Introduction to SMS PDU Mode

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 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 bit needed to send an alphanumeric text. By 5 bit coding you can contain 224 characters instatt 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 transmited 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 od send and receive of a SMS in text mode also.

Please note that there are a few differences of in the kind of implemetation 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  center, 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.

07

917238010010F5

040BC87238880900F100009930925161958003C16010

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. Ericssson 888?) the first three (colored) parts are omitted  when showing the message in PDU mode!

Octet(s)	Description
07	Length of the SMSC information (in this case 7 octets)
91	Type-of-address of the SMSC. (91 means international format of the phone number)
72 38 01 00 10 F5	Service center number(in decimal semi-octets). The length of the phone  number is odd (11), so a trailing F has been added to form proper octets. The  phone number of this service center is "+27831000015". See below.
04	First octet of this SMS-DELIVER message.
0B	Address-Length. Length of the sender number (0B hex = 11 dec)
C8	Type-of-address of the sender number
72 38 88 09 00 F1	Sender number (decimal semi-octets), with a trailing F
00	TP-PID. Protocol identifier.
00	TP-DCS Data coding scheme
99 30 92 51 61 95 80	TP-SCTS. Time stamp (semi-octets)
0A	TP-UDL. User data length, length of message. The TP-DCS field indicated  7-bit data, so the length here is the number of septets (10). If the TP-DCS  field were set to indicate 8-bit data or Unicode, the length would be the number of octets (9).
E8329BFD4697D9EC37	TP-UD. Message "hellohello" , 8-bit octets representing 7-bit data.

All the octets above are hexa-decimal 8-bit octets,  except the Service center 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:

Octet(s)	Description
00	Length of SMSC information. Here the length is 0, which means that the SMSC stored in the phone should be used. Note: This octet is optional. On some  phones this octet should be omitted! (Using the SMSC stored in phone is thus implicit)
11	First octet of the SMS-SUBMIT message.
00	TP-Message-Reference. The "00" value here lets the phone set the message  reference number itself.
0B	Address-Length. Length of phone number (11)
91	Type-of-Address. (91 indicates international format of the phone number).
6407281553F8	The phone number in semi octets (46708251358). The length of the phone  number is odd (11), therefore a trailing F has been added, as if the phone number were "46708251358F". Using the unknown format (i.e. the Type-of-Address 81 instead of 91) would yield the phone number octet sequence 7080523185 (0708251358). Note that this has the length 10 (A), which is even.
00	TP-PID. Protocol identifier
00	TP-DCS. Data coding scheme.This message is coded according to the 7bit default alphabet. Having "02" instead of "00" here, would indicate that the TP-User-Data field of this message should be interpreted as 8bit rather than 7bit (used in e.g. smart messaging, OTA provisioning etc).
AA	TP-Validity-Period. "AA" means 4 days. Note: This octet is optional, see bits 4 and 3 of the first  octet
0A	TP-User-Data-Length. Length of message. The TP-DCS field indicated 7-bit  data, so the length here is the number of septets (10). If the TP-DCS field were  set to 8-bit data or Unicode, the length would be the number of octets.
E8329BFD4697D9EC3 7	TP-User-Data. These octets represent the message "hellohello". How to do the  transformation from 7bit septets into octets is shown here

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 ( 7bit 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 messsage (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 acknowlege,  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 opertors 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 pefix *T# or *N# was or is neccary if you would like to get a acknowlege by the send of a SMS with a mobile GSM handset. Not all mobile phones can switch on the bit for a ackknowlege.

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 signaling 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 featurs are implemeted by all GSM operatos 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. By the German GSM operators is that serive 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 everytime 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 implementateion is not equal by evwery unit. Not all GSM phones, PCMIA modem cards or GSM modems offers all the features that are descriped 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 Email  address.  In some GSM networks it is possible to covert a short message to a fax or to an email.