Data Services on Signaling/Broadcast Channels—SMS

The GSM technology is the first in providing data services on signaling and broadcast channels. These services are commonly known as SMS. There are two different types of SMS: SMS point-to-point and SMS broadcast. We will look into both of these services in this section. Data services like SMS are also provided by other radio technologies, such as IS-136 and IS-95.SMS point-to-point is the data service that uses only the signaling channel (SDCCH) to transport the data over the air interface. In SMS, a short string of text (maximum 126 characters) is carried from one subscriber to another.

There is another type of SMS service called SMS broadcast, which is the only broadcast channel data service. As the name implies, the user terminal can only receive the data broadcasted from the network. This service transports data on a specially defined broadcast channel, CBCH, over the air interface. This service is also limited to a short text string based on the carrying capacity of the CBCH. This service was anticipated to be used by the broadcast data applications, such as traffic reports and weather alerts. This service didn't get much attention from service providers because it didn't provide a good revenue generation model.

SMS point-to-point (SMS p-p) is a dedicated service between two users. GSM has defined two teleservices for SMS: SMS-MO (mobile origination), and SMS-MT (mobile termination). Using this service, a user sends a short string of alphanumeric characters to another user. SMS can also be used by the network operators to notify a user about certain status (e.g., number of messages waiting in voice mail). SMS is based on the concept that the signaling channel capacity can be utilized for carrying a few bytes of user data. SMS has also opened up GSM PLMN for support of telematics. For example, with a reduced GSM user equipment, a vending machine can become a short message sending entity to send a message to the vendor when a specific supply is needed.
SMS is defined with a store-and-forward mechanism. In this way the message is saved in case the addressee is not reachable and is later delivered based on availability. A subscriber must subscribe to this service, and it is provisioned in the subscriber's profile in HLR. The SMS-GMSC is the gateway to a GSM PLMN for SM delivery addressed in the PLMN. The SMS-GMSC interrogates the HLR to locate the subscriber. The interrogation protocol is a generic protocol used also by the GSM call control procedures for locating a called party. The HLR informs about the current location in terms of MSC/VLR, which is used by the SMS-GMSC for eventual delivery of the SM to the MS. In case the MS is not reachable, the HLR sends a response back to the SMS-GMSC.