Short Message Service (SMS)

SMS provides paging functionality for alphanumeric messages of up to 160 characters to be exchanged with other GSM users. The network itself can also generate messages and broadcast to multiple MSs or to a specific MS. For example, a welcome message can be sent to a subscriber when he or she roams onto a new network; in addition, it can provide useful information, such as how to retrieve voicemail. The SMS service also transfers ring tones and logos to the MS.

The SMS slightly blurs the image of the user traffic being separate from signaling because, in a sense, the messages are user traffic; they are for human processing (written and read), rather than for communication between network entities.

The SMS does not have subcategories. It has the following operations:

• forwardSM

• sendRoutingInfoForSM

• reportSMDeliveryStatus

• readyForSM

• alertServiceCentre

• informServiceCentre

The following sections examine each of these.

forwardSM

Both the mobile originating (MO-SMS) and mobile terminating SMS (MT-SMS) procedures use the forwardSM operation to carry text messages between the MSC where the subscriber roams and the SMS-IWMSC or the SMS-GMSC, respectively. Figure 13-8 shows the MO-SMS procedure.

In Appendix L, Example L-6 contains a trace that shows the decode of a MAP operation forwardSM, including its SMS text.

sendRoutingInfoForSM

The SMS-GMSC uses this message during an MT-SMS to deliver an SMS to the MSC in whose area the subscriber is currently roaming. The message contains the subscriber's MSISDN, and the result contains the destination MSC's ISDN number. SCCP then uses this ISDN number to deliver the SMS using a forwardSM message. Figure 13-9 shows the MT-SMS procedure.

Figure 13-9. MAP Operations Involved in Sending an SMS from the SMS-SC to the MS

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In Appendix L, Example L-2 shows a trace showing a VLR's decode calling an HLR (to perform a location update).

reportSMDeliveryStatus

If the SMS-SC cannot deliver the MT-SMS to the MS (because the subscriber is not reachable, for example), then the SMS-SC returns a negative result to the SMS-GMSC. Upon receiving this result, the SMS-GMSC sends a reportSMDeliveryStatus to the HLR, which, in turn, sets a message waiting flag in the appropriate subscriber data. The HLR also sends an alertServiceCentre message to the SMS-IWMSC to inform it about the negative SM delivery and waits until the subscriber can be reached. When the VLR (also aware of SM delivery failure) detects that the subscriber is again reachable, it sends a readforSM message to the HLR. The HLR, in turn, sends an alertServiceCentre message to the SMS-IWMSC, which informs the SMS-SC. The delivery process then begins again with a forwardSM message.

informServiceCentre

If a sendRoutingInfoForSM is received for a subscriber that is currently unavailable, the HLR sends this message to the SMS-GMSC.