Long SMS Messages

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A standard SMS message will send messages of at most 160 7-bit characters to a recipient. But what is the message to be sent is longer than this. The PDU SMS protocol allows for this and the message is split into many SMS messages and then combined back together by the receiver. These are called long or concatenated SMS messages. When the message to be sent is longer than 160 7-bit characters then the client messaging app will split the message and send these part messages as indiidual SMS messages.

In the SMS message header the second octlet is set to 11 for an SMS message, this is changed to 41 for a concatenated or long SMS message, on each of the SMS messages making up the long message.

05Length of the header
00Type of Message
03Length of header data
A6Unique number for SMS message
02How many parts
01Part Number
XXXXXData
For messages that are part of a long SMS, the 160 SMS character limit is further reduced by 7 to allow for the UDH (User Data Header) Within this UDH there is a value 00 to FF to indicate the number of SMS messages making up the long message and a second number indicating the sequence number of this data. So theoretically a message of 37788 charactes can be sent over SMS as a single Long SMS but a more realistic limit is 5 SMS messages or 760 characters.

The sending App splits the long message into the correct number of SMS messages and sends these as individual SMS messages. A long SMS comprising 3 normal SMS messages will be charges as 3 messages. The receiving app will receive the individual SMS messages, and piece together the full long message for display on the receiver's device.