name: SAPI
full_form: Service Access Point Identifiers
context: The logical control channels are defined in 3GPP TS 45.002[ None ]  [32]. In the following those control channels are considered which carry signalling information or specific types of user packet information: i) Broadcast Control CHannel (BCCH): downlink only, used to broadcast Cell specific information; ii) Synchronization CHannel (SCH): downlink only, used to broadcast synchronization and BSS identification information; iii) Paging CHannel (PCH): downlink only, used to send page requests to Mobile Stations (MSs); iv) Random Access CHannel (RACH): uplink only, used to request a Dedicated Control CHannel; v) Access Grant CHannel (AGCH): downlink only, used to allocate a Dedicated Control CHannel; vi) Standalone Dedicated Control CHannel (SDCCH): bi-directional; vii) Fast Associated Control CHannel (FACCH): bi-directional, associated with a Traffic CHannel; viii) Slow Associated Control CHannel (SACCH): bi-directional, associated with a SDCCH or a Traffic CHannel; ix) Cell Broadcast CHannel (CBCH): downlink only used for general (not point to point) short message information; x) Notification CHannel (NCH): downlink only, used to notify mobile stations of VBS (Voice Broadcast Service) calls or VGCS (Voice Group Call Service) calls. Two service access points are defined on signalling layer 2 which are discriminated by their Service Access Point Identifiers (SAPI) (see 3GPP TS 44.006[ None ]  [19]): i) SAPI 0: supports the transfer of signalling information including user-user information; ii) SAPI 3: supports the transfer of user short messages. Layer 3 selects the service access point, the logical control channel and the mode of operation of layer 2 (acknowledged, unacknowledged or random access, see 3GPP TS 44.005[ None ]  [18] and 3GPP TS 44.006[ None ]  [19]) as required for each individual message.
description: Service Access Point Identifiers (SAPI) are identifiers used in the signalling layer 2 to discriminate between different service access points. These identifiers play a crucial role in selecting the appropriate service access point, logical control channel, and mode of operation for layer 2, based on the type of information being transferred, such as signalling information or user short messages.
explanation: SAPI is classified as an Identifier because it serves as a unique code to identify specific service access points in the signalling layer 2, enabling the selection of the appropriate channel and mode for information transfer. This aligns with the context provided from 3GPP TS 24.008, highlighting its role in managing logical channels and controlling the flow of information.
section_title: 1.5 Use of logical channels in A/Gb mode
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biotag_Description: B-Parameter,I-Parameter,I-Parameter,I-Parameter,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O,O