MAP Transport Protocols
While the MAP 3GPP specifications make some effort to divorce MAP from the layer that transports it, the typical transport is via TCAP which in turn is via SCCP/MTP and/or SIGTRAN protocols (SUA, M3UA etc.).
A MAP_OPEN construct therefore is directly related to a TCAP_BEGIN with a MAP application context, a MAP_CLOSE is a TCAP_END.
If a message is being delivered using MAP phase 2 or higher, and over MTP rather than SIGTRAN then the maximum MTP PDU size may cause the sender to instigate segmented message sending. This process is not related to concatenation, but simply means that the transaction with the MSC/SMSC/SGSN involves more steps than usual. The recommended way is an empty TCAP_BEGIN, followed by the MAP content within a TCAP_CONTINUE, and completing with a TCAP_END. The TCAP_BEGIN has TCAP related information that would otherwise cause the limit to be exceeded due to the additional fields added by MAP phase 2. The exact point that segmentation is required is dependent on factors such as the length of the addresses but is mainly dependent on the message length itself. 7 bit alphabet messages that are 140 characters or greater are typically subject to the MAP segmentation procedure.
This segmentation procedure is also increasingly followed, and optionally enforced, by carriers to avoid SMS spoofing impacting their customers. This works because the sending party must receive the responses in order to send a message and so their originating address must be correct.
Read more about this topic: Short Message Service Technical Realisation (GSM)
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