SMSC or Short Message Service Center
SMSC is a node located in the mobile operator’s home network (HPLMN), provides SMS services to mobile users. This is a key node in the telecom network. For sending SMS, the sim card has an SMSC number configured by the mobile network operator. SMSC provides reliable delivery of mobile originated and mobile terminated text message. When a mobile user sends a text to another mobile user. First, short messages reach the SMSC, this is called submitting an SMS on SMSC. After that SMSC starts the procedure for delivering text to the destination mobile user. For reaching a text to SMS from the handset, ss7 signalling is used.
Enabling SMS service:
Every service to the mobile sim card is configured in the subscriber profile on HLR. For SMS, the subscriber should be enabled for MO-SMS and MT-SMS Teleservices and there should be no operator determined barring of SMS service. Before allowing to send and receive a text to/from mobile, the roaming network checks for the subscription for SMS service. The roaming network downloads the subscription information at the time of the location update.
TeleService Codes for SMS:
- 21 – MT SMS
- 22 – MO SMS
- 20 – ALL SMS
Mobile Originated SMS :
In mobile-originated SMS, a mobile subscriber is allowed to submit a text on SMSC. While submitting, the roaming MSC checks for the MO-SMS teleservice subscription. If the subscriber is subscribed for Mobile Originated SMS and there is no outgoing call barring for SMS. The MSC starts a mobile originated SMS flow using GSM MAP protocol over SS7.
Mobile Terminated SMS.
In the mobile terminated procedure, SMSC first does an SRI for SM for a subscriber from the HLR of that subscriber. If MT-SMS teleservice code is enabled then HLR returns an IMSI and the visiting MSC number. SMSC starts the Mobile Terminated map protocol procedure using IMSI and MSC number.
Store and Forward.
While sending SMS from SMSC to the roaming MSC. There may be a temporary error from the HLR (e.g subscriber is not connected). The SMSC may store that SMS and keep trying after some time. How long the SMSC will keep the text, depends on the validity period of the text.
Message Waiting Data on HLR.
Priority message handling.