Introduction to SMS PDU Mode

Mobile network operators and SMS service providers use the SMS protocol to transfer SMS messages between network entities. The SMS protocol allows mobile devices to send and receive SMS. SMS sent by a mobile device is called Mobile Originated (MO) SMS. SMS received by the mobile device are called Mobile Terminated (MT) SMS. This guide presents information on how mobile phones encode the MO and MT SMS messages, in other words how do they formulate the Protocol Data Units (PDUs) to send and receive messages through the network.

Introduction

The PDU mode offers to send binary information in 7-bit or 8-bit format. That is helpful if you have to send compressed data, binary data or you like to build your own encoding of the characters in the binary bit stream. If you go back on the old encoding of a Fernschreiber, then there are only 5 bits needed to send an alphanumeric text. By 5-bit coding you can contain 224 characters instead of 160 characters in 7-bit Text mode. An others reason could be the sending of integer data.

If you would like to have the full control of your transmitted data in Text mode, you have to understand the PDU mode, because there are a few commands where you can set numeric parameters that change the kind of send and receive of a SMS in text mode also.

Please note that there are a few differences of in the kind of implementation of the PDU mode and by the other AT commands.

The SMS message, as specified by the Etsi organization (documents GSM 03.40 and GSM 03.38), can be up to 160 characters long, where each character is 7 bits according to the 7-bit default alphabet. Eight-bit messages (max 140 characters) are usually not viewable by the phones as text messages; instead, they are used for data in e.g. smart messaging (images and ringing tones) and OTA provisioning of WAP settings. 16-bit messages (max 70 characters) are used for Unicode (UCS2) text messages, viewable by most phones. A 16-bit text message of class 0 will on some phones appear as a Flash SMS (aka blinking SMS or alert SMS).

The PDU format

There are two ways of sending and receiving SMS messages; by text mode and by PDU (protocol description unit) mode. The text mode (unavailable on some phones) is just an encoding of the bit stream represented by the PDU mode. Alphabets may differ and there are several encoding alternatives when displaying an SMS message. The most common options are; "PCCP437", "PCDN", "8859-1", "IRA" and "GSM". These are all set by the at-command AT+CSCS, when you read the message in a computer application. If you read the message on your phone, the phone will choose a proper encoding. An application capable of reading incoming SMS messages, can thus use text mode or PDU mode. If text mode is used, the application is bound to (or limited by) the set of preset encoding options. In some cases, that's just not good enough. If PDU mode is used, any encoding can be implemented.

Receiving a message in the PDU mode

The PDU string contains not only the message, but also a lot of meta-information about the sender, his SMS service centre, the time stamp etc. It is all in the form of hexa-decimal octets or decimal semi-octets. The following string is what I received on a Nokia 6110 when sending the message containing "hellohello" from www.mtn.co.za.

This octet sequence consists of three parts: An initial octet indicating the length of the SMSC information ("07"), the SMSC information itself ("917238010010F5"), and the SMS_DELIVER part (specified by ETSI in GSM 03.40).

! Note: on some phones (e.g. Ericsson 888?) the first three (coloured) parts are omitted  when showing the message in PDU mode !

All the octets above are hexa-decimal 8-bit octets, except the Service centre number, the sender number and the timestamp; they are decimal semi-octets. The message part in the end of the PDU string consists of hexa-decimal 8-bit octets, but these octets represent 7-bit data (see below). The semi-octets are decimal, and e.g. the sender number is obtained by performing internal swapping within the semi-octets from "72 38 88 09 00 F1" to "27 83 88 90 00 1F". The length of the phone number is odd, so a proper octet sequence cannot be formed by this number. This is the reason why the trailing F has been added. The time stamp, when parsed, equals "99 03 29 15 16 59 08", where the 6 first characters represent date, the following 6 represents time, and the last two represents time-zone related to GMT.

Interpreting 8-bit octets as 7-bit messages

This transformation is described in detail in GSM 03.38, and an example of the "hellohello" transformation is shown here. The transformation is based on the 7-bit default alphabet , but an application built on the PDU mode can use any character encoding.

Sending a message in the PDU mode

The following example shows how to send the message "hellohello" in the PDU mode from a Nokia 6110.

AT+CMGF=0 //Set PDU mode

AT+CSMS=0 //Check if modem supports SMS commands

AT+CMGS=23 //Send message, 23 octets (excluding the two initial zeros) >0011000B916407281553F80000AA0AE8329BFD4697D9EC37There are 23 octets in this message (46 'characters'). The first octet ("00") doesn't count, it is only an indicator of the length of the SMSC information supplied (0). The PDU string consists of the following:

Introduction to SMS Text Mode

The Short Message Service SMS, as defined within the GSM 900 / 1800 / 1900 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 Centre (SMSC) instead. Each mobile telephone network that supports SMS has one or more messaging centres 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 acknowledges 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 a confirmation 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 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.

Another way is to send a prefix with the text message. These 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-Mobil 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 necessary if you would like to get an acknowledge by the send of a SMS with a mobile GSM handset. Not all mobile phones can switch on the bit for an acknowledge.

If you would like to understand the 3 parameters of this command, you have to understand the SMS in PDU mode. Another 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 takeover 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 operators worldwide. 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. In Germany, GSM operators include this service with every type of subscription.

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 included every time.

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, PCMIA 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 is usually another mobile phone, but it may also be a fax machine or an email address. In some GSM networks, it is possible to convert a short message to a fax or an email.