Construction of SMS PDU’s
LZT 123 7632 R1A
5
2 About
SMS
SMS, as you already probably know stands for Short Message Service.
SMS provides a means of sending messages of limited size from and to
GSM mobile stations. SMS makes use of something called a Service
Centre, which acts as a store and forward centre for the SMS messages.
Two different types of SMS services can be defined:
Mobile Originated (MO) – Mobile originated SMS will be transported from
a mobile station to the Service Centre. These may be destined for other
mobiles or may even be destined for other services which the GSM
network and Service Centre supports.
Mobile Terminated (MT) – Mobile terminated SMS will be transported from
a Service Centre to a mobile Station.
3 What is a PDU?
PDU is short for Protocol Data Unit (or it could even be Packet Data Unit,
the different meanings are in use). These data units represent how the
digital information is coded and structured when it sent over the air
interface.
When we use a mobile phone, we usually enter a text message by the
keypad of the phone, give it a phone number to be sent to, press the
“YES” button and, usually, receive a “MESSAGE SENT” notification from
the phone. With a GSM module this process has to be performed with AT
commands from a terminal of some kind, since there is no keyboard. In
some cases the module does not support text mode SMS, and the
message must be coded as a PDU.
Using the PDU mode gives the user much more power over the
information to be sent and how it is sent. For instance one may not wish to
send a text message, but may wish to send raw Data. PDU format will
allow you to do this.
There are different kinds of PDU involved in SMS messaging, from SMS-
REPORT, SMS-COMMAND etc. etc. but we will consider only two
different types here, probably the most important as well.
Related to Mobile originated SMS is the PDU of type
SMS-SUBMIT
. This
is a PDU that is sent from one Mobile terminal to the Service centre.
SMS-
SERVICE
CENTER
Mobile Station
Mobile Station
MO SMS
MT SMS
(SMS-SUBMIT PDU)
(SMS-DELIVER PDU)
