SMS Basics and Types Explained

Everyone is familiar with voice communication between mobile phones and between landlines and mobile phones. Text-based communication is increasingly replacing voice communication due to its lower cost. When someone wants to send brief information, they often prefer sending a message rather than making a call. This is known as SMS or short message service.

The key difference between voice and SMS is that voice communication is real-time, while SMS is non-real-time. SMS messages are typically limited to a maximum of 160 characters, which translates to about 140 bytes in GSM, where each character is encoded using 7 bits.

As mentioned, SMS is a non-real-time service, meaning that SMS messages sent from one mobile phone to another are stored on the SMSC (Short Message Service Center) and computers before being delivered. This is also referred to as a “store and forward” service.

For detailed information about the SMS path from the originating mobile phone to the terminating mobile phone, understand SMS MO/MT call flow. SMS can be transported using the signaling channel (i.e., SDCCH in GSM) when GPRS is not activated. If GPRS is activated, SMS can also be carried on the traffic channel.

Because SMS is carried by the signaling channel, it can be sent while voice or data communication is in progress. Voice and data communication use TCH or traffic channels.

Figure 1 depicts the working of SMS. As shown, user #1 transmits an SMS (meant for user #2), which goes to the SMS gateway. The SMS gateway forwards the message to the mobile service provider, which then retransmits the SMS to user #2.

SMS Types

Depending on the application and how SMS messages are sent from the originating system to the terminating system, SMS can be classified into P2P, A2P, P2N, and I2P categories. These are described below.

• Person to Person (P2P): This type of SMS is sent and received between two mobile subscribers. If the SMS is within the same RAT (Radio Access Technology) (i.e., GSM to GSM), the originating and terminating SMSC are the same. However, if the SMS is between two different RATs, the originating SMSC sends the message to the corresponding destination SMSC of the terminating RAT.

• Application/Advertiser to Person (A2P): In this type of SMS, advertising agencies contact the service operator to send their advertising details to the mobile subscribers registered with them. The service operator then broadcasts SMS messages to the mobiles with these advertising details. This method is also referred to as “push mode.”

• Person to Network (P2N) or Person to Application (P2A): In this type of SMS, content providers provide details such as ringtones, games, and pictures to be downloaded to the network service operator/provider. They both have an agreement and are charged accordingly. In this type, the service provider gives out short codes to the content provider.

• Internet to Person (I2P): In this type of SMS, some websites allow internet users to send SMS messages to mobile subscribers from the internet. Some websites also accept incoming messages from mobile subscribers.