How SMS are sent from internet/ websites to mobile phones

Since the invention of the Internet, a new era of convergence of technologies has ushered, bringing close distant technologies which could not have been thought to be related at all. Sure, we can send a mail from our GPRS activated cellular device, but is the reverse true? Can we send a text message from the Internet? The answer is yes and we all have used free online SMS sending websites either on a regular basis or just for testing if it works or is merely faking. The best and the most attractive part is the word ‘free’ and sometimes, ‘unlimited’ too. But have you ever wondered how this is made possible? We’re going to cruise along the path an online SMS would follow in the whole process in the next few pages.

The first and foremost requirement is a terminal with an internet browser and internet access. We visit any website offering free SMS service, say www.xyz.com and register for a free account (Some websites allow sending SMS as guest users too). This would be sufficient on the end users part to get started. To send an online SMS, an interface is presented to the user which allows them to enter the destination mobile number and the Text Message. Usually, the service is offered on a national basis through an Aggregator Gateway adopting Local Termination model, i.e. we are allowed to send messages on numbers from the same country only, but a few premium sites may offer international messaging too.

The life of an online SMS commences in the network jungle the moment you click on the send button. And that is where the user can cross fingers and hope it reaches the intended recipient while waiting for a delivery report. The internet is a quagmire of various protocols which help us to navigate through the World Wide Web. One of the most common formats adopted is the HTTP (Hyper Text Transfer Protocol), which does an effective cross-linking and navigation of various nodes of the webspace. The message entered by the user may travel the web in various protocols like HTTP, HTTPS (HTTP + SSL encryption), XML over HTTP / HTTPS, SMTP (email to SMS), and FTP. A simple HTTP request can be like:

The block diagram of the message flow is shown above.

1. The website server makes an entry of all the SMS details in its database record before it injects the message into the network. This may include the IP address of the user, user details, the time of login, message sent, destination number, time of sending message etc.

2. For a message to be accepted as a valid SMS in the mobile networks, it must have a few necessary parameters. These are Header (this identifies the type of message to be sent), Service Centre Timestamp, phone number of the sender, protocol identifier, data coding scheme, length of the message and the message itself. The message is input to a SMS gateway which handles the necessary protocol conversion. This protocol conversion is necessary because the internet request coming from a web server is generally in the form of an http request(Hyper Text Transfer Protocol), while the mobile networks make use of the SS7 signalling protocols. Thus, an SMS gateway acts as a bridge between the two network formats. For illustration, the message sent above as an http request may look like this in the CIMD protocol after conversion:

A SMS Gateway can be an application API written in programming languages like C/C++, Java etc. implemented in the server backend, or it can be a readymade solution provided by the software companies in the form of SaaS (Software as a Service). Websites usually employ Direct-to-SMSC gateway type of gateways to achieve this kind of conversion. Such Gateways are directly connected to the Mobile Operators SMS Centers (SMSC) through internet or via leased lines. It is the SMS centers which store the messages sent by a user and deliver it to the intended recipient when they are available.

3. All the SMS transactions on the mobile network are handled by SMS Centers (SMSCs) which can work on various protocols. It is necessary to convert the requests into the suitable protocols for successful communication as they do not understand the internet protocols. For example, Nokia uses CIMD protocol, CMG uses EMI while many others use SNPP protocol.

After going through a succession of steps of internal processing, the local GMSC sends an appeal to the HLR (Home Location Register) and gets the information required to route the message to the receiver. This includes the network information of the recipient so that it can be sent to Main Switching Center of the respective provider via the home MSC. The GMSC is the window of any network to the rest of the mobile networks.

4. The target MSC checks the receiver information using the VLR (Visitor Location Register) and does the necessary authentication and validation. The HLR and VLR are databases that keep entries of all the registered users in their specified coverage areas. The SMS uses a store and forward service. The MSC of the recipient forwards the message to the mobile server unit and stores the message there until received by the recipient or the maximum storage time expires, whichever comes first. If successful, the text is put on the internal memory of the SIM card until it is deleted by the user.

5. The MSC of the recipient then provides a form of acknowledgement to the SMS centre which would be reported back to the original sender as a delivery report if such service has been provided by the website.

In this way, a website can be used to send SMS to a valid mobile number.