No mandatory protocol between the Service Centre (SC) and the Mobile Switching Centre (MSC) below the transfer layer is specified by GSM; this is a matter of agreement between SC and PLMN operators.
This TR specifies three approaches to the specification of protocol stacks of communication protocols for the purpose of relaying short messages and alerts between Short Message Service Centres and Gateway/Interworking MSCs (GMSC) for the Short Message Service (SMS). One approach is based upon use of the complete OSI reference model (see X.200), another approach is based upon the use of only the lower three OSI layers, and another approach is based upon the use of CCITT Signalling System No. 7 (see Q.700).
Alternative protocol stacks are specified via ASN.1 encoding rules (see X.208 and X.209). These alternative protocol stacks are examples for the implementation of the Short Message Relay Layer (SM RL). The requirements placed upon the Short Message Relay Layer are briefly described in clause 9 of TS 03.40