SMS (Short Message Service) is specified by the ETSI (standards GSM 03.40₁ and 03.38₂ ). It can contain up to 160 characters, where each character is written according to the 7-bits GSM default alphabet.[ 7 bits default table ]

SMS also contains some meta-data, e.g.

1. Info about the senders ( Service center number, sender number)
2. Protocol information (Protocol identifier, Data coding scheme)
3. Timestamp

There are 2 ways to receive and send SMS messages _a, PDU (protocol discription unit) and Text mode. In this document we focus on PDU mode.

PDU format can be used on any encoding.
As an example:
07911326040000F0040B911346610089F600002080629 17314080CC8F71D14969741F977FD07

The above PDU string contains the message "How are you?" and was read from a Siemens C35i mobile phone.The string is built from hexadecimal-octets and semi decimal-octets. As mentioned, SMS contains metadata about itself.
We explain it using the example above:

Octet(s)	Description	format	In this example
07	Length of the SMSC information	hex-octet	7 octets
91	Type of address of SMSC	hex-octet	internation format
13 26 04 00 00 F0	SMSC number	decimal semi-octets b	31624000000
04	First octet of this SMS-DELIVER message.	hex-octet	TP-MMS
0B	Length of the sender address	hex-octet	11 (decimal)
91	Type of address of the sender number	hex-octet	...
13 46 61 00 89 F6	Sender number	decimal semi-octets	31641600986
00	Protocol identifier	hex-octets	...
00	Data encoding scheme	hex-octets	...
20 80 62 91 73 14 08	Time stamp c	decimal semi-octets	06-08-02 29:17:31
0C	Length of User data (SMS message)	hex-octets	12 (decimal)
C8 F7 1D 14 96 97 41 F9 77 FD 07	User data	8-bit octets respresenting 7-bit data	How are you?

PDU Encoder and Decoder

Hexadecimal PDU Message		7/8/16 Bit PDU Message (readable)
07911326040000F0040B911346610089F60000208062917314080CC8F71D14969741F977FD07
		Show User data translation (7 bit only)

String sms message		Hexadecimal PDU Message
SMSC Receiver Alphabet Size 7816 How are you?

Notes

a.	To see which modes your mobile supports, you can use the "AT+CMGF=?" command. You will get a response with the supported SMS formats  0: PDU mode, 1: Text mode
b.	To obtain data from a string that is written as a semi-octet, you have to swap the semi-octet string. If the length of a semi-octet string is odd, you have to add an extra "F" to make it even, so that you get a proper octet string. Example:  "13 46 61 00 89 F6" becomes "31 64 16 00 98 6F"
c.	Time stamp is represented in semi-octets (See note b). So "20 80 62 91 73 14 08" becomes "02 08 26 02 29 17 31 80". The first 6 characters represent the date, the next 6 characters represent the time, the last 2 characters represent the time-zone related to the GMT.
d.	For advanced PDU functions, PDUSpy from http://www.nobbi.com/pduspy.htm is highly recommended.