The Global System for Mobile Communication (GSM) is a standard for digital wireless communications.  In the GSM radio system, the mobile handset is called Mobile Station (MS). 
GSM has much more services than just voice telephony.  Today's second-generation GSM networks deliver high quality and secure mobile voice and data services (such as SMS/Text Messaging) with full roaming capabilities across the world. 
In mobile networks people can be contacted by calling to their mobile telephone number or by sending to that number a so called short message by e.g. making use of the Short Message Service (SMS).  Short Message Service (SMS) is the transmission of short text messages, to and from a mobile phone, fax machine and/or IP address.  SMS messages must be no longer than 160 alphanumeric characters and contain no images or graphics.  The point-to-point Short message service (SMS) provides a means of sending messages of limited size to and from GSM mobiles.  Detailed information can be found in the ETSI standard GSM 03.40 Version 5.3.0. 
The basic network structure of the SMS service comprises two entities, which may receive or send messages being the endpoints between which the SMS message is sent.  The entity can be located in a fixed network, a mobile station or an internet protocol network. 
A further component in the SMS network is the Mobile Switching Centre (MSC), which performs the switching functions of the system and control calls to and from other telephone and data systems.  Its most important functions are registration, updating of registration of whereabouts, hand-overs and roaming.  MSC is thus an interface between the radio system and various external networks, such as the public switched telephone network and packet switched Networks.  The MSC performs all signaling functions that are necessary to establish calls to and from mobile stations.  Communication between the different function authorities is based on signaling system # 7 (SS7).  An MSC that also handles the Gateway function into other networks is called Gateway-MSC (GMSC). 
Messages sent to and from mobile stations are received by a Short Message Service Centre (SMSC), which must then direct it to the appropriate mobile device if the message is to be sent to a mobile station or it must generally direct it to a recipient.  Before attempting delivery of a short message to a mobile device, the SMSC must receive routing information to determine the serving Mobile Switching Centre (MSC) for a given mobile station at the time of the delivery attempt.  In order to receive the routing information, the SMSC sends a routing request for SMS to the home location register (HLR) to find the customer.  Once the HLR receives the request, it will respond to the SMSC with the subscriber's status (either inactive or active) and the location for where the subscriber is roaming.  The SMSC then transfers the message to the serving MSC, which deliver the message to the receiving subscriber.  Recipients can then send a response message in a corresponding way. 
The SMS gateway MSC (SMS-GMSC) is an MSC capable of receiving a short message from an SMSC, interrogating a home location register (HLR) for routing information, and delivering the short message to the “visited” MSC of the recipient mobile station. 
The Home Location Register (HLR) is a database used for permanent storage and management of subscriptions and service profiles.  Upon interrogation by the SMSC, the HLR provides the routing information for the indicated subscriber. 
The Visitor Location Register (VLR) is a database that contains temporary information about subscribers and local subscriber capabilities.  This information is needed by the MSC in order to serve visiting subscribers. 
A Mobile Station (MS) is a wireless terminal capable of receiving and originating short messages as well as voice calls being an endpoint to or from which the SMS message is sent. 
The backbone of the wireless network signaling infrastructure is based on Signaling System No 7 (SS7).  SS7 is a global standard for telecommunications defined by the International Telecommunication Union (ITU) Telecommunication Standardization Sector (ITU-T).  The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wireline call setup, routing and control.  The SS7 network and protocol are used for e.g. basic call setup, management, wireless services, wireless roaming, and mobile subscriber authentication, enhanced call features thus providing efficient and secure worldwide telecommunications. 
SMS comprises two basic point-to-point services, i.e. the Mobile-Originated Short Message (MO-SM) and the Mobile-Terminated Short Message (MT-SM)
Mobile-originated (MO) short messages are transported from the MO-capable handset to the SMSC and can be destined to other mobile subscribers or for subscribers on fixed networks or Internet protocol (IP) networks (including the Internet and private e-mail networks).  Mobile-terminated (MT) short messages are transported from the SMSC to the handset and can be submitted to the SMSC by other mobile subscribers via MO-SM or by other sources such as voice-mail systems or operators. 
For MT-SM, a report is always returned to the SMSC either confirming the short message delivery to the handset or informing the SMSC of the short message delivery failure and identifying the reason for failure (cause code).  Similarly, for MO-SM, a report is always returned to the handset either confirming the short message delivery to the SMSC or informing of delivery failure and identifying the reason. 
Some of the potential applications of SMS technology, utilizing both MT-SM and MO-SM where appropriate, include Notification Services such as voice/fax message notification, e-mail notification, and reminder/calendar services, e-mail interworking, paging interworking, information services, such as weather reports, traffic information, entertainment information (e.g., cinema, theatre, concerts), financial information (e.g., stock quotes, exchange rates, banking, brokerage services), and directory assistance and WAP integration. 
SMS can support both (MT) and (MO) approaches to allow not only delivery under specific conditions but also delivery on demand, as a response to a request. 
The ETSI standard 3GPP TS 29.002 V3.10.0 Mobile Application Part (MAP) specification specifies the scenarios for mobile-terminated as well as mobile-originated short messages.  Other standards referred to are The ETSI standard 3GPP TS 23.040 5.3.0 Technical realization of the Short message Service (SMS) and The ETSI standard 3GPP TS 23.003 4.3.0 Numbering, Addressing and Identification. 
The GSM network was, however, never designed to handle interactive usage of SMS.  Interactive SMS usage of the GSM network is most often slow because of the lack of support for these types of applications, such as many mobile service applications, wherein interactive signaling is used. 
A common scenario in a mobile service application is the sending of a short message by a mobile station, such as a service request, and the sending of a reply for the request. 
Thus, the problem with e.g. a mobile originated short message coming from a handset is that the information required to quickly send back the answer is not available.  To be able to send back the answer, the Short Message Service Centre (SMS-C) must first request routing information from the HLR and not until then it can forward the short message to the right MSC.