This Technical Specification (TS) defines the architecture of layer 3 and its sublayers on the GSM Um interface, i.e. the interface between mobile station and network. It also defines the basic message format and error handling applied by the layer 3 protocols. The corresponding protocols are defined elsewhere: - the Radio Resource (RR) management protocol is defined in TS GSM 04.08; - the Mobility Management (MM) protocol is defined in TS GSM 04.08; - the Call Control (CC) protocol is defined in TS GSM 04.08; - the Supplementary Services (SS) protocol is defined in TS GSM 04.10, TS GSM 04.8x and TS GSM 04.9x; - The Short Message Service (SMS) protocol is defined in TS GSM 04.11. The communication between sublayers and adjacent layers and the services provided by the sublayers are distributed by use of abstract service primitives. But only externally observable behaviour resulting from the description is normatively prescribed by this Technical Specification.