The MAP layer defines the operations necessary to support SMS. Both American and international standards bodies have defined a MAP layer using the services of the SS7 TCAP. The American standard is published by Telecommunication Industry Association and is referred to as IS–41. The international standard is defined by the European Telecommunications Standards Institute (ETSI) and is referred to as GSM MAP.

The following basic MAP operations are necessary to provide the end-to-end SMS:

• Routing Information Request—Before attempting delivery of a short message, the SMSC must receive routing information to determine the serving MSC for the mobile device at the time of the delivery attempt. This is accomplished by way of an interrogation of the destination handset’s HLR, which is accomplished via the use of the SMSrequest and SendRoutingInfoForShortMsg mechanisms in IS–41 and GSM, respectively.
• Point-to-Point Short Message Delivery—The mechanism provides a means for the SMSC to transfer a short message to the MSC that is serving the addressed mobile device. After the address of said MSC has been obtained from the station’s HLR, the short message delivery operation provides a confirmed delivery service. The operation works in conjunction with the base station subsystem while the message is being forwarded from the MSC to the MS. Therefore, the outcome of the operation comprises either success (such as delivery to the mobile) or failure caused by one of several possible reasons. The point-to-point short message delivery is accomplished via the use of the short message delivery–point-to-point (SMD–PP) and forwardShortMessage mechanisms in IS–41 and GSM, respectively.
• Short Message Waiting Indication—he operation is activated when a short message delivery attempt by the SMSC fails due to a temporary failure, such as the station being unregistered, and provides a means for the SMSC to request the HLR to notify the SMSC when the indicated mobile device becomes available. This short message waiting indication is realized via the use of the SMS_notification indicator and set_message_waiting_data mechanisms in IS–41 and GSM, respectively.
• Service Center Alert—The operation provides a means for the HLR to inform the SMSC, which has requested a notification that a specific mobile device is now recognized by the mobile network to be available. This service center alert is accomplished via the use of the SMS_notification and alert_service_center mechanisms in IS–41 and GSM, respectively.

Service Elements
SMS is comprised of several service elements relevant to the reception and submission of short messages:

• Message Expiration—The SMSC will store and reattempt delivery of messages for unavailable recipients until either the delivery is successful or the expiration time—set on a per-message basis or on a platform-wide basis—arrives.
• Priority—This is the information element provided by an SME to indicate the urgent messages and differentiate them from the normal priority messages. Urgent messages usually take priority over normal messages, regardless of the time of arrival to the SMSC platform.
• Message Escalation—The SMSC stores the message for a period no longer than the expiration time (it is assumed that the escalation time is smaller than the expiration time associated with the message), and after said escalation time expires, the message will be sent to an alternate message system (such as a paging network or an e-mail server) for delivery to the user.

In addition, SMS provides a time stamp reporting the time of submission of the message to the SMSC and an indication to the handset of whether or not there are more messages to send (GSM) or the number of additional messages to send (IS–41).

Subscriber Services
SMS comprises two basic point-to-point services:

• Mobile-originated short message (MO–SM)
• 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 such as paging 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, paging networks, 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.

Depending on the access method and the encoding of the bearer data, the point-to-point short messaging service conveys up to 190 characters to an SME in GSM networks and from 120 to 205 in IS–41 networks.

In GSM networks, the type of messaging service is identified by the protocol identifier information element, which identifies the higher-level protocol or interworking being used. Examples are telex, group 3 telefax, X.400 messaging, European Radio Messaging System (ERMES), and voice telephone.

In IS–41 networks, the service type is distinguished by use of the teleservice identifier. Basic teleservices include the following:

• Cellular messaging teleservice (CMT)
• Cellular paging teleservice (CPT)
• Voice-mail notification teleservice (VMN)

CMT differs from the CPT due to the inclusion of a reply mechanism that enables a user or network acknowledgment to be selected on a per-message basis. The user acknowledgment includes a response code that paves the way for powerful interactive services between SMSCs.

Many service applications can be implemented by combining these service elements. Aside from the obvious notification services, SMS can be used in one-way or interactive services providing wireless access to any type of information anywhere. By leveraging new emerging technologies that combine browsers, servers, and new markup languages designed for mobile terminals, SMS can enable wireless devices to securely access and send information from the Internet or intranets quickly and cost-efficiently. One of these technologies where SMS can provide a cooperative, rather than a competitive, approach is the WAP, which allows transport of data for mobile wireless users.

A generic network infrastructure for realizing the innovative SMS services is depicted in Figure 2.

Figure 2. Network Infrastructure

Some of the potential applications of SMS technology, utilizing both MT–SM and MO–SM where appropriate, include the following:

• Notification Services—Notification services are currently the most widely deployed SMS services. Examples of notification services using SMS include the following:
  • Voice/fax message notification, which indicates that voice or fax mail messages are present in a voice mailbox
  • E-mail notification, which indicates that e-mail messages are present in an e-mail mailbox Reminder/calendar services, which enable reminders for meetings and scheduled appointments.
• E-mail Interworking—Existing e-mail services can be easily integrated with SMS to provide e-mail to short messaging and mobile e-mail and message escalation.
• Paging Interworking—Paging services integrated with SMS allow digital wireless subscribers to be accessible via existing paging interfaces, as well as escalation of messages.
• Information Services—A wide variety of information services can be provided by the SMS, including weather reports, traffic information, entertainment information (e.g., cinema, theater, concerts), financial information (e.g., stock quotes, exchange rates, banking, brokerage services), and directory assistance. SMS can support both push (MT) and pull (MO) approaches to allow not only delivery under specific conditions but also delivery on demand, as a response to a request.
• WAP Integration—SMS can deliver notifications for new WAP messages to wireless subscribers but can also be used as the transport mechanism for WAP messages. These messages can contain diverse information from sources that include databases, the World Wide Web, e-mail servers, etc.

Mobile Data Services
The SMSC can also be used to provide short wireless data. The wireless data may be in interactive services where voice calls are involved.

Some examples of this type of service include fleet dispatch, inventory management, itinerary confirmation, sales order processing, asset tracking, automatic vehicle location, and customer contact management. Other examples may be interactive gaming, instant messaging, mobile chat, query services, mobile banking, etc.

Customer Care and Management
The SMSC can also be used to transfer binary data that can be interpreted by the mobile device without presentation to the customer. This capability allows the operators to administer their customers by providing a mechanism for programming the mobile device. Examples of such services include mobile device programming, which allows customer profiles and subscription characteristics to be downloaded to the mobile device (customers can be activated/deactivated based on the data downloaded) and advice of charge, which enables the SMS to be used to report charges incurred for the phone call (e.g., calls made when roaming).

One interesting method to provide customer support is to offer a list of answers to frequently asked questions via short message. SMS also can be used to distribute general information about other products and services being offered by the service provider, thus guaranteeing maximum penetration of the advertising over the existing customer base. In a different scenario, a service provider may want to deliver short messages to subscribers to remind them of, for example, past-due payments, instead of reminding them over traditional mail or courier delivery, therefore reducing cost and ensuring that the message is delivered to its destination in a timely manner.