Short Message Service (SMS)

Short Message Service (SMS) is a communications protocol allowing the interchange of short text messages between mobile telephone devices. SMS text messaging is the most widely used data application on the planet, with 2.4 billion active users, or 74% of all mobile phone subscribers sending and receiving text messages on their phones. The SMS technology has facilitated the development and growth of text messaging. The connection between the phenomenon of text messaging and the underlying technology is so great that in parts of the world the term "SMS" is used as a synonym for a text message or the act of sending a text message, even when a different protocol is being used.

Short message service is a mechanism of delivery of short messages over the mobile networks. It is a store and forward way of transmitting messages to and from mobiles. The message (text only) from the sending mobile is stored in a central short message center (SMS) which then forwards it to the destination mobile. This means that in the case that the recipient is not available, the short message is stored and can be sent later. Each short message can be no longer than 160 characters. These characters can be text (alphanumeric) or binary Non-Text Short messages. An interesting feature of SMS is return receipts. This means that the sender, if wishes, can get a small message notifying if the short message was delivered to the intended recipient. Since SMS used signaling channel as opposed to dedicated channels, these messages can be sent/received simultaneously with the voice/data/fax service over a GSM network. SMS supports national and international roaming. This means that you can send short messages to any other GSM mobile user around the world. With the PCS networks based on all the three technologies, GSM, CDMA and TDMA supporting SMS, SMS is more or less a universal mobile data service.

The SMC (Short Message Center) is the entity which does the job of store and forward of messages to and from the mobile station. The SME (Short Message Entity) which can be located in the fixed network or a mobile station, receives and sends short messages.

The SMS GWMS (SMS gateway MSC) is a gateway MSC that can also receive short messages. The gateway MSC is a mobile network’s point of contact with other networks. On receiving the short message from the short message center, GMSC uses the SS7 network to interrogate the current position of the mobile station form the HLR, the home location register.

HLR is the main database in a mobile network. It holds information of the subscription profile of the mobile and also about the routing information for the subscriber, i.e. the area (covered by a MSC) where the mobile is currently situated. The GMSC is thus able to pass on the message to the correct MSC.

MSC (Mobile Switching Center) is the entity in a GSM network which does the job of switching connections between mobile stations or between mobile stations and the fixed network.

A VLR (Visitor Location Register) corresponds to each MSC and contains temporary information about the mobile, information like mobile identification and the cell (or a group of cells) where the mobile is currently situated. Using information form the VLR the MSC is able to switch the information (short message) to the corresponding BSS (Base Station System, BSC + BTSs), which transmits the short message to the mobile. The BSS consists of transceivers, which send and receive information over the air interface, to and from the mobile station. This information is passed over the signaling channels so the mobile can receive messages even if a voice or data call is going on.


Some of the common applications of SMS are:

  • Exchanging small messages like "See you at 8.30 tonight at xyz". SMS is particularly suited for these kinds of short messages because SMS is much cheaper than calling some one and giving the same message. Calling some one to give the same message would invariably take more time and hence more cost.
  • Many operators offer e-mail service over SMS. Every user is assigned an e-mail address at signup and any message delivered to that email is converted to short messages and delivered to the mobile.
  • It is possible to send e-mail messages (less than 160 characters) from a mobile phone to any e-mail address via SMS.
  • Information services like news, weather, entertainment and stock prices etc. can be availed just by sending a keyword like NEWS, WEATH etc to the short message center number.
  • SMS can be used by the network operators to provide services like balance enquiry in case of prepaid cards using SMS.
  • Mobile chatting is one more hot application of SMS
  • SMS can be used to notify users that they have received new voice-mail or fax messages.
  • It provides an alternative to alphanumeric paging services
  • Using SIM-Toolkit, now a part of GSM specifications, SMS can be used to have on the air activation of features. By sending codes embedded in short messages from the server network operators can remotely provision the user's wireless terminal
  • Internet e-mail alerts.
  • Downloading new ring tones.


Where does SMS go with services like WAP and GPRS? Well, The first thing to understand is that SMS is a bearer service. It is a mechanism of sending short messages. WAP provides the user with services and protocols that can be used on top of SMS. With the increased use of WAP, the SMS traffic in networks should see considerable increase.

GPRS, on the other hand, is a packet based data service that provides much higher throughput. Unlike SMS, it provides a real time data bearer. The users always stay connected to the network. But GPRS and SMS don’t really compete with each other in the real sense. The costs involved in sending small messages for an end user in case of SMS are expected to be lower than that in GPRS (packet data) service because the responsibility for sending the message to the recipient in case of SMS lies entirely on the short message center as opposed to the user in GPRS. In GPRS there is no concept of storage. Confirmation of delivery is a unique feature of SMS because of the very nature of short message service. Simultaneous transmission with GSM voice, data, and fax services is another distinguishing characteristic of SMS.

True, GPRS will be a much better option to use for services like WAP, but the availability of GPRS and GPRS-compliant handsets will take some time to pickup. Also, SMS needs no special network elements and handsets. It is something that almost every mobile user has and can use to send messages to any other mobile user without worrying about the capabilities of that mobile and its network!

SMS as used on modern handsets was originally defined as part of the GSM series of standards in 1985 as a means of sending messages of up to 160 characters (including spaces), to and from GSM mobile handsets. Since then, support for the service has expanded to include alternative mobile standards such as ANSI CDMA networks and Digital AMPS, as well as satellite and landline networks.[citation needed] Most SMS messages are mobile-to-mobile text messages, though the standard supports other types of broadcast messaging as well.

SMS Gateway providers

SMS gateway providers facilitate the SMS traffic between businesses and mobile subscribers, being mainly responsible for carrying mission-critical messages, SMS for enterprises, content delivery and entertainment services involving SMS, e.g. TV voting. Considering SMS messaging performance and cost, as well as the level of messaging services, SMS gateway providers can be classified as aggregators or SS7 providers.

The aggregator model is based on multiple agreements with mobile carriers to exchange 2-way SMS traffic into and out of the operator’s SMS platform (Short Message Service Centre – SMS-C), also known as local termination model. Aggregators lack direct access into the SS7 protocol, which is the protocol where the SMS messages are exchanged. SMS messages are delivered in the operator’s SMS-C, but not the subscriber’s handset, the SMS-C takes care of further handling of the message through the SS7 network

Another type of SMS gateway provider is based on SS7 connectivity to route SMS messages, also known as international termination model. The advantage of this model is the ability to route data directly through SS7, which gives the provider total control and visibility of the complete path during the SMS routing. This means SMS messages can be sent directly to and from recipients without having to go through the SMS-Centres of other mobile operators. Therefore, it’s possible to avoid delays and message losses, offering full delivery guarantees of messages and optimised routing. This model is particularly efficient when used in mission-critical messaging and SMS used in corporate communications.

SMS Spoofing

The GSM industry has identified a number of potential fraud attacks on mobile operators that are caused by abuse of SMS messaging services. The most serious of these threats is SMS Spoofing. SMS Spoofing occurs when a fraudster manipulates address information in order to impersonate a user that has roamed onto a foreign network and is submitting messages to the home network. Frequently, these messages are addressed to destinations outside the home network – with the home SMSC essentially being “hijacked” to send messages into other networks.

The only 100%-sure way of detecting and blocking spoofed messages is to screen incoming mobile originated messages to verify that the sender is a valid subscriber and that the message is coming from a valid and correct location. This can be implemented by adding an intelligent routing function to the network that can query originating subscriber details from the HLR before the message is submitted for delivery. This kind of intelligent routing function is beyond the capabilities of legacy messaging infrastructure.