SMS Protocol explained
08 April 2021 | Elemer TothIntroduction
To use the SMS you have to declare the number of the Short Message Service Centre (SMSC) in the Mobile Station (MS), provided that the MS supports Short Message Service-Mobile Originated (SMS-MO).
The SMS modem supports SMS-MO.
SMSC numbers for each country can be found in the SMS Service Centre Address table.At the SMS modem you enter the SMSC-number with the AT+Celular command:
at+csca = ”<SMSC-number>”
If the receiver of the SMS possesses a Telstra SIM card, the AT command has to be entered in the following way:
at+csca = "+61418706700"
With the command
at+csca?
you can question the current SMSC-number.
! Notice: In addition to the AT+CSCA command it is possible to enter the SMSC-number in front of the Protocol Data Unit (PDU).
Overview:
- MS: Mobile Station
- SME: Short Message Entity
- SMSC: Short Message Service Centre
- MMI: Man-Machine Interface
- PDU: Protocol Data Units
- SM-AL: Short Message Application Layer
- SM-TL: Short Message Transport Layer
- SM-RL: Short Message Relay Layer
- SM-LL: Short Message Link Layer
The MMI is based on the command set of AT+Cellular and could be realized by means of a terminal (for example Win-Terminal, HyperTerminal, etc) or the display of a handy.
The SM-TL provides a service to the Short Message Application Layer. This service enables the SM-AL to transfer short messages to its peer entity, receive short messages from its peer entity and receive reports about earlier requests for short messages to be transferred. The SM-TL communicates with its peer entity with six several PDUs (Protocol Data Units):
- SMS-DELIVER, conveying a short message from the SMSC to the MS
- SMS-DELIVER-REPORT, conveying a failure cause (if necessary)
- SMS-SUBMIT, conveying a short message from the MS to the SMSC
- SMS-SUBMIT-REPORT, conveying a failure cause (if necessary)
- SMS-STATUS-REPORT, conveying a status report from the SMSC to the MS
- SMS-COMMAND, conveying a command from the MS to the SMSC
The SMS modem supports the SMS-DELIVER and SMS-SUBMIT PDUs as described in the following sections.
2.1 SMS-DELIVER (Mobile Terminated)
MTI bit 1 = 0
bit 0 = 0
2.2 SMS-SUBMIT (Mobile Originated)
MTI bit 1 = 0
bit 0 = 1
! Notice: Any unused bits will be set to zero by the sending entity and will be ignored by the receiving entity !
| SCA | Service Centre Address – information element | Telephone number of the Service Centre |
|---|---|---|
| PDU Type | Protocol Data Unit Type | |
| MR | Message Reference | Successive number (0.255) of all SMS-SUBMIT Frames set by the modem |
| OA | Originator Address | Address of the originating SME |
| DA | Destination Address | Address of the destination SME |
| PID | Protocol Identifier | Parameter showing the SMSC how to process the SM (as FAX, Voice etc) |
| DCS | Data Coding Scheme | Parameter identifying the coding scheme within the User Data (UD) |
| SCTS | Service Centre Time Stamp | Parameter identifying time when the SMSC received the message |
| VP | Validity Period | Parameter identifying the time from where the message is no longer valid in the SMSC |
| UDL | User Data Length | Parameter indicating the length of the UD-field |
| UD | User Data | Data of the SM |
| RP | Reply Path | Parameter indicating that Reply Path exists |
| UDHI | User Data Header Indicator | Parameter indicating that the UD field contains a header |
| SRI | Status Report Indication | Parameter indicating if the SME has requested a status report |
| SRR | Status Report Request | Parameter indicating if the MS has requested a status report |
| VPF | Validity Period Format | Parameter indicating whether or not the VP field is present |
| MMS | More Messages to Send | Parameter indicating whether or not there are more messages to send |
| RD | Reject Duplicate | |
| MTI | Message Type Indicator | Parameter describing the message type 00 means SMS-DELIVER 01 means SMS-SUBMIT |
Parameter description
3.1 Service Centre address information element (SCA info element)
len:
The octet "len" contains the number of octets required for the number of the Service Centre plus the 1 byte "type of number".
type of number:
81H: the following number is national
91H: the following number international
(For further information see GSM 04.08 chapter 10.5.4.6)
octet:
One octet includes two BCD-digit Fields. If the called party BCD number contains an odd number of digits, the last digit shall be filled with an end mark coded as ”FH”.
Example:
if you have the SC-number +61418706700 you have to type:
style="font-size: 14.0pt">07911614786007F0
! Notice: If the "len" field is set to Zero the SMS modem takes the default value of the Service Centre address set by the AT+CSCA command
3.2 Protocol Data Unit Type (PDU Type)
SMS-SUBMIT: SMS-DELIVER:
! Notice: you have to write the PDU-type in Hex-Format, a possible example is "11H" !
RP: 0 Reply Path parameter is not set in this PDU
1 Reply Path parameter is set in this PDU
UDHI: 0The UD field contains only the short message
1 The beginning of the UD field contains a header in addition of the short message
SRI: (is only set by the SMSC)
0 A status report will not be returned to the SME
1 A status report will be returned to the SME
SRR: 0 A status report is not requested
1 A status report is requested
VPF: bit4 bit3
0 0 VP field is not present
0 1 Reserved
1 0 VP field present an integer represented (relative)
1 1 VP field present an semi-octet represented (absolute)
any reserved values may be rejected by the SMSC
MMS: (is only set by the SMSC)
0 More messages are waiting for the MS in the SMSC
1 No more messages are waiting for the MS in the SMSC
RD: 0 Instruct the SMSC to accept an SMS-SUBMIT for an short message still held in the SMSC which has the same MR and DA as a previously submitted short message from the same OA.
1 Instruct the SMSC to reject an SMS-SUBMIT for a short message still held in the SMSC which has the same MR and DA as a previously submitted short message from the same OA.
MTI: bit1 bit0 Message type
0 0 SMS-DELIVER (SMSC ==> MS)
0 0 SMS-DELIVER REPORT (MS ==> SMSC, is generated automatically by
the modem, after receiving a SMS-DELIVER)
0 1 SMS-SUBMIT (MS ==> SMSC)
0 1 SMS-SUBMIT REPORT (SMSC ==> MS)
1 0 SMS-STATUS REPORT (SMSC ==> MS)
1 0 SMS-COMMAND (MS ==> SMSC)
1 1 Reserved
(The fat-marked lines represent the features supported by the SMS modem)
! Notice: not every PDU Type is supported by the Service Centre !
3.3 Message Reference (MR)
The MR field gives an integer (0..255) representation of a reference number of the SMS-SUBMIT submitted to the SMSC by the MS.
Notice: at the SMS modem the MR is generated automatically, -anyway you have to generate it - a possible entry is for example ”00H”
3.4 Originator Address (OA) Destination Address (DA)
OA and DA have the same format explained in the following lines:
len:
The octet "len" contains the number of BCD digits
type of number:
81H: the following number is national
91H: the following number international
(For further information see GSM 04.08 chapter 10.5.4.6)
BCD-digits:
The BCD-digit Field contains the BCD-number of the Destination e.g. the Originator.
If the called party BCD number contains an odd number of digits, the last digit shall be filled with an end mark coded as ”FH”.
Example:
if you have the national number 1234567 you have to type:
style="font-size: 14.0pt">0781214365F7
3.5 Protocol Identifier (PID)
The PID is the information element by which the Transport Layer either refers to the higher layer protocol being used or indicates interworking with a certain type of telematic device.
Here are some examples of PID coding:
00H: The PDU has to be treat as a short message
01H: The PDU has to be treat as a telex
02H: The PDU has to be treat as group3 telefax
03H: The PDU has to be treat as group4 telefax
(For further information see GSM 03.40 chapter 9.2.3.9)
! Notice: it is not guaranteed that the SMSC supports every PID coding!
3.6 Data Coding Scheme (DCS)
The DCS field indicates the data coding scheme of the UD (User Data) field and may indicate a message class. The octet is used according to a coding group which is indicated in bits 7..4. The octet is then coded as follows:
|
Coding group: Bits 7..4 |
bits 3..0 |
|
0000 |
Alphabet indication Unspecified message handling at the MS 0000 Default alphabet (7-bit data coding in the User Data) 0001-1111 reserved |
0001-1110 |
Reserved coding groups |
1111 |
Data Coding/message class bit 3 is reserved, set 0 bit 2 (message coding) 0 Default alphabet (7-bit data coding in the User Data) 1 8-bit data coding in the User Data bit 1 bit 0 (message class) 0 0 Class0 immediate display 0 1 Class1 ME (Mobile Equipment)- specific 1 0 Class2 SIM specific message style="margin-left: 3.6pt">1 1 Class3 TE (Terminate Equipment)- specific |
Default alphabet indicates that the UD (User Data) is coded from the 7-bit alphabet given in appendix A. When this alphabet is used, eight characters of the message are packed in seven octets, and the message can consist of up to 160 characters (instead of 140 characters in 8-bit data coding)
In 8-bit data coding, you can relate to the INTEL ASCII-HEX table.
In Class 0 (immediate display) the short message is written directly in the display, as the SMS modem has no display the Class 0 message can be realised only in a roundabout way.
In Class 1 to Class 3 the short message is stored in the several locations: ME, SIM-card and TE.
In time the Class 2 is supported, if you choose Class 1 or Class 3 the short message is treated the same way as a Class 2 message.
! Note: It is recommended to use the Class2 message, or the coding group ”0000 bin” !
3.7 Service Centre Time Stamp (SCTS)
The SCTS is the information element by which the SMSC informs the recipient MS about the time of arrival of the short message at the Transport Layer entity of the SMSC. The time value is included in every SMS-DELIVER being delivered to the SMSC, and represents the local time in the following way:
The Time Zone indicates the difference, expressed in quarters of an hour, between the local time and GMT (Greenwich Main Time).
3.8 Validity Period (VP)
The Validity-Period is the information element which gives an MS submitting an SMS-SUBMIT to the SMSC the possibility to include a specific time period value in the short message. The Validity Period parameter value indicates the time period for which the short message is valid, i.e. for how long the SMSC shall guarantee its existence in the SMSC memory before delivery to the recipient has been carried out.
The VP field is given in either integer or semi-octet representation. In the first case, the VP comprises 1 octet, giving the length of the validity period, counted from when the SMS-SUBMIT is received by the SMSC. In the second case, the VP comprises 7 octets, giving the absolute time of the validity period termination. In the first case, the representation of time is as follows:
| VP Value | Validity period value |
|---|---|
| 0-143 | (VP + 1) x 5 minutes (i.e. 5 minutes intervals up to 12 hours) |
| 144-167 | 12 hours + ((VP-143) x 30 minutes) |
| 168-196 | (VP-166) x 1 day |
| 197-255 | (VP - 192) x 1 week |
In the second case, the representation of time is identical to the representation or the SCTS (Service Centre Time Stamp).
The case of representation is set in the VPF (Validity Period Format) in the PDU-type.
3.9 User Data Length (UDL) and User Data (UD)
The UDL field gives an integer representation of the number of characters within the User Data field to follow.
PDU Examples
Here are two examples of how to send a short message with AT+Cellular (refer to Appendix B for more details on how to send the SMS messages):
First enter PIN-number and the Service Centre Address:
at+cpin="XXXX"enter the PIN-number OK at+csca="+61418706700" enter the Service-Centre-Address (Telstra) OK
1st example:
at+cmgs=18 enter ”send message”, 18 is the actual length of the PDU message in octet > 0011000A81 4090793444 00000105E8329BFD06
type the PDU (SMS-SUBMIT) and finish with ”ctrl Z” the thin-typed characters are the Destination Address e.g. the own tel.-number(0409974344) the Service Centre address is the same as set via at+csca command
+CMGS: 0 OK at+cpms?are messages stored on the SIM-Card?
+CPMS: "SM" , 1 , 7 , "SM" , 1 , 7 on this SIM-Card is 1 message stored OK you can store at most 7 messages at+cmgr=1read stored message in location 1
+CMGR: 0,,24 07911614786007F0040B911604994743F400009930139100406B05E8329BFD06 This is a PDU (SMS- OK -DELIVER) sent by the
Service Centre
2nd example:
at+cmgw=18 write message in the memory of the SIM-card > 07911614786007F011000781409079344400F6AA0568656C6C6F type the PDU (SMS-SUBMIT) and finish with ”ctrl Z” the thin-typed characters are the Destination Address e.g. the own tel.-number (0409974344). The Service Centre Address is „+61418706700“ +CMGW: 2 OK at+cmgr=2 read stored message in location 2+CMGR: 2,,18
07911614786007F011000A81407008090500F6010568656C6C6F this is the PDU stored in location 2
OK
at+cmss=2 send the message stored in location 2
+CMSS: 3 OK at+cmss=2,“0407485455“,129 send the message stored in location 2 to the national (129 = 81H) destination address „0407485455“ at+cmss=2,“+61419877302“,145 send the message stored in location 2 to the international (145 = 91H) destination address „+61419877302“ at+cpms? are messages stored on the SIM-Card?+CPMS: "SM" , 3 , 7 , "SM" , 3 , 7 on this SIM-Card are 3 message stored
OK you can store at most 7 messages
at+cmgr=3 read stored message in location 3
+CMGR: 0,,24 07911614786007F0040B911604994743F400009930139100406B05E8329BFD06 This is a PDU (SMS- OK -DELIVER) sent by the Service Centre
Appendix A - Default Alphabet
Abbreviations:
- MS Mobile Station
- SME Short Message Entity
- SMSC Short Message Service Centre
- MMI Man-Machine Interface
- PDUs Protocol Data Units
- SM-AL Short Message Application Layer
- SM-TL Short Message Transport Layer
- SM-RL Short Message Relay Layer
- SM-LL Short Message Link Layer
- PDU Type Protocol Data Unit Type
- MR Message Reference
- OA Originator Address
- DA Destination Address
- PID Protocol Identifier
- DCS Data Coding Scheme
- SCTS Service Centre Time Stamp
- VP Validity Period
- UDL User Data Length
- UD User Data
- RP Reply Path
- UDHI User Data Header Indicator
- SRI Status Report Indication
- SRR Status Report Request
- VPF Validity Period Format
- MMS More Messages to Send
- RD Reject Duplicate
- MTI Message Type Indicator
- ME Mobile Equipment
- TE Terminal Equipment
- SIM Subscriber Identity Modul
- 0 phone failure
- 1 no connection to phone
- 2 Phone-adaptor link reserved
- 3 operation not allowed
- 4 operation not supported
- 5 PH-SIM PIN necessary
- 10 SIM not inserted
- 11 SIM PIN required
- 12 SIM PUK required
- 13 SIM failure
- 14 SIM busy
- 15 SIM wrong
- 16 incorrect password
- 20 memory full
- 21 invalid index
- 22 not found
- 23 memory failure
- 24 text string too long (+CPBW)
- 25 invalid characters in text string
- 26 dial string to long
- 27 invalid characters in dial string
- 30 no network service
- 31 network timeout
- 100 unknown
- 265 PUK for theft protection necessary
- 266 PUK2 for SIM necessary
- 267 PIN2 for SIM necessary
More information
- How to receive an SMS Message with a modem using AT commands
- How to send an SMS with a GSM modem using AT commands
- SMS Protocol explained
- Introduction to SMS PDU Mode
- How to use an SMS modem on a virtual machine
- Ozeki 4G SMS Modem
- Ozeki 4G SMS Modem Dual Sim
- Ozeki SMS Gateway One
- Ozeki SMS Gateway PLC
- Compare SMS devices
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