Mobiletidings

Combining SMS Messages: The Complete Guide

As discussed in a previous post, many features of SMS messages become available if you can set the User Data Header (UDH) field.

When using a GSM modem in text mode you can’t send a UDH, so you need to use PDU mode.

How do you add a UDH?

There are 2 things you need to do:

  • Set the UDH bit in the first octet of the PDU. For an SMS-SUBMIT PDU (the only one we’ve been using so far) the value is normally 0×01. To indicate that a UDH is present we need to set bit 6 (0×40). So for an SMS-SUBMIT with UDH present, we set the PDU type to 0×41.
  • With the UDH bit set, this first octet of the payload (or User Data = UD) needs to indicate the length of the UDH in octets. This field is known as UDHL.

The confusing part here is that the UDHL in octets, but the User Data Length (UDL) is in septets (in the case of a GSM-7 encoded text message). Here is what a payload would look like:

Size Field Description
1 octet UDL size in septets (counting UDHL + UDH + text)
1 octet UDHL size in octets (counting only UDH)
n octets UDH
6 or fewer bits filler to assure the UD septets that follow are on a septet boundary.
m septets text GSM-7 encoded characters
GSM-7 encoded charcaters always start on a septet boundary, so if you have a UDH you might have to add some padding bits to have you text start at a septet boundary.

If the text is UCS-2 encoded it would look simpler:

Size Field Description
1 octet UDL size in octets (counting UDHL + UDH + text). Each character in the text occupies a short (=2 octets).
1 octet UDHL size in octets (counting only UDH)
n octets UDH
m shorts text UCS-2 encoded characters.

 

What is in a UDH?

The UDH itself consists of one or more information elements (IE). An item is encoded in 3 fields:

Size Field Description
1 octet IEI Information element identifier
1 octet IEDL Information element data length
n octets IED Information element data

 

There can be many information elements present in a UDH. The one IE we need to construct a big SMS message from several smaller messages is the IE with identifier (IEI) 0×00.

This IEI has 3 values as part of its data:

  • A concatenated short message reference number. If you send multiple concatenated SMS messages the receiving end will need to be able to figure out which part belongs to which large message. This reference number solves that problem.
  • The total number of SMS messages in the large message.
  • The sequence number of the current SMS message

So in more detail, if an SMS message is number 2 of 3, this information element should be in the UDH field:

Size Field Value Description
1 octet IEI 0×00 Information element identifier for a concatenated short message
1 octet IEDL 0×03 Information element data length
1 octet 0xa1 A reference number (must be the same for all parts of the same larger messages)
1 octet 0×03 This message has 3 parts
1 octet 0×02 This is part 2.

 

Here is a complete set of AT commands to send a message consisting of 3 parts:

First part:

AT+CMGS=153<crlf>
0041000B915121551532F40000A0050003000301986F79B90D4AC3E7F53688FC66BFE5A0799A0
E0AB7CB741668FC76CFCB637A995E9783C2E4343C3D4F8FD3EE33A8CC4ED359A079990C22BF41
E5747DDE7E9341F4721BFE9683D2EE719A9C26D7DD74509D0E6287C56F791954A683C86FF65B5
E06B5C36777181466A7E3F5B0AB4A0795DDE936284C06B5D3EE741B642FBBD3E1360B14AFA7E7
<ctrl-Z>
Field Value Description
PDU-TYPE 0×41 SMS-SUBMIT + UDH bit
UDL 0xA0 Total payload length in septets (160)
UDHL 0×05 User data header length
UDH 0×0003000301 Concatenated message IE:

  • IEI = 0×00
  • IEDL = 0×03
  • Ref nr = 0×00
  • Total  = 0×03
  • This part = 0×01

 

Second part:

AT+CMGS=153<crlf>
0041010B915121551532F40000A005000300030240EEF79C2EAF9341657C593E4ED3C3F4F4DB0
DAAB3D9E1F6F80D6287C56F797A0E72A7E769509D0E0AB3D3F17A1A0E2AE341E53068FC6EB7DF
E43768FC76CFCBF17A98EE22D6D37350B84E2F83D2F2BABC0C22BFD96F3928ED06C9CB7079195
D7693CBF2341D947683EC6F761D4E0FD3CB207B999DA683CAF37919344EB3D9F53688FC66BFE5
<ctrl-Z>

 

Field Value Description
PDU-TYPE 0×41 SMS-SUBMIT + UDH bit
UDL 0xA0 Total payload length in septets (160)
UDHL 0×05 User data header length
UDH 0×0003000302 Concatenated message IE:

  • IEI = 0×00
  • IEDL = 0×03
  • Ref nr = 0×00
  • Total  = 0×03
  • This part = 0×02

 

Third part:

AT+CMGS=139<crlf>
0041020B915121551532F4000090050003000303CAA0721D64AE9FD3613AC85D67B3C32078589
E0ED3EB7257113F2EC3E9E5BA1C344FBBE9A0F7781C2E8FC374D0B80E4F93C3F4301DE47EBB41
70F93B4D2EBBE92CD0BCEEA683D26ED0B8CE868741F17A1AF4369BD3E37418442ECFCBF2BA9B0
E6ABFD9EC341D1476A7DBA03419549ED341ECB0F82DAFB75D<ctrl-Z>
Field Value Description
PDU-TYPE 0×41 SMS-SUBMIT + UDH bit
UDL 0×90 Total payload length in septets (144)
UDHL 0×05 User data header length
UDH 0×0003000303 Concatenated message IE:

  • IEI = 0×00
  • IEDL = 0×03
  • Ref nr = 0×00
  • Total  = 0×03
  • This part = 0×03

Note that the other fields of the PDU remain the same, with the exception of the ‘message reference’ (which is the octet after the PDU type). The message reference should be different with each SMS-SUBMIT PDU sent.

These three messages together make “Lorem Ipsum”  (see Lorem Ipsum). I am pretty sure this is the first Lorem Ipsum in SMS speak.

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