SMS messages are created by mobile phones or other devices
(e.g.: personal computers). These devices can send and receive SMS messages
by communicating with the GSM network. All of these devices have at least one
MSISDN number. They are called Short Messaging Entities. To understand
their roles in the GSM SMS network, please take a look at Figure 1 below:
The SMEs are the starting points (the source) and the
end points (the receiver) for SMS messages. They always communicate with
a Short Message Service Center (SMSC) and never communicate directly with
each other. An SME can be a mobile telephone. Depending on the role of
the mobile phone in the communication, there are two kinds of SMS
messages: Mobile-originated (MO) messages and Mobile-terminated (MT)
messages. MO messages are sent by the mobile phone to the SMSC.
Mobile-terminated messages are received by the mobile phone. The two
messages are encoded differently during transmission.
An SME can also be a computer equipped with a messaging
software, such as Ozeki NG - SMS Gateway, which can communicate directly
with the SMSC of the service provider. For this communication, a mobile
phone attached to the PC with a phone-to-pc data cable (Figure 2) or a
direct IP link (Figure 3) can be used.