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, for example
 - Info about the senders ( Service center number, sender number) 
 - Protocol information  (Protocol identifier, Data coding scheme)
 - 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:

07911326040000F0040B911346610089F60000208062917314080CC8F71D14969741F977FD07

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