What is SMSC or Short Message Service Center?
SMSC is a node in the mobile operator’s home network (HPLMN) that provides SMS services to subscribers. This is a key node in the telecom network. The sim card has an SMSC number configured by the mobile network operator for sending SMS.
SMSC provides reliable delivery for mobile-originated and mobile-terminated text messages. 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 delivering text to the destination mobile user. For reaching a text to SMS from the handset, ss7 signaling 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 sending and receiving a text to/from mobile, the roaming network checks for the subscription for SMS service. The roaming network downloads the subscription information at the location update time.
TeleService Codes for SMS:
- 21 – MT SMS
- 22 – MO SMS
- 20 – ALL SMS
Mobile Originated SMS :
In mobile-originated SMS, a mobile subscriber can 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 the 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. HLR returns an IMSI and the visiting MSC number if the MT-SMS teleservice code is enabled. SMSC starts the Mobile Terminated map protocol procedure using IMSI and MSC.
Store and Forward.
While sending an SMS from SMSC to the roaming MSC. There may be a temporary error from the HLR (e.g., the 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.