The Short Message Service SMS, as defined within the GSM and UMTS digital mobile phone standard has several unique features:
A single short message can be up to 160 characters (7 bit coded ) or 140 characters (8 bit coded) of text in length. Those 140 / 160 characters can comprise of words or numbers or an alphanumeric combination. Non-text based short messages (for example, in binary format) are also supported. More about that binary mode you will find at the link PDU mode.
The Short Message Service is a store and forward service, in other words, short messages are not sent directly from sender to recipient, but always via an SMS Center (SMSC) instead. Each mobile telephone network that supports SMS has one or more messaging centers to handle and manage the short messages. More about SMSC you can read at the link SMSC.
The Short Message Service features confirmation of message delivery. This means that unlike paging, users do not simply send a short message and trust and hope that it gets delivered. Instead the sender of the short message can receive a return message back notifying them whether the short message has been delivered or not. The default factory parameter of this acknowledge from the transmitter of a SMS to the receiver of a message by most GSM modem is OFF, so that you will get no confirmation from the receiver. If you turn it on. then you get an conformation that the SMSC has got the message and after the delivery of the short message to the receiver you will get an additional, second message (SMS backward) that the message is delivered to the a GSM phone or modem. In this automatic generated message is the data and time of the delivery coded. The acknowledge, the coding scheme the time of storage of a short message in the SMSC and a lot of more will be set with the command AT+CSMP.
A other way is to send a prefix with the text message. This prefixes are not equal by the different GSM operators in the world. By the German GSM operator Vodafone you have to add *N# and by the GSM operator T-Mobile you have to add *T#. The notation with AT+CSMP is equal in all SMSC. The handling with the prefix *T# or *N# was or is neccary if you would like to get a acknowledge by the send of a SMS with a mobile GSM handset. Not all mobile phones can switch on the bit for a acknowledge.
If you would like to understand the 3 parameters of this command, you have to understand the SMS in PDU mode. An other important command is AT+CNMI. It tells the GSM modem how to handle an incoming short message.
Short messages can be sent and received simultaneously with GSM voice, Data and Fax calls. This is possible because whereas voice, Data and Fax calls take over a dedicated radio channel for the duration of the call, short messages travel over and above the radio channel using the signalling path. As such, users of SMS rarely if ever get a busy or engaged signal as they can do during peak network usage times. If you switch on the simultaneously receive of a SMS during a data call, then you will get a SMS string during a fax or data call.
Ways of sending multiple short messages are available. SMS concatenation (stringing several short messages together) and SMS compression (getting more than 160 characters of information within a single short message) have been defined and incorporated in the GSM SMS standards. Not all that possible features are implemented by all GSM operates world-wide. Single message should work everywhere.
To use the Short Message Service, users need the relevant subscriptions and hardware, specifically:
A subscription to a mobile telephone network that supports SMS. By the German GSM operators is that service by every different kind of subscription included.
Use of SMS must be enabled for that user (automatic access to the SMS is given by some mobile network operators, others charge a monthly subscription and require a specific opt-in to use the service). In Germany that is every time included.
A mobile phone or GSM modem that supports SMS. Today this is supported by every GSM phone or GSM modem.
Knowledge of how to send or read a short message using their specific model of mobile phone or GSM modem. The implementation is not equal by every unit. Not all GSM phones, PCMCIA modem cards or GSM modems offers all the features that are described in the ETSI.
A destination to send a short message to, or receive a message from. This is usually another mobile phone but may be a fax machine or an e-mail address. In some GSM networks it is possible to covert a short message to a fax or to an e-mail.