The exchange of messages between an MS and an SME involves the entities shown in Figure 4. The architecture for providing SMS over a generic IP CAN is described in TS 23.204 (see clause 5). The basic network structure of the SMS is depicted in Figure 5.

*: **: ***: The reference points of Figure 5 support the short message transfer in the following way:

• message transfer on reference point 1 is described in clause 5;
• the operations performed on reference point 2 are described in TS 29.002 for MAP based interface and in TS 29.338 for Diameter based interface (S6c);
• the operations performed on reference point 4 are described in TS 29.002 for MAP based interface;
• message transfer on reference point 3 is described in clause 4.2;
• message transfer on reference point 5 is supported by the protocol described in TS 24.011.
• the operations performed on reference points 6 are described in TS 29.118.

Reference point 3 is used to support communications between:

• MSC ↔ SMS-GMSC / SMS-IWMSC;
• SGSN ↔ SMS-GMSC / SMS-IWMSC;
• SMS Router ↔ SMS-GMSC;
• SMS Router ↔ MSC;
• SMS Router ↔ SGSN.

If any of the above two end nodes are in the same PLMN, the definition of this reference point is left to the operators. For example, this reference point may use:

• PSPDN; or
• CCITT SS no 7 (according to TS 29.002).

If any of the above two end nodes are in different PLMNs, this reference point shall use CCITT SS no 7 according to TS 29.002, unless otherwise bilaterally agreed.

One SC may be connected to several PLMNs, and may be connected to several MSCs (SMS-GMSCs or SMS-IWMSCs) within one and the same PLMN. The SC is addressed from the mobile by an E.164 [17] number in the numbering plan of the PLMN to which the SC is connected. This E.164 [17] number shall uniquely identify the SC to that PLMN. There may be an intermediate network between the PLMN and the SC; in this case the PLMN must autonomously make a connection to the SC using the SC address in this intermediate network. No mandatory protocol between the SC and the MSC below the transfer layer is specified by GSM/UMTS; this is a matter for agreement between SC and PLMN operators. However, Annex A provides an example protocol stack which could be used.

The SC should be capable of:

• submitting a short message to an MS, retaining the responsibility of the message until
  1. the report has been received; or
  2. the Validity-Period expires.
• receiving a report from the PLMN;
• receiving a short message from an MS;
• returning a report to the PLMN for a previously received short message.

The detailed functionality of the SC is outside the scope of the present document, and is for the SC operator to define. However, the following functional requirements are mandatory for all SCs in order to support the SM-TP (see clause 9) towards the PLMN:

1. To identify each SMS-DELIVER sent to an MS in a unique way, a time stamp value is included in the field TP-Service-Centre-Time-Stamp, TP-SCTS, of the SMS-DELIVER. The time stamp gives the time when the message arrived at the SC with the accuracy of a second. If two or more messages to the same MS arrive at the SC within one second, the SC shall modify the time stamp of those messages in such a way that:
   1. all messages to the MS contain different time stamps;
   2. the modification of the time stamps is kept to a minimum.
2. The SC is only allowed to have one outstanding SMS-DELIVER (i.e. a message for which a report has not been received) to a specific MS at a given time.
3. The SC shall be able to initiate overwriting of short messages previously received by the SC if requested by the same originating address (MS or any other source) by use of the same message type.

An SME has the ability of determining which data formats within the Extended Object IE are supported by a specific terminal. The SC has the option of supporting this feature using an SMS-DELIVER PDU. This SMS-DELIVER PDU shall contain the EMS Data Format Delivery Request IE, and be marked for automatic deletion by the mobile station.

The MS, when equipped for SMS, should be capable of:

• submitting a short message TPDU to an SC, retaining the responsibility of the message until:
  1. the report arrives from the network; or
  2. a timer expires.
• receiving a short message TPDU from an SC;
• returning a delivery report to the network for a previously received short message;
• receiving a report from the network;
• notifying the network when it has memory capacity available to receive one or more short messages when it has previously rejected a short message because its memory capacity was exceeded;
• notifying the SC when a short message is intended to replace a short message the MS has previously submitted to the same destination address.

It is recommended that an MS supporting both replying and automatic SC selection (as specified in clause D.2 of Annex D) follows procedures specified in Annex D when replying to MT short messages with MO short messages. It is recommended that an MS supporting a capability for requesting a reply path follows procedures specified in Annex D.

The reference configuration is assumed as in Figure 6, i.e. only the case where the terminal is integrated in the MS is considered.