When a user sends a text message (SMS message) to another user, the message gets stored in the SMSC (short message service center) which delivers it to the destination user when they are available. This is a store and forward option.

An SMS center (SMSC) is responsible for handling the SMS operations of a wireless network.
When an SMS message is sent from a mobile phone, it will reach an SMS center first.
The SMS center then forwards the SMS message towards the destination.
The main duty of an SMSC is to route SMS messages and regulate the process. If the recipient is unavailable (for example, when the mobile phone is switched off), the SMSC will store the SMS message.

It will forward the SMS message when the recipient is available.
I. Validity Period of an SMS Message - An SMS message is stored temporarily in the SMS center if the recipient mobile phone is offline. It is possible to specify a cutoff period after which the SMS message will be deleted from the SMS center. Once deleted, the SMS message will no longer be available for dispatch to the recipient mobile phone (even if it becomes online).

II. Message Status Reports - The SMS sender needs to set a flag in the SMS message to notify the SMS center that he wants the status report about the delivery of this SMS message. This status report is sent to the SMS sender in the form of an SMS....

The Short Message Service is realised by the use of the Mobile Application Part (MAP) of the SS#7 protocol, with Short Message protocol elements being transported across the network as fields within the MAP messages. These MAP messages may be transported using 'traditional' TDM based signalling, or over IP using SIGTRAN and an appropriate adaptation layer. The Short Message protocol itself is defined by 3GPP TS 23.040 for the Short Message Service - Point to Point (SMS-PP),[2] and 3GPP TS 23.041 for the Cell Broadcast Service (CBS).

Four MAP procedures are defined for the control of the Short Message Service:
Mobile Originated (MO) short message service transfer;
Mobile Terminated (MT) short message service transfer;
Short message alert procedure;
Short message waiting data set procedure.