2 Overview of Short Message Service (SMS) support

04.113GPPPoint-to-Point (PP) Short Message Service (SMS) support on mobile radio interfaceRelease 1996TS

The purpose of the Short Message Service is to provide the means to transfer messages between a GSM PLMN Mobile Station (MS) and a Short Message Entity via a Service Centre, as described in GSM 03.40. The terms "MO" ‑ Mobile Originating ‑ and "MT" ‑ Mobile Terminating ‑ are used to indicate the direction in which the short message is sent.

This ETS describes the procedures necessary to support the Short Message Service between the MS and the MSC and vice versa, as described in GSM 03.40.

The procedures are based on services provided by the Mobility Management sublayer as described in GSM 04.07/04.08.

2.1 Protocols and protocol architecture

The hierarchical model shows the layer structure of the MSC and the MS.

MSC

MS

SM‑AL

SM‑TL

SM‑RL

SMR

< SM‑RP protocol >

SMR

CM‑sublayer

SMC

< SM‑CP protocol >

SMC

MM‑sublayer

RR‑sublayer

Figure 2.1/GSM 04.11: Protocol hierarchy

The CM‑sublayer, in terms of the Short Message Service Support, provides services to the Short Message Relay Layer.

On the MS‑side the Short Message Relay Layer provides services to the Short Message Transfer Layer. The Short Message Relay Layer is the upper layer on the network side (MSC), and the SM‑user information elements are mapped to TCAP/MAP.

The peer protocol between two SMC entities is denoted SM‑CP, and between two SMR entities, SM‑RP.

Abbreviations:

SM‑AL Short Message Application Layer

SM‑TL Short Message Transfer Layer

SM‑RL Short Message Relay Layer

SM‑RP Short Message Relay Protocol

SMR Short Message Relay (entity)

CM‑sub Connection Management sublayer

SM‑CP Short Message Control Protocol

SMC Short Message Control (entity)

MM‑sub Mobility Management sublayer

RR‑sub Radio Resource Management sublayer

2.2 Use of channels

The short message service will be supported by an SDCCH or SACCH, depending on the use of a TCH:

‑ when a TCH is not allocated, the short message service will use an SDCCH;

‑ if a TCH is allocated during a short message transaction on an SDCCH, the short message transaction will stop and continue on the SACCH associated with the TCH;

‑ if a TCH is allocated for the short message service, the short message service will use the associated SACCH;

‑ when an entity using a TCH finishes its transaction, the RR‑sublayer may choose to continue an ongoing short message transfer on the SACCH, or optionally transfer it to an SDCCH.

Table 2.1/GSM 04.11 summarizes the use of channels for the short message service. Arrows indicate changes of channel.

Table 2.1/GSM 04.11: Channels used for short message transfer

Channel dependency

Channel used

TCH not allocated

SDCCH

TCH not allocated ‑> TCH allocated

SDCCH ‑> SACCH

TCH allocated

SACCH

TCH allocated ‑> TCH not allocated

SACCH ‑> SACCH opt. SDCCH³

2.3 Layer 2 SAPI 3 handling

General rule:

The Radio Resource Management (RR reference GSM 04.08) in the Mobile Station and on the network side (i.e. in the BSC) shall establish the acknowledged mode of operation on SAPI 3 whenever needed, i.e. when a message requiring SAPI 3 transfer shall be transmitted.

RR shall control the layer 2 also for SAPI 3, and keep knowledge of the mode.

The network side may initiate release of the acknowledged mode for SAPI 3 either explicitly (by the use of DISC‑ and UA‑frames, see GSM 04.06) or indirectly by channel release (see GSM 04.08).

This means:

‑ the Mobile Station side will initiate establishment of SAPI 3 acknowledged mode in the case of mobile originating short message transfer;

‑ the network side will initiate establishment of SAPI 3 acknowledged mode in the case of mobile terminating short message transfer;

‑ the network side may choose to keep the channel and the acknowledged mode of operation to facilitate transfer of several short messages for or from the same Mobile Station. The queuing and scheduling function for this should reside in the MSC.