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Figurę 3: SCTP and M3UA

the middle of this, the other nodes ąueries the HLR for information about where the ME is, if the subscribers is barred or not and so on.

2.3.1 SMS delivery Example

To illustrate how the GSM network and the HLR works we will show how an SMS is received and propagated on the terminating end. This is also shown in figurę 4.

The terminating side is morę interesting sińce the originating subscribers location is known and does not need to do a lookup to find out which MSC to use. The figurę shows the message in transit, it has finished the originating part of the procedurę and arrived at a SMS-SC, an SMS Service Centre. This in tura hands over the SMS to the SMS-GMSC, which can identify which HLR to contact by use of the MSISDN. The HLR, in tura, knows which MSC the terminating subscriber currently is connected to and if it is available for short messages. The HLR also holds information whether the subscriber is allowed to receive messages, it might be barred due to unpaid bills or a plan that does not allow messages. In this example, the subscriber is available and no barring is in place. Therefore the HLR returns the MSC address to the SMS-GMSC which forwards the message using an forwardSM operation to the MSC. The MSC then forwards the SM to the subscriber