile originating short message
1. An MS establishes a connection to the network, as in the case of a normal call set-up. This step is not performed if the MS is in active mode, since the connection already exists.
2. If authentication is successful, the MS sends the short message using SDCCH to the SMS–C via the MSC/VLR. The SMS–C in turn forwards the short message to its destination. This could be an MS or a terminal in the fixed network, such as a PC.
MOBILE TERMINATED SMS
Mobile terminated SMS has the capability to transfer a short message from the SMS–C to an MS.
Mobile terminating short message
1. A user sends a message to a SMS–C.
2. The SMS–C sends the message to the SMS–GMSC.
3. The SMS–GMSC queries the HLR for routing information.
4. The HLR returns routing information to the SMS-GMSC.
5. The SMS-GMSC re-routes the message to the MSC/VLR.
6. The MS is paged and a connection is set up between the MS and the network, as in the normal call set-up case.
7. If authentication is successful, the MSC/VLR delivers the message to the MS. Short messages are transmitted on the allocated signaling channel, SDCCH.
8. If the delivery was successful, a report is sent from the MSC/VLR to the SMS–C, if not, the HLR is informed by the MSC/VLR, and a failure report is sent to SMS–C.
In the case of an unsuccessful delivery, the SMS-C informs the HLR and VLR that there is a message waiting to be delivered to the MS. The HLR then informs the SMS–C when the MS becomes available. Mobile terminated SMS can be input to the SMS–C via a variety of sources, e.g. speech, telex, facsimile or internet.
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