How is a Multipart SMS Constructed?

Question

  • How is a multipart SMS constructed?
  • How many characters are included in a single part of a concatenated SMS?
  • How are long messages split into parts?

Answer

The message body of a long SMS is split into chunks of 132 bytes (152 GSM characters or 66 Unicode characters), excluding headers.

The maximum length of a single SMS body is 140 bytes.  The length of the body of a multi-part SMS is reduced because those extra 8 bytes are needed to hold 'segmentation information' that is sent along with the body of the sms, so that the handset knows how to put all the parts back together again into a single long message.

Due to the nature of SMS, and networks in general, the different parts may arrive in any order, so this segmentation information is vital for the handset to be able to re-construct the message properly.

One final thing to consider with long SMS, it's not always the case that 160 characters will fit into a single GSM-encoded SMS. There are some characters in the GSM alphabet that require 2 characters because they are 'extended alphabet' characters. So they need an 'escape' char + a char. A good example is the £ sign or Euro symbol. These 'escape' characters are also taken into account when deciding where to split the message into parts. 

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