Long messages are transmitted as multiple SMSs, segmented by the application and then reassembled on the phone.
Each fragment as a
Reference number - identifying the message to which the fragment belongs
Number of Fragments
Fragment Number or sequence number
GSM supports fragmentation through a user data header (UDH) in the front of the SMS message. The structure is specified in the 03.40 GSM/3GPP specification. In the specification, fragment/sequence number always starts at 1.
CDMA support fragmentation also, though the encoding standard is different (CDMA does not support GSM user data headers). For CDMA encoding, the first byte is zero (message type) and this indicates that fragmentation (and port numbers) follow.
Implementation of the Wireless Datagram Protocol (WDP) of WAP, which carries WAP Push over CDMA specifies fragmentation and reassembly. The specification can be found at Wap Forum. The specification indicates that fragment/sequence numbers start at 0 (unlike 1 for GSM).
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