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Introduction to SMS

SMS appeared on the wireless scene in 1991 in Europe. The European standard for digital wireless, now known as the Global System for Mobile Communications (GSM), included short messaging services from the outset.

In North America, SMS was made available initially on digital wireless networks built by early pioneers such as BellSouth Mobility, PrimeCo, and Nextel, among others. These digital wireless networks are based on GSM, code division multiple access (CDMA), and time division multiple access (TDMA) standards.

Network consolidation from mergers and acquisitions has resulted in large wireless networks having nationwide or international coverage and sometimes supporting more than one wireless technology. This new class of service providers demands network-grade products that can easily provide a uniform solution, enable ease of operation and administration, and accommodate existing subscriber capacity, message throughput, future growth, and services reliably. Short messaging service center (SMSC) solutions based on an intelligent network (IN) approach are well suited to satisfy these requirements, while adding all the benefits of IN implementations.

SMS provides a mechanism for transmitting short messages to and from wireless devices. The service makes use of an SMSC, which acts as a store-and-forward system for short messages. The wireless network provides the mechanisms required to find the destination station(s) and transports short messages between the SMSCs and wireless stations. In contrast to other existing text-message transmission services such as alphanumeric paging, the service elements are designed to provide guaranteed delivery of text messages to the destination. Additionally, SMS supports several input mechanisms that allow interconnection with different message sources and destinations.

A distinguishing characteristic of the service is that an active mobile handset is able to receive or submit a short message at any time, independent of whether a voice or data call is in progress (in some implementations, this may depend on the MSC or SMSC capabilities). SMS also guarantees delivery of the short message by the network. Temporary failures due to unavailable receiving stations are identified, and the short message is stored in the SMSC until the destination device becomes available.

SMS is characterized by out-of-band packet delivery and low-bandwidth message transfer, which results in a highly efficient means for transmitting short bursts of data. Initial applications of SMS focused on eliminating alphanumeric pagers by permitting two-way general-purpose messaging and notification services, primarily for voice mail. As technology and networks evolved, a variety of services have been introduced, including e-mail, fax, and paging integration, interactive banking, information services such as stock quotes, and integration with Internet-based applications. Wireless data applications include downloading of subscriber identity module (SIM) cards for activation, debit, profile-editing purposes, wireless points of sale (POSs), and other field-service applications such as automatic meter reading, remote sensing, and location-based services. Additionally, integration with the Internet spurred the development of Web-based messaging and other interactive applications such as instant messaging, gaming, and chatting.

1. Benefits of SMS

In today's competitive world, differentiation is a significant factor in the success of the service provider. Once the basic services, such as voice telephony, are deployed, SMS provides a powerful vehicle for service differentiation. If the market allows for it, SMS can also represent an additional source of revenue for the service provider.

The benefits of SMS to subscribers center around convenience, flexibility, and seamless integration of messaging services and data access. From this perspective, the primary benefit is the ability to use the handset as an extension of the computer. SMS also eliminates the need for separate devices for messaging because services can be integrated into a single wireless device—the mobile terminal. These benefits normally depend on the applications that the service provider offers. At a minimum, SMS benefits include the following:

  • Delivery of notifications and alerts
  • Guaranteed message delivery
  • Reliable, low-cost communication mechanism for concise information
  • Ability to screen messages and return calls in a selective way
  • Increased subscriber productivity

More sophisticated functionality provides the following enhanced subscriber benefits:

  • Delivery of messages to multiple subscribers at a time
  • Ability to receive diverse information
  • E-mail generation
  • Creation of user groups
  • Integration with other data and Internet-based applications

The benefits of SMS to the service provider are as follows:

  • Ability to increment average revenue per user (due to increased number of calls on wireless and wireline networks by leveraging the notification capabilities of SMS)
  • An alternative to alphanumeric paging services, which may replace or complement an existing paging offer
  • Ability to enable wireless data access for corporate users
  • New revenue streams resulting from addition of value-added services such as e-mail, voice mail, fax, and Web-based application integration, reminder service, stock and currency quotes, and airline schedules
  • Provision of key administrative services such as advice of charge, over-the-air downloading, and over-the-air service provisioning
  • Protection of important network resources (such as voice channels), due to SMS’ sparing use of the control and traffic channels
  • Notification mechanisms for newer services such as those utilizing wireless application protocol (WAP)

2. Network Elements and Architecture

External Short Messaging Entities

An ESME is a device that may receive or send short messages. The short message entity (SME) may be located in the fixed network, a mobile device, or another service center.

  • VMS—The VMS is responsible for receiving, storing, and playing voice messages intended for a subscriber that was busy or not available to take a voice call. It is also responsible for sending voice-mail notifications for those subscribers to the SMSC.
  • Web—The growth of the Internet has also affected the world of SMS. Therefore, it is almost mandatory to support interconnections to the World Wide Web for the submission of messages and notifications. The increasing number of Internet users has a positive impact on the SMS traffic increment experienced in the last few years.
  • E-Mail—Probably the most demanded application of SMS is the ability to deliver e-mail notifications and to support two-way e-mail, using an SMS–compliant terminal. The SMSC must support interconnection to e-mail servers acting as message input/output mechanisms.
  • Others—There are several other mechanisms to submit short messages to the SMSC that include, but are not limited to, paging networks, specialized software for PC–based messaging and operator bureaus.


SMSC is a combination of hardware and software responsible for the relaying and storing and forwarding of a short message between an SME and mobile device.

The SMSC must have high reliability, subscriber capacity, and message throughput. In addition, the system should be easily scalable to accommodate growing demand for SMS in the network.

Normally, an IN–based solution will allow for a lower entry cost compared to point solutions because it can support other applications on a single hardware platform and share resources, thereby spreading the deployment cost over several services and applications.

Another factor to be considered is the ease of operation and maintenance of the application, as well as the flexibility to activate new services and upgrade to new software releases.

Signal Transfer Point

The STP is a network element normally available on IN deployments that allows IS–41 interconnections over signaling system 7 (SS7) links with multiple network elements.



The 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. Also, if the destination station was not available when the message delivery was attempted, the HLR informs the SMSC that the station is now recognized by the mobile network to be accessible, and thus the message can be delivered.

Visitor Location Register (VLR)

The visitor location register is a database that contains temporary information about subscribers homed in one HLR who are roaming into another HLR. This information is needed by the MSC to service visiting subscribers.


The MSC performs the switching functions of the system and controls calls to and from other telephone and data systems. The MSC will deliver the short message to the specific mobile subscriber through the proper base station.

Air Interface

The air interface is defined in each one of the different wireless technologies (GSM, TDMA, and CDMA). These standards specify how the voice or data signals are transferred from the MSC to the handset and back, as well as the utilization of transmission frequencies, considering the available bandwidth and the system’s capacity constraints.

The Base Station System

All functions related to the transmission of electromagnetic radio signals between the MSC and the mobile devices are performed in the base station (BS). The BS consists of base station controllers (BSCs) and the base transceiver stations (BTSs), also known as cell sites or simply “cells.” The BSC may control one or more BTSs and is in charge of the proper resource assignment when a subscriber moves from one sector of one BTS to another, regardless of whether the next sector lies within the same BTS or in a different one.

The Mobile Device

The mobile device is the wireless terminal capable of receiving and originating short messages. Commonly, these devices have been digital cellular phones, but more recently the application of SMS has been extended to other terminals such as POS, handheld computers, and personal digital assistants (PDAs). The wireless network signaling infrastructure is based on SS7. SMS makes use of the mobile application part (MAP), which defines the methods and mechanisms of communication in wireless networks and employs the services of the SS7 transactional capabilities application part (TCAP). An SMS service layer makes use of the MAP signaling capabilities and enables the transfer of short messages between the peer entities. The capabilities of the terminal vary depending on the wireless technology supported by the terminal. Some functionality, although defined in the SMS specification for a given wireless technology, may not be fully supported in the terminal, which may represent a limitation in the services that the carrier can provide. This trend, however, is disappearing as service providers’ merger and acquisition activity demands uniform functionality across all the constituents of the parent companies. Also, some manufacturers may include additional functionality, not considered in the specification, attempting to offer a more attractive product for service providers as well as end users. This will be the case more often as service provider continue to incorporate SMS into their revenue-generating and customer-loyalty strategies.

3. Signaling Elements

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.

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.

4. Mobile-Terminated Short Message Example

  1. The short message is submitted from the ESME to the SMSC.
  2. After completing its internal processing, the SMSC interrogates the HLR and receives the routing information for the mobile subscriber.
  3. The SMSC sends the short message to the MSC using the forward short message operation.
  4. The MSC retrieves the subscriber information from the VLR. This operation may include an authentication procedure.
  5. The MSC transfers the short message to the MS.
  6. The MSC returns to the SMSC the outcome of the forwardShortMessage operation.
  7. If requested by the ESME, the SMSC returns a status report indicating delivery of the short message.
  1. The short message is submitted from the ESME to the SMSC.
  2. The SMSC sends an acknowledgement to the ESME, indicating reception of the short message.
  3. After completing its internal processing, the SMSC interrogates the HLR.
  4. The HLR sends the routing information for the mobile subscriber to the SMSC.
  5. The SMSC sends the short message to the MSC using the SMSDPP Invoke operation.
  6. The MSC transfers the short message to the MS.
  7. The MS returns an acknowledgement to the MSC.
  8. The MSC returns to the SMSC the outcome of the SMSDPP operation.
  9. If requested by the ESME, the SMSC returns a delivery receipt indicating successful delivery of the short message.

5. Mobile-Originated Short Message Example

  1. The MS is powered on and registered with the network.
  2. The MS transfers the SM to the MSC.
  3. The MSC interrogates the VLR to verify that the message transfer does not violate the supplementary services invoked or the restrictions imposed.
  4. The MSC sends the short message to the SMSC using the forwardShortMessage operation.
  5. The SMSC delivers the short message to the SME (and optionally receives acknowledgment).
  6. The SMSC acknowledges to the MSC the successful outcome of the forwardShortMessage operation.
  7. The MSC returns to the MS the outcome of the MO-SM operation.
  1. The MS transfers the SM to the MSC.
  2. The MSC interrogates the home SMSC to verify that the message transfer does not violate the supplementary services invoked or the restrictions imposed. The MSC sends the short message to the home SMSC using the SMSPP Invoke operation.
  3. The SMSC delivers an acknowledgment to the MSC.
  4. The MSC returns order release to the MS.
  5. The SMSC queries the HLR for the location of the destination MS.
  6. The HLR returns the destination (MSC) serving the destination MS.
  7. The SMSC delivers SM to the MSC serving the destination MS.
  8. The SMSC delivers the short message to the MS.
  9. The MS acknowledges to the MSC the successful outcome of the SMSDPP operation.
  10. The MSC returns to the SMSC the outcome of the MO–SM operation (delivery successful).

6. SMS Applications

SMS was initially designed to support limited-size messages, mostly notifications and numeric or alphanumeric pages. While these applications are and will continue to be widely used, there are more recent niches that SMS still can exploit.

Short bursts of data are at the heart of many applications that were restricted to the world of data networks with fixed terminals attached to a local-area network (LAN) or wide-area network (WAN). However, many of these applications are better served if the data communication capabilities could be added to the mobility of the station. Thus, a waiter who can charge a customer's credit card right at the table, at any time, instead of going to a fixed POS terminal located by the register will be able to help customers in a faster, more convenient way.

Also, the ability to track the location of a moving asset such as a truck or its load is very valuable for both providers and clients. This application, again, just needs to interchange small amounts of information, such as the longitude and latitude at a current time of the day, and perhaps other parameters like temperature or humidity.

This application does not necessarily require the monitored entity to be in movement. The requirements are basically short, bursty data and a location that has digital network coverage. For example, in a neighborhood, it would be faster, easier, and cheaper to drive a truck from the local power company, which interrogates intelligent meters to obtain their current readings and then forwards them via short message to a central data processing center to generate the billing. Similarly, delivery trucks could be alerted of the inventory of a customer running low, when the truck is close to the customer’s facilities. The truck driver could place a quick phone call to the customer to offer a short-time replenishment at a low cost for the distributor.

Another family of applications that can use SMS as a data transport mechanism is banking. It is no secret that automated teller machine (ATM) and Internet transactions are less costly than transactions completed at a branch. Internet transactions are even cheaper than ATM transactions. Therefore, enabling wireless subscribers to check their balances, transfer funds between accounts, pay their bills and credit cards is valuable, not only for the subscriber but also for financial institutions.

Entertainment applications are also good drivers of SMS usage. Examples of these are simple short message exchanges between two parties (“texting”) or between multiple participants (“chat”). Also, delivery of information that the subscriber can tailor to his or her lifestyle represents an attractive proposition for wireless users.

Wireless Web browsing allows the users to search for information without the physical restrictions of a PC. College students certainly appreciate not having to go to the computer lab or their dorm to check e-mail or find out what the required book is for the semester that is about to start.

E-mail continues to be by far the most used wireless data application. However, handsets are evolving quickly and are including more and more functionality that supports newer applications at the same time that user friendliness increases. Probably the next big success beyond wireless Web will be Internet shopping and other e-commerce applications such as electronic coupons, advertising, etc.

The potential for applications is enormous, and new needs appear to arise constantly, demanding a solution that may travel over SMS.


ATMasynchronous transfer mode
BSbase station
BSCbase station controller
BTSbase transceiver station
CDMAcode division multiple access
CMTcellular messaging teleservice
CPTcellular paging teleservice
ERMESEuropean Radio Messaging System
ESMEexternal short message entities
ETSIEuropean Telecommunications Standards Institute
GSMGlobal System for Communications
HLRhome location register
INintelligent network
IPInternet protocol
LANlocal-area network
MAPmobile application part
MOmobile originated
MO–SMmobile-originated short message
MSCmobile switching center
MTmobile terminated
MT–SMmobile-terminated short message
PDApersonal digital assistant
POSpoint of sale
PPpoint to point
SIMsubscriber identity module
SMshort message
SMDshort message delivery
SMD–PPshort message delivery–point to point
SMEshort messaging entity
SMSshort message service
SMSCshort message service center
SS7signaling system 7
STPsignal transfer point
TCAPtransactional capabilities application part
TDMAtime division multiple access
VLRvisitor location register
VMNvoice-mail notification
VMSvoice-mail system
WANwide-area network
WAPwireless application protocol