No mandatory protocol between the Service Centre (SC) and the Mobile Switching Centre (MSC) below the transfer layer is specified by GSM/UMTS specifications; this is a matter of agreement between SC and PLMN operators. However, SC and PLMN operators can base these protocols on the following principles:
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provide the service defined for SM-RL (see clause 9.3);
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be based on widely accepted telecommunications protocols in the public domain;
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permit open interconnection.
Annex B held information that is now contained in
TS 23.038.