This TS defines the principal architecture of layer 3 and its sublayers on the GSM Um
interface, i.e. the interface between Mobile Station (MS) and network; for the CM sublayer, the description is restricted to paradigmatic examples, call control, supplementary services, and short message services for non-GPRS services. It also defines the basic message format and error handling applied by the layer 3 protocols.
to see the ToC for advanced Rel-15 Word version